Understanding Prolamins: The Unique Proteins of Cereal Grains

Prolamins stand as distinctive proteins found exclusively in grasses, particularly cereals, encompassing a varied array of molecules such as glutelin, glutenin, and gliadin. These proteins, with subunits ranging from 30 to 90 kDa, serve as the primary nitrogen reservoir within the endosperm of cereal seeds. This role is facilitated by extensive gene duplication in endoreduplicated cells, leading to massive accumulation levels.

Comprising notable quantities of proline and glutamine, prolamin proteins derive their name from these amino acids. However, they contain relatively low levels of arginine, lysine, and histidine. Structurally, prolamins feature α-helix globular domains, enriched with 6-8 cysteine residues and 3-4 disulfide bonds, rendering them highly soluble in ethanol (40-70%).

Prolamins play a pivotal role in determining the hard texture of mature cereal seeds, a critical trait for grains like maize. Nonetheless, deficiencies in essential amino acids within prolamins contribute to subpar grain protein quality.

Across various cereal species, prolamins exhibit diversity. In wheat, barley, and rye, prolamins are categorized into sulfur-rich (α-, β-, γ-gliadins), sulfur-poor (ω-gliadins), and high-molecular-weight proteins. Furthermore, these proteins are named based on their cereal source, such as secalin (rye), hordein (barley), and avenin (oats), among others.

However, the significance of prolamins extends beyond their nutritional attributes. In wheat and related grains, these proteins are implicated in celiac disease, a condition sometimes termed as gluten-sensitive enteropathy due to its association with the wheat prolamin fraction known as gluten.

Understanding prolamins not only sheds light on the intricate biology of cereal grains but also underscores their implications for human health. From their pivotal role in nitrogen storage to their influence on grain texture and protein quality, prolamins stand as a testament to the complexity and significance of plant proteins in agriculture and nutrition. Moreover, their involvement in conditions like celiac disease underscores the importance of continued research into these unique proteins and their impacts on human health.
Understanding Prolamins: The Unique Proteins of Cereal Grains

The Essential Role of Transport Proteins in Cellular Function

Transport proteins are vital components within cells that play a crucial role in regulating the flow of substances, including nutrients, into and out of cells. These proteins serve as carriers or provide protein-lined passages through which water-soluble materials of small molecular weight may diffuse, facilitating essential cellular processes.

Moreover, transport proteins are not confined solely within cells; many traverse body fluids, transporting nutrients and other molecules between organs. Notably, lipoproteins serve as carriers for various molecules, ensuring their delivery to specific tissues.

Specialized transport proteins also facilitate the transportation of crucial nutrients and molecules. For instance, certain proteins are dedicated to carrying fat-soluble vitamins, water-soluble vitamins, and minerals throughout the body.

Among the most pivotal transport proteins is hemoglobin, found within red blood cells. Hemoglobin is an iron-containing protein responsible for transporting oxygen from the lungs to tissues throughout the body and returning carbon dioxide from tissues to the lungs. Remarkably, over 98% of the oxygen in the blood is bound to hemoglobin molecules.

Hemoglobin's structure consists of a protein component called globin and a red pigment component known as heme. The iron within hemoglobin imparts the characteristic red color to blood.

Abnormalities in hemoglobin levels can have profound health implications. Low hemoglobin levels, indicative of anemia, can result from various factors, including blood loss, nutritional deficiencies (such as iron, vitamin B12, or folate acid), bone marrow disorders, or abnormal hemoglobin variants like sickle cell anemia. Additionally, deficiencies or imbalances in minerals like zinc, copper, and vitamins A, B complex, and C can contribute to anemia, particularly in females.

Conversely, elevated hemoglobin levels may occur in individuals living at high altitudes or those who smoke. Furthermore, dehydration can artificially inflate hemoglobin levels, which normalize upon rehydration.

The significance of transport proteins extends beyond hemoglobin, as other proteins facilitate the movement of molecules across cell membranes, ensuring proper cellular function and homeostasis.

In conclusion, transport proteins are indispensable for maintaining cellular integrity and functionality. From regulating nutrient transport within cells to facilitating the circulation of vital molecules throughout the body, these proteins play a multifaceted role in sustaining life. Understanding the mechanisms by which transport proteins operate is crucial for comprehending various physiological processes and addressing associated health conditions effectively.
The Essential Role of Transport Proteins in Cellular Function

The Diverse Functions of Proteins in Living Systems

Proteins are integral to the functioning of living organisms, serving as essential structural elements with a wide array of biological roles. With numerous variations, each protein carries out specific functions crucial for the survival and operation of organisms.

A primary function of proteins resides in their enzymatic roles. Comprising enzymes, proteins act as catalysts, greatly accelerating biochemical reactions without undergoing degradation. Enzymes are indispensable for processes such as food digestion, facilitating the breakdown of carbohydrates, fats, and proteins to ensure efficient nutrient absorption.

Transport proteins play a crucial role in the movement of vital molecules within organisms. They facilitate the transfer of iron, fats, minerals, and oxygen, guaranteeing their distribution to cells and tissues where they are necessary for various physiological processes.

Structural proteins provide the framework for cellular and organismal architecture, forming the basis of cell structures and imparting stability and shape to tissues and organs. The intricate association of protein subunits with themselves and other molecules contributes to the dynamic organization of biological systems.

Hormonal proteins serve as chemical messengers, regulating physiological activities to maintain internal equilibrium. For example, insulin, a critical hormone produced by the pancreas, regulates blood glucose levels by facilitating glucose uptake into cells, ensuring stable energy metabolism.

Defensive proteins fulfill a vital function in protecting organisms against foreign invaders. These proteins, including antibodies produced by specialized white blood cells, play a pivotal role in the immune system by recognizing and neutralizing pathogens, safeguarding the organism from infection and disease.

While carbohydrates serve as the primary energy source, proteins can act as an alternative fuel reserve. In times of need, such as prolonged fasting or intense physical activity, proteins can be metabolized to provide energy, though this is typically a last resort due to their essential role in cellular functions.

Moreover, proteins contribute significantly to the maintenance and repair of body tissues. As the building blocks of cells, proteins are essential for tissue growth, repair, and regeneration. The constant turnover of tissues necessitates the continuous synthesis and replacement of proteins to ensure the integrity and functionality of biological structures.

In conclusion, proteins display a remarkable diversity of functions essential for the survival and operation of living organisms. From enzymatic catalysis to structural support, hormonal regulation, immune defense, and energy provision, proteins serve as indispensable components of biological systems, highlighting their significance in maintaining life processes.
The Diverse Functions of Proteins in Living Systems

The Essential Role of Proteins in Human Health and Functionality

Muscles, the powerhouses of our physical activity, rely heavily on proteins for their growth and maintenance. Proteins, constructed from amino acids, are the building blocks that ensure our muscles remain healthy and capable of performing work. However, the significance of proteins extends far beyond muscle development alone; they play a fundamental role in various vital functions within the human body.

Upon consumption, proteins undergo digestion, breaking down into amino acids that are then distributed to all cells. These amino acids are integral to the synthesis of biologically active proteins, which serve diverse purposes ranging from structural support to enzymatic activity. The body's intricate metabolic pathways, known as anabolism and catabolism, orchestrate this process, ensuring the assembly of proteins necessary for cellular functions.

Structural proteins constitute a significant portion of the body's protein repertoire, forming the foundation of muscle and connective tissues. Enzymes, another vital protein category, optimize biological processes, making them more efficient and conserving energy. Additionally, proteins participate in immune responses, act as carrier molecules, and facilitate DNA translation, underscoring their multifaceted roles in maintaining health and homeostasis.

Transport proteins, specialized in moving nutrients and molecules across cell membranes, ensure essential substances reach their intended destinations. The critical role of proteins is particularly pronounced during periods of growth and maturation, such as infancy, where ample protein intake is crucial for development. The intricate interplay between DNA and proteins dictates which amino acids are utilized for cellular construction and function, highlighting the complexity of biological systems.

The quality of dietary protein is paramount, influencing growth in children and the overall health of adults. Factors such as digestibility and amino acid composition determine protein quality, emphasizing the importance of a balanced diet. Furthermore, protein consumption must be accompanied by essential vitamins and minerals to optimize its utilization and support overall health.

In summary, proteins are indispensable molecules that underpin numerous physiological processes essential for human health and functionality. From muscle development to enzymatic catalysis and immune defense, proteins play diverse and critical roles throughout the body. Understanding the significance of proteins and ensuring their adequate intake is vital for maintaining optimal health and well-being.
The Essential Role of Proteins in Human Health and Functionality

Protein Sources and Complementation

Protein plays a fundamental role in human nutrition, contributing significantly to various bodily functions. Amino acids, which serve as the fundamental building blocks of proteins, are indispensable for protein synthesis and the development of tissues and organs.

Proteins are categorized into two main groups: essential and nonessential. Essential proteins, or amino acids, are those that the body cannot produce independently and must be acquired through dietary means. These encompass nine crucial amino acids, including histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Conversely, nonessential proteins are internally synthesized by the body.

Complete proteins, containing all nine essential amino acids in adequate proportions, are notably found in animal-derived sources such as meat, fish, eggs, and dairy products. These food items offer a well-rounded amino acid profile essential for optimal health.

In contrast, incomplete proteins lack one or more essential amino acids. Examples of incomplete protein sources encompass grains, fruits, and vegetables. While these dietary components are beneficial, they necessitate strategic combinations to ensure the acquisition of all essential amino acids.

Animal-based protein sources present a diverse array of options, including beef, lamb, poultry, fish, eggs, and dairy products, which are widely consumed worldwide. Not only do these foods deliver high-quality protein, but they also furnish essential vitamins and minerals crucial for overall well-being.

Increasingly, plant-based protein options are gaining popularity due to their health benefits and eco-friendly nature. Soy and spirulina exemplify plant-based complete proteins. Soy-based products like tofu and tempeh furnish a comprehensive amino acid profile, catering to the dietary needs of vegetarians and vegans.

Protein complementation emerges as a strategic approach to address deficiencies in amino acid profiles. By combining incomplete proteins from varied sources, such as pairing beans with rice or legumes with grains, individuals can achieve a balanced intake of essential amino acids, ensuring optimal protein consumption.

In conclusion, comprehending the classification of proteins, encompassing essential and nonessential amino acids, is pivotal for sustaining a well-rounded diet. Embracing a diverse array of protein sources, spanning both animal and plant-based options, facilitates the fulfillment of essential nutrient requirements. Protein complementation stands as an effective method to address dietary gaps and foster overall health and vitality.
Protein Sources and Complementation

Understanding Proteins in the Human Body

Proteins serve as the fundamental building blocks of the human body, playing a crucial role in various physiological functions. Comprised of smaller molecules known as amino acids, proteins undergo digestion upon consumption, eventually reaching the body's cells where they are reassembled to fulfill specific needs.

The human body harbors a myriad of proteins, each possessing a distinctive structure and function. Primarily, proteins function as structural components, constituting essential tissues like muscles and connective tissues. Collagen, the most abundant protein in mammals, imparts elasticity and strength to tissues such as skin and bone. Similarly, keratin forms the basis of hair and nails, showcasing a tightly coiled helical structure.

Beyond structural roles, proteins serve as enzymes, facilitating efficient biochemical reactions. Digestive enzymes, for instance, catalyze the breakdown of complex food molecules into simpler constituents like glucose. Amylase, a notable digestive enzyme, aids in starch digestion, ensuring the release of vital nutrients for energy production.

Moreover, proteins play pivotal roles in immune responses, acting as carriers or transport molecules, and aiding in DNA translation. Enzymes derived from dietary proteins execute specific tasks essential for cellular functions, requiring specific vitamins and minerals for optimal performance.

The significance of proteins is particularly pronounced during periods of growth and development, such as infancy. Newborns necessitate ample proteins for growth and maturation, with genetic instructions dictating the synthesis of specific proteins tailored to meet structural and functional demands.

Dietary protein contributes substantially to the amino acid pool, alongside protein turnover and liver biosynthesis. This pool serves as a reservoir of amino acids crucial for various physiological processes, ensuring a steady supply of building blocks necessary for protein synthesis and cellular function.

In essence, proteins form the cornerstone of human biology, playing indispensable roles in structural integrity, enzymatic activities, immune responses, and growth. Understanding the multifaceted functions of proteins underscores their significance in sustaining overall health and well-being.
Understanding Proteins in the Human Body

The Essential Role of Protein in Human Diets

Protein plays a vital role in human diets, constituting approximately 10-15% of the energy intake across diverse dietary patterns. Its significance extends beyond mere sustenance, permeating the cellular structure of the body and serving as essential components for enzymes, molecules involved in substance transportation, and certain hormones.

Aside from their nutritional importance, proteins also exert significant influence over the sensory characteristics of food, particularly those sourced from animals, dictating textures in a variety of culinary offerings from meats to dairy products.

Various food groups, including meat, poultry, legumes, eggs, nuts, milk, yogurt, and cheese, are rich sources of high-quality protein. Even seemingly lesser contributors such as vegetables and grains accumulate substantial protein content, with wheat and flour being key indicators of bread quality.

Proteins often form intricate complexes with carbohydrates and lipids within food matrices, each combination imbuing distinct properties. Glycoproteins and lipoproteins, for example, not only impact food texture but also serve practical functions as edible emulsifiers.

Despite their incorporation into food, proteins undergo further transformations during heating processes like boiling, baking, or roasting. These processes can lead to the degradation of amino acid side chains or interactions with other components, resulting in unique flavors. However, excessive heat exposure can diminish the nutritional value of proteins, highlighting the delicate balance between culinary techniques and preserving nutritional integrity.

The quality of dietary protein significantly influences both child growth and adult health maintenance. High-quality proteins provide essential amino acids necessary for optimal bodily function, whereas low-quality counterparts may fall short, impairing physiological processes.

Digestibility and amino acid composition are critical factors determining protein quality, influencing the ease of breakdown and the range of essential amino acids available. This underscores the intricate interplay between dietary choices and overall health outcomes.

In essence, proteins transcend their basic role as nutrients, serving as architects of both culinary enjoyment and physiological well-being. As we navigate the complexities of nutrition, understanding and harnessing the power of protein remains crucial for promoting health and vitality.
The Essential Role of Protein in Human Diets

Hard wheat flour

There is no difference between bread flour and strong bread flour, strong flour, or hard flour or hard wheat flour. In the US, “bread flour” is the most common label for hard flour. “Strong flour” is what it called in the UK and other regions.

Hard wheat flour is made solely from hard wheat grains. Hard grains refer to kernels that have a high protein content. When strong flour is used to make dough, it requires a lot of kneading, as it has a high protein content.

Hard wheat flour is typically used to make breads. But this type of flour also can be used in several other types of recipes. Here are a few uses for bread flour: Sourdough bread, white bread, pretzels, dinner rolls, cinnamon buns, bagels.

Hard wheat flour tastes similar to other flours, but it’s slightly off-white and feels coarser and denser.

While hard wheat flours contain around 12 to 15 percent protein, soft wheat flours, like all-purpose flour, are usually about 8 to 12 percent protein. The high protein content also gives hard flour a slightly higher gluten content than other types of wheat flour, which makes it ideal for breads that need to rise well. Gluten is what holds the dough together, gives elasticity, allows the dough to maintain its shape, to trap the carbon dioxide inside when the dough starts to rise.
Hard wheat flour

Protein C and protein S

Protein C and protein S are glycoproteins, predominantly synthesized in the liver, that are important components of the natural anticoagulant system in the body. They are Vitamin K dependent and serve as essential components in the maintenance of physiologic hemostasis.

Proteins C is a precursor of the serine protease, activated protein C (APC). Its proteolytic activation by thrombin occurs on the surface of endothelial cells and involves thrombomodulin and endothelial PC receptor (EPCR).

Proteins S acts as a principal cofactor to protein C. The two work together to help regulate and control blood clot formation by inactivating specific coagulation factors (factors V and VIII) that are required to generate and form blood clots.

The protein C anticoagulant system provides important control over the blood coagulation cascade. Any alteration in this pathway, either hereditary, iatrogenic, or otherwise, may interfere with normal coagulation. Deficiencies in protein C and protein S are associated with thrombotic complications.
Protein C and protein S

Structure of protein

In most biological processes on the molecular, cellular or organism level, proteins are the main actors, and it is therefore important to understand their function. The function of a protein is determined by its structure.

Proteins contain Carbon, Hydrogen, Oxygen and Nitrogen as the major components while Sulfur and Phosphorus are minor constituents. Nitrogen is characteristic of proteins. The variety of protein structures is made possible by the order and arrangement of the amino acid building blocks. All amino acids have the same basic structure.

Proteins are polymers: similar molecules (called monomers) are repeated many times to form a chain. Chains that are less than 40-50 amino acids or residues are often referred to as polypeptide chains since they are too small to form a functional domain. Larger than this size, they are called proteins.

Each polypeptide chain has a unique amino acid sequence decided by the genes. The primary structure is maintained by the covalent bonds of the peptide linkages.

Amino acids (except glycine) contain a central chiral carbon, i.e., a carbon atom covalently linked to four different groups of atoms, often called carbon α (Cα). This central chiral atom is linked to an amino group and a carboxylic acid group, thus the term amino acid.

The sequence of amino acid side chains gives proteins unique properties. They not only determine the fold of the protein, but they also determine surface properties that are important for selective interactions with other molecules and catalysis of chemical reactions.

Generally, the polypeptide chains are linear. However branching points in the chains may be produced by interchain disulphide bridges. The covalent disulphide bonds between different polypeptide chains in the same protein (interchain) or portions of the same polypeptide chain (intrachain) are also part of the primary structure.
Structure of protein

Protein: General structure and functions

The word protein is derived from Greek word, “proteios” which means primary. As the name shows, the proteins are of paramount importance for biological systems. Proteins are biochemical molecules consisting of polypeptides joined by peptide bonds between the amino and carboxyl groups of amino acid residues.

Proteins are made up of hundreds or thousands of smaller units known as amino acids. Most organisms use 20 naturally-occurring amino acids to build proteins. The linear sequence of the amino acids in a protein is dictated by the sequence of the nucleotides in an organisms’ genetic code. Amino acids can combine to form long linear chains known as polypeptides. Each type of polypeptide chain has a unique amino acid sequence.

The sequence of amino acids determines each protein’s unique 3-dimensional structure and its specific function such as catalysis of biochemical reactions, mechanical support and immune protection, movement, transport of ligand, transmits nerve impulses, and control growth and differentiation.

The proteins function to regulate specific steps in metabolism – one step, one protein. Hence, many proteins are needed.

The polypeptide must fold into a specific three-dimensional structure before it can perform its biological functions. The function of all proteins depends on their ability to specifically interact with other molecules. Such specificity is possible because polypeptides with different amino acid sequences fold into different tertiary structures.

Proteins are not entirely rigid molecules. They undergo conformational changes upon ligand binding. Each kind of protein evolved to interact with a specific molecule or ligand. For example, transport proteins (such as hemoglobin) bind to specific ligands (in this case oxygen) and transport the ligand to a site where it is needed. Hemoglobin, the transporter of oxygen is a tetrameric protein (alpha 2, beta 2), with each monomer having a heme unit. Binding of oxygen to one heme facilitates oxygen binding by other subunits.

Storage proteins such as myoglobin, another oxygen-binding protein, allow the cell to store higher concentrations of the ligand than otherwise would be possible.

Catalytic proteins— the enzymes—convert the ligands into other molecules. They act as biochemical catalysts. The first step in enzymatic catalysis is the binding of the enzyme to the substrate. This, in turn, depends on the structural conformation of the active site of the enzyme, which is precisely oriented for substrate binding

Many proteins have structural or mechanical functions. Structural proteins interact with specific molecules, often endowing the bound molecules with special biological properties. For instance, one class of proteins, the histones, binds to DNA to form compact nucleoprotein structures called nucleosomes, while a second class of proteins combines with RNA to form the ribonucleoprotein complex known as the ribosome.

Structural proteins collagen is the most abundant protein in mammals and is the main fibrous component of skin, bone, tendon, cartilage and teeth.

Proteins are also important in cell signaling, immune responses, cell adhesion, and the cell cycle.
Protein: General structure and functions

Wheat prolamins

Mature cereal grains contain about 8-15% protein. Prolamins, the major class of storage proteins in cereals are synthesized at the endoplasmic reticulum during seed development and deposited into subcellular structures of the immature endosperm, the protein bodies.

Prolamins are defined as proteins extractable in aqueous ethanol. Prolamins of wheat are the major components of gluten, the properties of which determine the quality of wheat flour for various technological processes including breadmaking.

Wheat prolamins consist of a mixture of monomeric gliadins and polymeric glutenin subunits, which are present in approximately equal amounts. Together they form 80% of the total storage protein content in the wheat kernel. The remainder is albumins (12%) and globulins.

The glutenins are composed of a group of high molecular weight subunits, as well as a group of low molecular weight subunits. the low molecular weight glutenins are structurally similar to the S-rich gliadins and are therefore termed as aggregated gliadins. The monomeric gliadin subunits range in size from about 30 to 60 kDa and were originally classified into groups of α, β, γ and ω gliadins, based on their migration in acidic gels.

The prolamin fractions of wheat and related grains are implicated in the causation of celiac disease. The prolamins are the major storage proteins in these grains, so all of these grains contain ample gluten and are considered hazardous for celiac sufferers.
Wheat prolamins

General physical properties of protein

Proteins (Greek proteios, “primary” or “of first importance”) are biochemical molecules consisting of polypeptides joined by peptide bonds between the amino and carboxyl groups of amino acid residues.

Proteins are the building blocks of life and there are estimated to be almost one million different proteins in a normal cell. Each protein has very different and unique physical properties.

*Proteins are colorless and usually tasteless. These are homogeneous and crystalline.

*Size. Proteins are very large polymers of amino acids with molecular weights that vary from 6000 amu to several million amu. Proteins are too large to pass through cell membranes, and are contained within the cells where they were formed unless the cell is damaged by disease or trauma.

*Structure. The proteins range in shape from simple crystalloid spherical structures to long fibrillar structures.
A. Globular proteins- These are spherical in shape and occur mainly in plants, esp., in seeds and in leaf cells. These are bundles formed by folding and crumpling of protein chains. e.g., pepsin, edestin, insulin, ribonuclease etc.
-dissolve in water or form stable suspensions.
-not found in structural tissue but are transport proteins, or proteins that may be moved easily through the body by the circulatory system
-e.g., hemoglobin and transferrin.
B. Fibrillar proteins- These are thread-like or ellipsoidal in shape and occur generally in animal muscles. Most of the studies regarding protein structure have been conducted using these proteins. e.g., fibrinogen, myosin etc.
-insoluble in water
-major components of connective tissue, elastic tissue, hair, and skin
-e.g., collagen, elastin, and keratin.

*Aggregation. Physical protein aggregation results from the association of unfolded proteins. According to the Lumry-Eyring model, the native protein is transformed in a transitional protein species that is prone to associate and to form protein aggregate. The protein aggregation pathway occurs in three steps:
(1) protein unfolding;
(2) association of unfolded monomers in oligomers; and
(3) nucleation, growth and condensation in aggregates

*Denaturation. Denaturation refers to the changes in the properties of a protein. In other words, it is the loss of biologic activity. In many instances the process of denaturation is followed by coagulation— a process where denatured protein molecules tend to form large aggregates and to precipitate from solution.

*Amphoteric. Like amino acids, the proteins are amphoteric, i.e., they act as acids and alkalies both. Depending on pH, they can exist as polyvalent (cations, anions or zwitter ions). These migrate in an electric field and the direction of migration depends upon the net charge possessed by the molecule. The net charge is influenced by the pH value. Each protein has a fixed value of isoelectric point (pl) at which it will move in an electric field.

By definition the net charge is zero and the total charge is maximal at the isoelectric point. Lowering or raising the pH tends to increase the net charge towards its maximum, while the total charge always becomes less than at the isolectric point.

*Solubility. The solubility of proteins is influenced by pH. Solubility is lowest at isoelectric point and increases with increasing acidity or alkalinity. This is because when the protein molecules exist as either cations or anions, repulsive forces between ions are high, since all the molecules possess excess charges of the same sign. Thus, they will be more soluble than in the isoelectric state.

Two different types of protein solubility are distinguished. True or thermodynamic solubility refers to the concentration of the protein in solution, which is in equilibrium with a crystalline solid phase of that protein. In contrast, apparent solubility corresponds to the concentration of protein in a solution, which is in equilibrium with a solid amorphous precipitate of the same protein.

*Optical activity. The optical activity of proteins is due not only to asymmetry of amino acids but also to the chirality resulting from the arrangement of the peptide chain.
General physical properties of protein

Common properties of protein

Proteins play a fundamental role not only in sustaining life, but also foods derived from plants and animals. Proteins exhibit a number of common properties that just be accounted for in any definition of these compounds:

*There are polymeric of high molecular weight, which are built up by the linking together of a large number of small molecules.

*They are amphoteric, i.e., they being able to act as an acid or a base. This enables them to resist small changes in pH. 

*Following complete hydrolysis of a protein, the hydrolysate consists entirely of amino acids (except that additional groups, such as heme, iron, copper, may also be found in the case of a conjugate protein). It is commonly recognized that amino acids being linked by peptide bonds formed between α-amino and α-carboxylic acid groups of neighboring amino acids in the polypeptide sequence.

*In their polymeric structures, the amino acid units of proteins are joined together in definite sequences and exist in definite three-dimensional conformations. This sequence built from a limited number of well-defined building blocks, the 20 genetically determined amino acids and a smaller number of posttranslational modifications of them.
Common properties of protein

Peptide bonds of protein

All proteins yield amino acids when hydrolyzed and all but two of these are α-amino carboxylic acids. Proline and hyroxyproline are α-imino acids.

In proteins, amino acids are united through amide linkages between the α-carboxyl and α-amino functional groups of adjacent amino acid residues. Such bonds are called peptide bonds. Linkage of many amino acids through peptide bonds results in an unbranched chain called a polypeptide. Each component amino acid in a polypeptide is called a “residue” or “moiety”.

Proteins thus have a repeating backbone from which 20 different possible kinds of side chains protrude.

Chemically, the peptide bond is a covalent bond that is formed between a carboxylic acid and an amino group by the loss of a water molecule. In the cell, the synthesis of peptide bonds is an enzymatically controlled process that occurs on the ribosome and is directed by the mRNA template.

The peptide bond has a partial double-bond character, that is, it is shorter than a single bond and is rigid and planar. The bonds between the α-carbons and the α-amino or α-carboxyl groups can be freely rotated and this allows the polypeptide chain to assume a variety of possible configuration.

A peptide chain has directionality because its two ends are different. There is an N-terminal end and a C-terminal end. By convention, the direction of the peptide chain is always

N-terminal end S C-terminal end 

The N-terminal end is always on the left, and the C-terminal end is always on the right.
Peptide bonds of protein

Protein carbonylation

The oxidation of food proteins is one of the emerging research topics within food science, in particular, in the field of meat and meat products.

In cells where the redox balance is disturbed, excessive oxidation of proteins can take place. One of the most damaging of these modifications is protein carbonylation in which proteins are modified through the addition of carbonyl groups to specific side chains located in their active center.

Protein carbonylation is present at physiological level in living cells, and increases with ageing or during pathological conditions.

Carbonylation is generally recognized as one of the most remarkable chemical modification in oxidized proteins. Protein carbonylation increased in trout muscle kept at room temperature for 48 hrs compared to fresh muscle, showing in the myofibriallar protein fraction the biggest increase in carbonylation during fish tainting.

Protein carbonylation can be generated by conjugation with aldehydes produced during peroxidation of polyunsaturated fatty acids or by reacting with carbonyl derivatives generated by reactions with carbohydrates.
Protein carbonylation

What is gliadin?

Seed storage proteins (about 85% of endosperm proteins) are traditionally classified into monomeric (gliadins) and polymeric (glutenins) proteins, based on their solubility.

On the basis of electrophoretic mobility at low pH the wheat gliadins may be separated into the slowest group named the ω-gliadins and three faster groups, named the α, β and γ-gliadins.

The amino acids sequences of some gliadins are known. A very common N-terminal sequence found in α, β and γ-gliadins is Val-Arg-Val-Pro-Glu-Leu-Glu-Pro.

Due to the differences in cysteine content the gliadin components may be divided into S-rich (α, β and γ-gliadins) and S-poor (ω-gliadins) components.

All gliadin components have extremely high glutamic acid content. In some ω-gliadins the glutamic acid content is higher than 50%.

Although not major participants in the gluten polymer, the monomeric gliadins contribute to dough physical/chemical properties by extensive hydrogen bonding with the gluten polymer.

Gliadins are rather hydrophobic, hence their solubility in water or salt solution. More hydrophobic gliadins (the γ-gliadins) increase bread loaf volume, while gliadins from more hydrophilic end of the spectrum decrease loaf volume.
What is gliadin?

Glycoprotein

Glycoproteins may be defined as biopolymers having amino acids and sugar reduces link covalently to each other. They are widely distributed in nature. Found in higher animals, plants and microorganisms, glycoproteins contain a protein chain of any of the 20 naturally occurring L- α -amino acid units.

Proteoglycans are a subclass of glycoproteins with distinctive structure. Glycoproteins differ from monosaccharides protein structures in that they contain considerably less carbohydrate.

The mucopolysaccharides however may be considered a type of glycoprotein. Formed in the Golgi apparatus in the process of glycosylation, glycoproteins are important components of plasma membranes in which they extend throughout the lipid layer.

The molecular weights of glycoprotein range from 15,000 up to million. Some glycoproteins have only one glycochain in a molecule, whereas other glycoprotein possesses many glycochains in a molecule. The carbohydrate chains of glycoproteins are short, consisting perhaps eighth to ten saccharides units.

D-galactose, N-acetyl-D-galactosamine, D-glucose,  N-acetyl-D-glucosamine, D-mannose, L-fucose, and sialic acid, comprise the bulk of the carbohydrate moiety.
Glycoprotein

The protein of monellin

Monellin is an intensely sweet-tasting protein present in the berry of Dioscoreophyllum cumminsii which is known as the serendipity berry because after eating them, a sweet sensation remains in the mouth for some time.

The protein monellin, derived from these berries, is at least 2500 times as sweet as sucrose and causes sour substances to taste sweet.

Monellin composed of two subunits, one being 45 residues (A chain) and the other 50 residues (B chain). X-0ray and NMR methods indicate the structure of monellin has a five-strand antiparallel beta-sheet and a 15 residue alpha-helix.

Monellin undergoes irreversible thermal unfolding at pH 2.5 and 85 °C. leading to an unfolded-like conformation.

It was not commercially viable in its natural form since the protein is unstable, even within a few days at 20 °C and is lost in heating.

Monellin was first purified in 1972. It is heterodimer consisting of 44 amino acid residue A chain and a 50 amino acid residue B chain, held together by non-covalent interactions.

The overall molecular mass is 11kDa and the protein displays a pI value of 9.0 – 9.4.
The protein of monellin

Biological value of protein

Since proteins are of such great importance to animals and man, many plants are grown because of nutritional value of their proteins.

However not all proteins have the some biological value. Biological value can be defined as the percentage of the absorbed nitrogen retained in the body.

Protein quality refers to the ability of a dietary protein to supply the amino acid needs of the body. The fact that a specific food is a rich source of protein does not indicate that the food has any particular value in supporting growth or maintenance. Some proteins are rich in certain essential amino acids, and thus have a high biological value, whereas other proteins are devoid of some of these amino acids or contain them in very small amounts.

For example, gelatin is a protein that is sometimes used in cooking. This protein is available in a pure powdered form; however, the use of gelatin as a food and as the sole source of protein cannot supply the body’s amino acid needs.

In those areas of the world where the main protein source is vegetable protein that lacks certain amino acids, protein deficiency diseases often occur, particularly in children.

 A protein with biological value of 70 or more is considered capable of supporting growth, assuming caloric value of the diet is adequate. This means that 70% of the nitrogen absorbed is retained.
Biological value of protein