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 importance of sulphur

Sulphur is an essential component of all living cells. Sulphur is a major inorganic element with biological importance across species. Found in protein molecules in muscles, hair, eyes, brain and other organs. It is abundant in human body and makes up about a quarter of a percentage of human weight.

Sulphur has an atomic weight 32.064, an atomic number of 16, and is represented by the chemical symbol “S”.

Sulphur is found in insulin and thus helps to regulate the metabolism of sugar. It assists in maintenance of oxygen balance necessary for brain function.

Most of the dietary sulphur is provided by the sulfur-containing amino acids methionine and cysteine, with an estimated requirement for young men of ~14 mg/day per kg body weight. Although cysteine is not an essential amino acid, it can be synthesized only from methionine, and it is conventional to consider the sum of methionine plus cysteine (the sulphur amino acids) in consideration of protein quality.

Proteins contain between 3 and 6% of sulfur amino acids. A very small percentage of sulfur comes in the form of inorganic sulfates and other forms of organic sulfur present in foods such as garlic, onion, broccoli, etc.

Allicin is a sulphur-containing compound (diallyl thiosulphinate; thio-2-propene-L-sulfinic acid-5-allyl ester), partially responsible for the flavor of garlic. Formed by the action of allinase on alliin (S-(2-propenyl)-L-cysteine sulphoxide) when the cells are disrupted, releasing the enzyme to act on the substrate. It has antibacterial properties.

Sulphur intake is directly correlational to protein intake source. Chicken, fish and beef proteins contain about 5% of sulphur -containing amino ccid. Dairy products contain around 4% because of casein’s lower sulphur -containing amino acid content.
The importance of sulphur

Methionine structural formula

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

Nonessential amino acid L-Citrulline in food

L-Citrulline is a neutral, non-essential alpha-amino acid that is an important component of the urea cycle in the liver and kidneys. As a non-protein amino acid, L-citrulline is rarely found in food, but is highly concentrated in watermelon.

Citrulline has been isolated in other cucurbitaceous fruits including bitter melon, cucumber, muskmelon, pumpkin, bottle gourd, dishrag gourd and wax gourd. The concentration of L-citrulline in watermelon grown in the United States can range from 1.6 to 3.5 g/kg of fresh watermelon

L-citrulline is a nonessential amino acid that can be metabolized to L-arginine, an essential amino acid for humans, which produces nitric oxide (NO), improving athletic performance and relieving muscle soreness; it also has cardioprotective effects among other properties.

L-Citrulline may improve vascular function through increased L-arginine bioavailability and nitric oxide synthesis. Citrulline is used in the nitric oxide system in humans and has potential antioxidant and vasodilatation roles. L-Citrulline is a precursor of L-arginine. It is effectively converted to L-arginine in the urea cycle.

Nonessential amino acid L-Citrulline in food

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

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 

What is quality protein maize (QPM)?

Maize is a major food for millions of the poor in Africa and Latin America. The protein in normal maize is however, of poor nutritional value due to the limited concentrations of two essential amino acids, lysine and tryptophan.

During the last few decades scientists have developed and improved quality protein maize which looks and tastes like normal maize and yields as much or more.

Quality protein maize contains opaque-2, a single-gene mutations that alters the protein composition of the endosperm portion of the kernel and nearly doubles lysine and tryptophan concentrations.

The name ‘opaque-2’ was coined for the gene because it gave the kernels a chalky appearance.

The nutritive value of QPM protein approaches that of protein from milk. The biological value of common maize protein is equal to about 40% that of milk protein, whereas the biological value of QPM protein is about 90% that of milk proteins.

QPM can help reduce protein deficiencies, particularly in young children, in settings where maize dominates diets.
What is quality protein maize (QPM)?

Properties of peptide bond

Chemical bonds identify the linkage between two atoms. The amino acids are linked by peptide bonds which are formed by a condensation reaction (the loss of a water molecule) between the backbone carboxyl group of one amino acid and the amino group of another.

The amino acids line up so that carboxylic acid group of one is next to the amino group of the other. The bond is formed with removal of water.

The peptide bonds link together to form long chains of amino acids called polypeptide chains. Proteins are long, coiled complex polypeptide chains made of many different amino acids together, end-to-end.

The properties of peptide bond have important effects on the stability and flexibility of polypeptide chains in water.

The stability of the peptide bond, as well as other properties important for the behavior of polypeptides, is due to resonance, the decolonization of electrons over several atoms.

The bond is essential flat, or planar: that is to say that the carbon, nitrogen, and carbonyl oxygen atoms involved in the bond all essentially lie in the same plane. This limits rotation around the bond.

The peptide bonds in proteins can be broken by hydrolysis. Proteins are hydrolyzed into peptides and then to amino acids by boiling with dilute acid or by using a protease enzyme.
Properties of peptide bond

Amino acid of L-Glutamine

L-Glutamine, the most abundant amino acid in muscle tissue, is partly responsible for the transport of nitrogen into the cell for muscle growth and the extraction of ammonia way from muscle tissue. The ‘L’ indicates a natural form of the amino acid glutamine.

It is nonessential amino acid which is synthesized from branched chain amino acids (BCAAs) by transamination requiring the enzyme glutamine synthase.

L-Glutamine makes up approximately 50-60% of the free amino acid within cells but less than 10% of the structural protein within skeleton muscle.

L-Glutamine provides an important contribution muscle growth through protein synthesis an increased growth hormone levels in a recent study by the American Journal of Clinical Nutrition.

In American Journal of Clinical Nutrition 1995, scientists reported that a single 2-gram dose of L-Glutamine can elevated circulating growth hormone levels by over 430%.

Growth hormone is responsible for glucose and amino acid uptake within the body, muscle growth due to protein synthesis and the utilization of fat stores for energy.

L-Glutamine is present in large amounts in the lining of the digestive tract. When muscles or the lining of the intestinal tract are damaged, glutamine plays a vital role in the process that fixes the damage.

It can be exceptionally useful in cases of food poisoning or infection through is action to slow diarrhea. In general it slows down transit time by restoring mucosal function.

L-Glutamine   is metabolized to glutamate, aspartame, lactate and pyruvate and is a substrate for the production of glutathione which is important in antioxidant intracellular defense mechanisms.
Amino acid of L-Glutamine    

Amino acid of cystine, cysteine and methionine

Among the 20 amino acids that constitute the primary structure of proteins, methionine and cysteine, contain a sulphur atom.

Therefore, methionine, cysteine and cystine are the principle sources of sulphur in the body.

Cystine, cysteine and methionine are source of a part of the structure of insulin and the keratin of hair and are involved with oxidation-reduction reactions in the body.

While protein eaten in excess of that required for growth or cellular repair may be utilized as a source of energy, it is not considered that they are efficiently utilized for this purpose.

Cystine is a dimer of cysteine, in which the two cysteine residues are connected via disulfide bond. This due to cysteine is unstable in solution and readily oxidized to the dimer form.

Methionine is an essential amino acid and has to be supplied in the diet. Cysteine is not essential can be synthesized in the body from methionine. The presence of cysteine and cystine in the diet reduces the requirement of methionine.
Amino acid of cystine, cysteine and methionine

Amino acid of tryptophan

Tryptophan is a precursor for serotonin (a neurotransmitter), melatonin (a neurohormone), and niacin. 

Serotonin is a monoamine, and derived from tryptophan is found in the gastrointestinal tract, platelets and in the central nervous system. It is thought to be a contributor to feelings of well-being and happiness. 

Tryptophan is an amino acid from which a substance involved in the constriction of blood vessel is formed, and is present in components of blood involved in clotting.

Like tyrosine, tryptophan is a neutral amino acid that also gains entry into the brain by the large neutral amino acid transporter. Thus, plasma tryptophan will competed with other neutral main acids such as tyrosine and phenylalanine.

It means that the concentration of brain tryptophan will be determined not only by the concentration of tryptophan in plasma but also the plasma concentration of other neutral amino acids.

Tryptophan is the least abundant anion acid in most proteins, accounting on the average of 1 to 1.5% of the total amino acids in typical plant (1%) and animal (1.5%) proteins.

In human infants, the requirement for growth is roughly 12 to 40 mg/kg. In adult humans, the minimum daily requirement has been estimated to be 250 mg/d in males and 160 mg/d in females.
Amino acid of tryptophan

Amino acid of phenylalanine and tyrosine

Phenylalanine is an essential amino acid that serves as a precursor in the biosynthesis of other amino acids including tyrosine.

Phenylalanine and tyrosine together lead to the formation of thyroxin and epinephrine. In addition, tyrosine is the precursor from which the formation of melanin, pigment of skin, hair and part of the eye is made.

Thus, most phenylalanine is converted to tyrosine. Excess phenylalanine is normally eliminated from the body by hydroxylation to tyrosine.

Phenylalanine and tyrosine are used by the body to make the hormones adrenaline and thyroxine and are also involved in the formation of melanin, a pigment present in the skin, hair and part of the eye.

In phenylketonuria (PKU), the missing or defective protein is a liver enzyme that converts the essential amino acid phenylalanine to the amino acid tyrosine.

Without this enzyme, phenylalanine and its metabolites accumulate and damage the developing nervous system.

Phenylalanine and tyrosine share a common pathway of degradation, which occurs in liver, and ultimately yields fumarate and acetyl CoA.
Amino acid of phenylalanine and tyrosine

Amino acid of Arginine

Arginine is found in foods such as meat, dairy products, poultry and fish. The body uses arginine to create nitric oxide, a substance used to relax the blood vessel.

Arginine is a conditionally essential amino acid, which means that under most circumstances, the human body synthesis its own arginine as needed.

It is considered to be essential in newborn infants and during early childhood development when production may not occur quickly enough to keep up with requirements. 

Arginine is required for the detoxification of ammonia and amine resulting in the production of urea.

Arginine is also a precursor for nitric oxide, a simple gas made up of nitrogen and oxygen that penetrates and crosses the membranes of almost all cells in human body and helps regulates many cellular functions.

Arginine also helps the body to manufacture protein and is needed for the production of certain hormones, including growth hormones. It is one of acclaimed five amino acids and it is crucial for muscle growth.

For athlete, arginine is actually a conditionally essential a like intense training in the gym, increases body’s demand for this amino acid.

Deficiency of arginine may occur in the presence of excessive ammonia, excessive lysine, rapid growth, pregnancy, trauma, or protein deficiency and malnutrition.

Symptoms of arginine deficiency include poor wound healing, muscular weakness, impotence, hair loss, skin rash, constipation and fatty liver.
Amino acid of Arginine

Amino acid of Histidine

Histidine was first isolated in 1896 from various proteins. It is an essential amino acid for humans. Histamine is the product of the alpha-decarboxylation of the proteinogenic amino acid histidine.

Histamine plays several important roles in human metabolism, among them serving as the signaling compound for the allergic response.

As a signaling compound, it is normally present in the human body, but its location and concentration are carefully regulated. Histidine is used by many proteins as a regulatory mechanism, changing the conformation and behavior of the polypeptide in acid regions such as the late endosome or lysosome.

Histidine is needed for growth and for repair of human tissues and is converted to a substance that stimulates the secretion of hydrochloric acid in the stomach to facilitate gastric function.

Most of the histidine processed in the body is derived from the diet. Natural sources of histidine include beans, dairy products, eggs, fish, meat nuts, seeds, soy and whey.
Amino acid of Histidine

Amino acid glycine in human body

The proteins coded for by an allele are also molecules that consist of chains of smaller molecules. The subunits of protein are celled amino acids.

The collagen of skin, bone and connective tissue needs to be tough and stretchy, so it has repetitive structure that utilizes the amino acids glycine and hydroxylproline.

Glycine is a simple amino acid and is utilized by the liver to detoxify certain components of foods, such as benzoic acid.

It may also be involved in the synthesis of several body components, such as bile acids. Glutamic acid may act as a source material for the synthesis of other amino acids.

Glycine is a non-chiral molecule. Glycine can also be obtained from different sources such as the diet, protein breakdown, or synthesis from different precursors such as i-serine, glyoxylate, sarcosine, or from condensation of CO2, NH3, and 5,10-MTHF.

Glycine may also be manufactures from threonine through a metabolic process called degradation.

Glycine is a precursor of proteins, porphyrins, bile acids, and creatine; it acts as a neurotransmitter and a conjugating substance that aids the excretion of xenobiotics by making them more water-soluble.
Amino acid glycine in human body

Amino acid cysteine

Cysteine is found extensively in the plant foods human ingest. It is found plentifully in the grasses and thus in the meat of domesticated grazing animals.

Cysteine takes its name from cystine, named after the Greek kustis meaning bladder – cystine was first isolated for kidney stones.

Cystine is the stable form of the sulfur rich amino cysteine. The body is capable of converting one of the other as required.

The sulfur in cysteine molecules plays a crucial role in folding proteins into their correct shapes. For examples sulfur independent proteins are keratin – part of hair, skin and nails and collagen – part of connective tissue like cartilage.

Though sulfur itself does not improve nail growth it was found that cystine may have a positive effect on the growth of hypnochium cells.

The amino acid contains a sulfur group that help to function as antioxidant. Cysteine also can be combined with glutamic acid and glycine in liver cells to form glutathione, which is a principle water soluble antioxidant in cells and the blood.

The human body synthesizes the amino acid cysteine for homocysteine and it is part of human hair, skin and nails as the protein keratin.

Cysteine can be found in red pepper, garlic, onions, broccoli, brussel sprouts, oats and wheat germ.

Amino acid cysteine

Amino acids

Amino acids, peptides and proteins are important constituent of food. They supply the required building blocks for protein biosynthesis.

In another word proteins are polymers of amino acids. Amino acids contain an amino group (-NH2) and an acid group (-COOH). There are twenty amino acid that are found in proteins.

Protein molecules join resemble linked chains, with the links being amino acids joined by peptide bonds.

A peptide bond in formed by the condensation of the amino group (-NH2) or the amino acid with the acid group (-COOH) of another amino acid resulting in the loss of water.

Condensation reactions involve the removal of water (H2O) and formation of a bond. The reversal of this is hydrolysis, which involves the addition of water.

All amino acids have the same basic structure – a carbon with three groups attached to it: an amine group –NH2, and acid group –COOH, and a hydrogen atom H.

Peptide bonds are not easily broken, Cooking would not normally results in the breaking of peptide bonds to yield amino acids from proteins.

The amino acids form proteins due to the reaction between the amino group of one amino acid and the carboxyl group of another.

Of the 22 amino cods, only about half are essential for human nutrition, the amounts of these essential amino acids present in a protein and their availability determine the nutritional quality of the protein.

High quality proteins provide enough of all of the essential amino acids needed by the body to create its own working proteins, whereas low quality proteins don’t.

Protein contributes significantly to the physical properties of food through their ability to build or stabilize gels, foams, emulsion and fibrillar structure.
Amino acids

Amino Acids

Food Science
What Are Amino Acids?Amino acids are chemical structures chained together to compose various types of proteins.

There are 21 different amino acids recognized in science to have some kind of useful function, ranging from providing energy for the body to protecting against cardiovascular disease.

Not one cell in the body could survive without the amino acids that proteins provide.

In fact, next to water, protein is the second most abundant substance in our bodies. How Do Amino Acids Work?When we eat protein rich foods, such as meat, dairy and legumes, our bodies break down the proteins into smaller components, allowing nutrients to be absorbed across the intestinal wall.

Specific proteins contain specific amino acids arranged in a specific order. Once these chains are broken down during digestion into the individual amino acids, the body can recombine them in different patterns to produce different proteins that the body needs for various other functions.

The body uses these new proteins and other individual amino acids not only to provide energy for the body, but also to maintain and repair muscles, bones, tissues and cells, as well as hair, skin and nails.

Amino acids are also essential for the production of neurotransmitters, body fluids, enzymes and many different hormones, such as insulin - the hormone that regulates our blood sugar levels.


The 21 different amino acids are broken down into two very important groups - essential and nonessential. Essential amino acids are those that you must obtain from your diet daily to keep your body in good supply. Nonessential amino acids are those that the body can produce on it's own without your help.

ESSENTIAL AMINO ACIDS
· Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine.
· Must be obtained from food daily!
· Found most commonly in these foods:
· Brown rice, wheat, lentils, nuts, seeds, soy, beef, fish, eggs, pork, poultry, dairy, potatoes, mushrooms, and avocadoes

NONESSENTIAL AMINO ACIDS
· Alanine, Arginine, Asparagines, Aspartic Acid, Cysteine, Glutamic Acid, Glutamine, Glycine, Proline, Serine, Taurine, and Tyrosine.
· Made daily by the body, so no need to worry!

Remember, all of these amino acids are used by the body on a daily basis, so it is important to include protein in your diet. Deficiency is most commonly caused by a low-protein diet, although it can also be brought on by medical conditions such as stress, infection or trauma, all of which force the body to use extra protein for recovery purposes.

Certain medication and chemical imbalances can also cause deficiencies. Symptoms of amino acid deficiency show up as fatigue, reduced metabolism, insomnia, hair-skin-and-nail problems, undue stress and poor health in general.

If you don't think you are getting enough protein in your everyday foods, think about taking an amino acid supplement, which can compensate for inadequate amino acid intake from food. Amino acid supplements can also be used for therapeutic purposes, such as for the prevention of heart disease. Dosages are different depending on which amino acid you're taking and for what reason, so just ask your doctor or local pharmacist about your needs.
Food Science