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

Polysaccharide of chitin

Chitin is a major structural polysaccharide found in inveterate animals and lower plants. It is widely available biopolymer obtained principally from shrimp and crab shell waste.

It is occurs as highly ordered microfibrils in many species, in a variety of arrangements, from diatom spines to cell was components of many fungi and yeasts.

Chitin is a type of polysaccharide composed of an amino sugar and consisting of a straight chain molecule by binding D-glucosamines in which each amino sugar is acetylated by β-1,4 linkage.

Chitin

Among the three forms, α-chitin is the most abundant, crystalline, tightly compacted and stable form in which the chains are arranged in an anti-parallel fashion. α-chitin is found where extra hardness proves essential, whereas β- and γ-chitin provide toughness, flexibility and mobility.

Chitin is considered the second most plentiful biomaterial following cellulose. When chitin is deacetylated to about 50% of the free amine form, it is referred to as chitosan.

Chitin is produced by removing calcium carbonate and proteins from the shells. In the production of chitin, calcium carbonate is first dissolved by stirring the shells in dilute hydrochloric acid at ambient temperature.

Proteins are then extracted from the decalcified shells by threatening them with dilute aqueous sodium hydroxide; crude chitin is then obtained.
Polysaccharide of chitin

The importance of gluten in flour for baking

The principle functional protein of wheat flour is gluten. Gluten has the important property that when it is moistened and worked by mechanical action, it forms an elastic dough.

This dough may be stretched in two directions and form sheets or films, or it may be stretched in all directions under the pressure of expanding gas and form bubbles as does bubble gum. Gluten from strong flour is more perfect colloidal gel, as shown by its greater hydration capacity than gluten from soft flour.

Baking quality of flour depends on both the quantity and quality of gluten that can be formed from it.

Yeats breads are prepared from soft dough using hard wheat flours to form gluten structure that is strong and elastic.  The structure may contain starch and sugar or other ingredients such as eggs and fat. The yeast is responsible for the production of CO2 within the gluten structure.

The wet gluten is a cohesive, elastic mass that expands greatly on baking to form a light porous ball. Baked gluten contains about 85% protein, 8% lipids and variable amounts of starch depending on the thoroughness of washing.

Wheat starch does not form elastic films as does gluten; rather, the moistened starch, when heated forms a paste and stiffens or more correctly gelatinizes.

Baking the gluten shows that it expands greatly as the steam within it expands and that it coagulates when heated to form the structure of the baked product.

Generally, cookies are produced using soft wheat flour that ahs relatively weak gluten strength (e.g. pastry flour, a blend of soft red and soft wheat).

The cookies usually consist of high fat and sugar contents, and low moisture content.
The importance of gluten in flour for baking

Cell wall structure

The cell is the basic structural unit if all plant tissues. These cell are surrounded by cell walls that provide an elastic support for retaining the contents of the cell.

The cell also has a membrane layer, which is located just inside the cell wall and which controls the passage of liquids into and out of the cell. The cell is filled with jelly-like substance, termed the cytoplasm, which is composed of protein, sugars, salts and other substance dispersed in water.

Mature ells also contain vacuoles, which are separate compartments filled with a fluid, cell-sap and which are composed of dissolved sugars, salts, organic acids, pigments and other materials.

Also located within the cytoplasm are separate inclusion bodies , called plastids, which contains the pigment of chlorophyll. These plastids are only both 4 to 10 nm in diameter.
Cell wall structure

Sphingolipids

Sphingolipids, phosphosphingolipids, and glycosphingolipds, constitute a family of molecules particularly abundant in the nervous system.

Sphingolipids are a diverse group of over 100 bioactive lipids involved in many aspects of cellular function. 

They all share a common sphingosine backbone, which may be linked via an amide bond to a fatty acid, or phosphorylated to form sphingosine-1-phosphate.

As sphingolipids are almost exclusively membrane components, particularly of the plasma membranes, that are suitable to interact with external ligands or other cells though they hydrophilic groups and with protein and lipids partners of the membrane through their hydrophobic groups.

Sphingolipids are found to be associated with plasma lipoproteins and their presence can be influence lipoprotein, including high density lipoprotein, metabolism.

Meat, milk and fish are important sources of sphingolipids in the diet. The sphingolipids is in plants are mainly cerebrosides with glucose, galactose, mannose and inositol.
Sphingolipids

Proteins in Food

Proteins in Food
Proteins are present in all living plant and animal tissues
Composition
Proteins, like fats and carbohydrates, contain carbon hydrogen and oxygen. In addition they contain about sixteen per cent nitrogen which distinguishes the proteins for carbohydrates and fats.

Proteins are built from simpler compounds called amino acids. The amino acids contain a basic (amino – NH2) and an acidic (carboxyl –COOH) group in their molecules.

Some of amino acids cannot be synthesized in the body, but are essential for maintenance in human beings. These amino acids, which have to be supplied in the food, are called essential amino acids.

The remaining amino acids, which can be synthesized from others in the body, are termed as non essential amino acid, because our body does not have to depend for their supply in the foods we eat.

Structure of Proteins
The structure of a protein molecule is dependent on the number and kind of amino acids in it. The sequence of amino acids and the manner on which these are linked indicates the structure of protein.

Early experiments on protein quality ere carried out with purified proteins. On the basis of these experiments, proteins were classified as complete, partially complete and incomplete proteins depending on their ability to maintain life and promote growth.

Thus, animal proteins from milk, eggs and meat were all included in the list of complete proteins.
Gliadin, one of the proteins from wheat was found able to maintain life, but lack sufficient amounts of some amino acids necessary for growth.

Therefore, it was reported to be partially incomplete protein.

Zein one of the proteins from corn and gelatin are the examples of proteins, which are incapable of maintaining tissues or supporting growth. Therefore, these are totally incomplete proteins.

However, later experiments with diets made up of mixed protein from plants sources were found to maintain life sand support growth of animals.
Proteins in Food