Proteins occupy a central position in the architecture and functioning of living matter. They are intimately connected with all phases of physical and chemical activity, that constitutes life. They are constituted of 51% carbon, 25% oxygen, 7% hydrogen, 16% nitrogen and 4% sulfur. Sometimes small quantities of phosphorous is also present in proteins. They are present in the protoplasm in the form of a dense colloidal solution and in seed in the solid state in the form of protein granules. All enzymes are proteins and also they form part of unit membrane. Cytochromes which are essential for electron transport are made up of iron and protein. Proteins are also present as storage substances as crystalloid in the aleurone grains of cereals.
Structure of Protein
The different structures of proteins are :
1. Primary Structure : The peptide bond and the definite sequence of amino acids give the protein its primary structure. Also important in the primary structure of a protein is the disulphide bond. Since many proteins contain more than one polypeptide chain, a connection between them other than a dipeptide bond is a necessary feature of the protein molecule. The disulphide (-S-S-) bond in the amino acid cysteine is important in this respect.
2. Secondary Structure : Peptide and disulphide bonds are not the only links involved in protein structure. Available evidence indicates that the polypeptide chain have a coiled or helical structure. This arrangement is maintained by noncovalent links called hydrogen-bonds, which occur as a result of hydrogen atom sharing electrons with 2 oxygen atoms. The helical structure of the polypeptide chain constitutes the secondary structure of the protein molecule. In addition to hydrogen bonds, salt links and Van der Waals forces also help maintain the helical structure.
3. Tertiary Structure : It has also been determined that the helix itself may be folded into different specific patterns. The folding of the helix in such a manner is usually referred to as tertiary structure of the protein molecule, and is maintained primarily by hydrogen bonding. Salt links and Vander Walls forces however are also involved.
It is now thought that the secondary and tertiary structure of protein molecule are intimately associated with the molecules biological function. In many instances where these features are disrupted certain specific functions (e.g.. enzyme – activity) are irreversibly lost. This may occur when proteins are exposed to high temperature, changes in pH, U V radiation.
4. Quaternary Structure : (Protein-protein interactions) A quaternary molecule may consist of several polypeptide chains associated either end to end or laterally to form a single unit indicating quaternary structure. e.g, haemoglobin (animal protein) consists of 4 polypeptide chain two designated as alpha and two beta each type with a characteristic amino acid sequence. The polypeptide chain takes a configuration or characteristic shape within the haemoglobin molecule.
Classification of proteins based on composition and solubility :
Now-a-days it is the most accepted system of classifications. This system divides proteins into 3 major groups based on their composition viz. Simple, Conjugated and Derived.
A.. Simple proteins :
These proteins yield only their constituent amino acids on hydrolysis. They are further classified as follows on their solubility properties :
(a) Albumins : Soluble in water and dilute salt solution. They are more abundant in seeds, e.g., Legumilin of legume seeds, Leucosin of cereal seeds.
(b) Globulins : Soluble in dilute salt solutions.Coagulate on heating e.g., Legumin-in peas, Tuberin-in potato tuber.
(c) Glutelins : Soluble in dilute acids and alkalies. Not coagulated by heat, e.g., : Glutelin-in Maize, Glutenin in wheat.
(d) Prolamines : Soluble in 70-90% alcohol e.g. Gliadin-in wheat, Hordein-in-barley, Zein-in corn.
(e) Histones : Rich in basic aminoacids eg. argine, lysin, soluble in water. Associated with nucleic acids.
(f) Protamines : Rich in basic aminoacids and soluble in water, found in nucleic acids.
B.Conjugated (= complex) protein :
In addition to aminoacids, the conjugated protein is associated with a nonaminoacid component. This additional component is referred to as : prosthetic group. Their further classification is based on the nature of prosthetic group as follows :
(a) Metalloproteins : Require a metal as activator e.g : many respiratory enzymes.
(b) Chromoproteins : Protein coupled with a coloured pigment e.g., : Cytochromes.
(c) Nucleoproteins : Protein coupled with nucleic acid.
(d) Lipoproteins : Protein forming complexes with lipids e.g., :Lecithin, Cephalin.
(e) Glycoproteins : These protein contain carbohydrates as prosthetic group.
(f) Phosphoproteins : Proteins linked with phosphoric acid.
C. Derived Proteins :
These are derivatives of proteins resulting from action of heat, enzymes or chemical reagents e.g., proteases, peptones.