Proteins

Proteins, which constitute about 10 to 12% of the cell contents, are macromolecules having a significant role in the metabolism of a cell.

Proteins are polymers of amino acids. Each protein is a linear, unbranched, polypeptide molecule. The order in which amino acids occur is specific for each polypeptide. The protein molecules are very large and highly complex macromolecules. Insulin (human) has 53 amino acids arranged in two polypeptide chains of 22 and 31 amino acids. Human serum albumin has 582 amino acids in its polypeptide chain.

The number and variety of proteins varies from species to species and within a species from cell to cell. A bacterium Escherichia coli may have about 3000 types of proteins. A human liver cell may have millions of proteins. However, all these proteins are synthesised from the same twenty protein amino acids.

Proteins show four structural levels namely, primary, secondary, tertiary and quaternary.

• The linear sequence of amino acids in a polypeptide chain represents the primary structure. The enzyme ribonuclease and the protein myoglobin function only in their primary structure.

• If the polypeptide chain is coiled into a spiral or helix to have a three-dimensional structure, then it is called secondary structure. e.g., keratin of skin.

fig. 14.4 - Levels of Structure in Proteins

• If the helical polypeptide molecule is folded on itself assuming a complex but specific structure such as spherical or rod like, then it is called tertiary structure. e.g., globulins of blood.

• Some proteins have two or more polypeptides, each with primary, secondary and tertiary structures then it is called quaternary structure. e.g., Insulin and hemoglobin.

Proteins are classified on different basis such as structure and composition. The following table gives a classification of proteins.

fig. 14.5 - Classification of Proteins

Proteins serve a number of significant functions in the cells. The most significant of these is their role as biological catalysts enzymes. Apart from this, proteins are important to a cell as structural components necessary for growth, repair and regeneration. Proteins also have significant role in the transport of respiratory gases, as antibodies in the defence mechanism, as biological buffers and so on.

The following table lists the functions of proteins in various capacities in a cell.

PROTEIN CLASSIFICATION BASED ON FUNCTION
Type	Examples	Occurance	Function
Structural	Collagen Sclerotonin Keratin Elastin Mucoproteins Viral coat proteins	Connective tissue Bodywall of insects Skin of mammals Connective tissue Synovial joints Surrounding the nucleic acid of virus	Providing rigidity Protection Protection Providing flexibility Lubrication Protection
Hormones	Insulin Glucagon Adreno corticotropic hormone	Islets of Langerhans in pancreas Pituitary gland	Glucose metabolism Stimulating of adrenal cortex
Transport	Haemoglobin Ilemocyanin Myoglobin Serum albumin	Vertebrate blood Invertebrate blood Skeletal muscle Vertebrate blood	Transport of oxygen Transport of oxygen Transport of oxygen Transport of lipids and fatty acids
Defensive	Antibodies Fibrinogen Prothrontin	In blood In blood In blood	Form complexes with foreign proteins Involved in clotting mechanism
Contractile	Actin Myosin	Muscle fibres Muscle fibres	Moving filaments Stationary filaments
Storage	Ovalbumin Casein	Egg yolk Milk	Reserve food Reserve food
Toxins	Snake venom Diptheria toxin	Salivary juice of snakes Dipthena patients	As enzymes Produced by diptheria bacteria

fig. 14.6- Functions of Proteins

Apart from these functions, proteins are also important in membranes functioning as carriers and transport sites. Most proteins function as enzymes.