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Proteins are polymeric molecules in which the subunits are monomers of amino acids which are covalently linked to each other by peptide bonds. These are the most abundant chemical compounds of the living systems. Proteins are versatile, complex molecules with high molecular weight. They play very important roles in every aspect of the structural and function of the living cell. They are the essential constituents of the protoplasm and also form the physical basis of life.

Proteins are responsible for most of the fundamental features of structure and function of living organisms and are also the cause of the tremendous diversity of living systems. Enzymes are proteins which catalyze the different chemical reactions in the cell. As important components of muscle, tendon, bone, membrane, and skin, proteins are involved in structure and motility of organisms. As specific proteins such as haemoglobin, they are involved in the transport of materials. Proteins defend the body against infections as antibodies and control metabolism as various hormones.

Basic Structural Units of Protein Molecules

When proteins are carefully dismantled chemically, they are found to contain chains of smaller units called amino acid. Amino acids are linked together in the protein molecule by peptide bonds formed between the carboxyl group of one amino acid and the amino group of another. Thus proteins are polymers of amino acids, but the amount the number and the arrangement of the amino acids may differ in different protein molecules. There are 20 different amino acids which are found in various proteins. Proteins may be regarded as very large and elaborate polypeptides. However, there is a great difference between proteins and other biologically active macromolecules such as polysaccharides and lipids. Polysaccharides and lipids are composed of smaller subunits which are often identical or at least very similar. Functions of these macromolecules are largely as structural components in the biological system or as storage compounds. Proteins, on the other hand, are composed of 20 different amino acids which are very varied and may also be present in greater or smaller amounts in one protein.

Consequently, there is a great variety and range of proteins. Certain proteins function as structural elements in the biological system. But the majority of them play a dynamic role in the cells, that is, as enzymes catalyzing various biochemical reactions. To fulfill this functional role, proteins must have a particular three-dimensional structure (conformation). Protein structure is understood in the three levels of organization of the protein molecule with increasing complexity.

These are
  1. The primary structure,
  2. The secondary structure and
  3. The tertiary structure of proteins.
Primary Structure
The primary structure of proteins refers to the number, nature and sequence of amino acids along the peptide chains.

Secondary Structure
Although the peptide bonds, the sequence of amino acids and the disulphide cross linkages constitute the primary structure of all proteins, these characteristics do not explain the behaviour of most proteins in solution. Other physicochemical characteristics of proteins depend on the fact that the polypeptide chain is held in a coiled shape by hydrogen bonds. If the peptide chain is coiled like a spiral staircase with three and a half amino acids per turn the structure is called the
α-helix. This is a highly stable structure of protein (Fig. 2.3.3).

Fig.2.3.3 Secondary Structure of Proteins: Arrangement of Polypeptide Chains in α-helix

α-helix is found in proteins such hair nails and in the muscle protein myosin and also in short regions of many globular or non-fibrous proteins. Another variation of the secondary structure of proteins is called the β-pleated sheet. In this structure two or more peptide chains are joined together by intermolecular hydrogen bonds. Hydrogen bonding occurs at right angles to the main chains (Fig2.3.4). Silk is the best example for this type of protein structure. This structure of protein is also found in feathers and claws.

Individual Polvpeptide Strands  

Fig.2.3.4 Secondary Structure of Proteins-pleated Sheets

Fig 2.3.5 Tertiary Structure of Protein Myoglobin

Tertiary Structure
The tertiary structure of protein is defined as its total three-dimensional structure; that is the coiling of long peptide chain into the final compact structure as in the case of globular proteins such as myoglobin (Fig 2.3.5). This sort of structure is also found in many complex enzymes and it has been found that the total three-dimensional tertiary structure is essential for the functional activity of the protein molecule. Three main types of bonds, hydrogen bonds, ionic bonds and hydrophobic bonds are responsible for the formation of the tertiary structure of a protein.

Classification of Proteins

Proteins are classified in different ways:
  1. according to the structure of the protein molecule there are two Fundamental types:
    1. fibrous proteins and
    2. globular proteins;
  2. according to the chemical composition proteins are of three principal types:
    1. simple proteins,
    2. conjugated proteins,
    3. derived proteins.
Simple proteins are formed of amino acids only. Examples are albumin, globulin, histone, globins, etc. Conjugated proteins contain in addition to amino acids, some other non-protein substances (prosthetic groups) in their molecules. For example, the protein globin is combined with an iron containing porphyrin haeme to form haemoglobin. Conjugated proteins are classified according to the nature of the prosthetic group as follows:

Lipoproteins: These proteins are composed of amino acids as well as lipids (e.g. lipovitellin of egg yolk, serum, protein of brain, etc.).

Nucleoproteins: These proteins are made up of amino acids and nucleic acids. Nucleoproteins of the chromosomes contain nucleic acids DNA and RNA and proteins such as histones and protamines.

Glyco- or mucoproteins: These proteins such as those contained in the mucin of saliva and hormones from the pituitary gland, contain amino acids and carbohydrates in their molecules.

Chromoproteins: These proteins contain amino acids and coloured pigments in their molecules e.g. haemoglobin,haemocyanin, cytochrome, etc.

Phosphoproteins: These proteins contain amino acids and phosphoric acid in their molecules e.g. casein of milk and ovovitellin of egg.

Metalloproteins: These proteins contain metals in addition to amino acids in their molecules e.g. enzyme tyrosinase contains copper; carbonic anhydrase contains zinc.

The derived proteins include those which are derived from the original proteins by the action of heat, enzymes or chemical reagents. Best examples of such type of proteins are proteases and peptones. Proteins are also classified according to their solubility properties.

Functions of Proteins

Proteins are the most important constituents of the cell. Apart from being important structural elements, proteins function in the cell in various capacities. The major functions of proteins can be summarised as follows:
  1. They act as biological catalysts (enzymes) for biochemical reactions in the cell.
  2. Proteins control and regulate various biological activities such as growth, differentiation, reproduction and metabolism, in the form of hormones.
  3. Proteins help in the transportation of materials through blood. Various steroid hormones and many other substances are transported through the blood, conjugated to proteins. The transport of oxygen and carbon dioxide depend on the protein haemoglobin in the erythrocytes.
  4. Proteins protect the body from infection as they combat the foreign disease causing agents, in the form of antibodies.
  5. Proteins control the hereditary transmission of characters as they are part of the chromosome as nucleoproteins.

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