Proteins are the main structural components of the cell. All proteins contain C, H, O and N. Some contain P, S, and few have Fe, I and Mg incorporated into the molecule.
Amino acids: Amino acids are the building blocks of proteins. Some other types of molecules may be attached to proteins e.g. nucleic acids, lipids and carbohydrates. There are many amino acids known to occur, but only 20 are commonly found in proteins. Plants re able to make all amino acids from simpler substances The amino acids are built on a common plan Each contains a carbon atom It is called alpha carbon, to this a hydrogen atom , an amino group - NH2, a Carboxyl group -COOH and a variable group known as - R group, are attached. The R group has a different structure in each of the 20 biologically important amino acids and determines their individual chemical properties.
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| general structure of amino acids Chains of amino acids Peptide Bond: How amino acids fit together? The bond formed to unite two amino acids is called peptide bond. It is between amino group of one amino acid and carboxyl group of another amino acid. Thus the bond is between C-N. The linkage of C=O and NH is called amide or peptide linkage. Water is removed in this process. The chain of amino acids joined by peptide bonds is called polypeptide chain. Proteins have levels of structures: An analysis of protein shape shows that proteins can have up to four levels of structures (1) Primary structure (2) Secondary structure (3) Tertiary structure (4) Quaternary structure.  Primary structure The primary structure of a protein is the sequence of amino acids joined by polypeptide bonds. In 1953 Fredric Sanger determined the amino acid sequence of hormone insulin. It was a laborious ten years task. Insulin is a small protein. The protein is constructed by two polypeptide chains of 21 and 30 amino acids. There is also a disulphide bridge between two cysteine of the smaller chain.  Secondary structure The secondary structure of a protein comes about when the polypeptide takes a particular orientation in space. The two possible patterns of amino acids within a polypeptide are cc (alpha) helix structures, beta pleated sheet. α (Alpha) helix: The polypeptide chain is loosely coiled in a regular spiral shape called an α -helix.  The twisting of the chain: There is a slightly negative charge on the oxygen and nitrogen and slightly positive charge on the hydrogen associated with a peptide bond. These charges make it possible for hydrogen bonding to occur between C = o of one amino acid and the N - H of another amino acid in a polypeptide. Hydrogen bonding between every fourth amino acid holds the spiral shapes of an helix. Thus amino acid at 1 would be bonded to amino acid 5, number 2 to number 6, and so on. The secondary structure is usually studied by technique of X-ray crystallography. In this process X-ray is passed through a purified crystal of protein, when this is done X-rays are scattered by the crystal and form a characteristic pattern which can be recorded on a photographic plate. Using mathematical technique, the structure of the protein can be inferred from the pattern it produces. X-ray diffraction data indicate that the helix makes a complete turn for every 3.6 amino acids. Example of helix protein is keratin (found in hair, nails) wool, collagen (found in skin). 13- pleated sheet: In the 13 -pleated structure of proteins, the polypeptide chains are more extended and lie parallel with hydrogen bonding between chains. It is the main protein component of the silk.  Tertiary structure: Usually the polypeptide chain bends and folds extensively forming a precise compact globular shape called the tertiary structure of proteins. The structure is maintained by the interaction of ionic bonds, hydrogen and disulphide bonds as well as hydrophobic interaction e.g. myoglobin.  Quaternary structure: Many highly complex proteins consist of more than one polypeptide chains. The separate chains are held together by hydrophobic interaction of hydrogen and ionic bonds. This is known as quaternary structure. It can be more understandable by seeing the structure of hemoglobin. It is the oxygen carrying red pigment found in the red blood cells of vertebrates. It consists of four separate polypeptide chains of two types, namely two alpha and two beta chains. Each alpha contains 141 amino acids and each beta chain contains 146 amino acids.  Significance of Sequence of Amino acids: A protein molecule may have 51 to 3000 amino acids. All the amino acids must be in proper position in the polypeptide chain. If the proper site of even a single amino acid is changed, the normal structure and function of the protein is changed e.g. sickle cell anemia. Hemoglobin consists of two alpha and two beta chains. The fault occurs in the sixth amino acid in the beta chain. The glutamic acid of the normal hemoglobin is replaced by valine in the hemoglobin of a sickle cell.   Shapes of protein molecules: The shapes of protein molecules are in accordance with their function. Thus shape of protein molecules has a significant role. The shapes may be fibrous, globular and intermediate. Fibrous: These proteins have long parallel polypeptide chains cross-linked at intervals forming long fibers or sheets. These have secondary structures physically tough and insoluble in water. These perform structural function e.g. collagen (tendons, bones, and connective tissue), myosin (in muscle), silk (Spider’s web) and keratin (hair, horn, nail, feathers). Globular: Polypeptide chains are tightly folded to form spherical shape, having tertiary structure. These are the most important ones and are easily soluble. These form enzymes, antibodies, and some hormones e.g. insulin. Intermediate: These proteins are intermediate in shape between globular and fibrous protein and are soluble e.g. fibrinogen which forms insoluble fibrin when blood clots. Functions of proteins Proteins play important functions in the living organisms. A brief account of functions of proteins is given as follows. 1. Proteins play an important role in membranes where they function as enzymes, receptors, and transport sites. 2. Proteins form the structural part in the organisms, such as collagen is the component of connective tissue of bones, tendons and cartilage. Keratin forms feathers, nails, hair and horn. Elastin forms elastic connective tissues in ligaments. Viral coat proteins wrap up the nucleic acid of virus. 3. Enzymes are proteins e.g. trypsin catalyses hydrolysis of proteins. Some hormones like insulin, glucagon (a pancreatic hormone that raises blood sugar by promoting conversion of glycogen to glucose in the liver) and ACTH 4. (Adrenocorticotropic hormone secreted by anterior lobe of pituitary gland) are proteins which help to regulate glucose metabolism. 5. Respiratory pigment hemoglobin transports oxygen in vertebrate’s blood and myoglobin stores oxygen in muscles. 6. Some proteins are antibodies, fibrinogen and thrombin .have protective functions. Antibodies form complexes with foreign particles. Fibrinogen form fibrin in blood clotting. Thrombin takes part in blood clotting mechanism. 7. Protein fibers like actin and myosin take part in muscle contraction. 8. Ova albumen is egg white protein and casein is milk protein. Their function is storage. 9. Snake venom are enzymes and diphtheria toxin is made by diphtheria causing bacteria |
