Everything you need to know about proteins. Some of the most frequently asked questions are as follows:-
Q.1. What are proteins?
Ans: A protein is a macromolecule composed of one or more polypeptide chains, each with a characteristic sequence of amino acids linked by peptide bonds. The cells of organisms contain thousands of different proteins each with a different function or biological activity. These functions include enzymatic activity, molecular transport, nutrition, cell or organismal motility, structural roles, defense regulation etc.
The protein consists of very long polypeptide chains with 100 to 2000 amino acid residues joined by peptide linkages. However, all proteins are made from the same set of 20 amino acids. The genetic information of a cell is ultimately expressed as protein. There is a segment of DNA (a gene) for each protein which encodes information specifying its sequence in amino acids.
Q.2. How do fireflies produce light in darkness? Also give some other examples of bioluminescence.
Ans: The production of light by a firefly is as a result of light producing reaction involving luciferin a complex carboxylic acid and ATP which is catalyzed by enzyme luciferase. Some fungi that include some mushrooms, marine dinoflagellates, jellyfish and crustaceans are also able to generate bioluminescence which needs a considerable amount of energy.
In the firefly ATP is used in a set of reactions which converts chemical energy into light energy. McElroy and his colleagues at Johns Hopkins University isolated the major biochemical components involved luciferin, a complex carboxylic acid and luciferase an enzyme.
The regeneration of light flashes or bioluminescence needs activation of luciferin by an enzymatic reaction with ATP in which a pyrophosphate cleavage of ATP takes place to form luciferyl adenylate. This compound is then acted upon by molecular oxygen and luciferase to bring about the oxidative decarboxylation of the luciferin to produce oxyluciferin. This reaction which has some intermediate steps is accompanied by emission of light as shown in Fig. 39.1.
Q. 3. How can proteins be classified according to their biological roles?
Ans: The proteins according to their biological functions can be classified into the following types:
(i) Enzymes or the proteins with catalytic activity.
(ii) Transport proteins e.g., hemoglobin of erythrocytes.
(iii) Nutrient and storage proteins, e.g., proteins of seeds, ovalbumin the major protein of egg white and casein (the milk protein).
(iv) Contractile or motile proteins, e.g., actin and myosin in skeletal muscle and protein dynein in flagella and cilia.
(v) Structural proteins, e.g., collagen (protein in leather), keratin (in hair, nails and feathers), fibroin (silk fibres and spider webs), and resilin (hinges of some insects).
(vi) Defense proteins, e.g., immunoglobulin’s or antibodies; fibrinogen and thrombin (blood clotting proteins); and snake venoms, bacterial toxins and plant proteins like ricin.
(vii) Regulatory proteins, e.g., insulin which regulates sugar metabolism and growth hormones of pituatory.
(viii) Monellin, a protein of an African plant which has intensely sweet taste.
(ix) Antifreeze proteins, found in Antarctic fish.
Q.4. What are conjugate proteins? How are they classified?
Ans: Some proteins also have chemical components in addition to amino acids. These are called conjugated proteins. The non-amino acid part of the conjugated protein is usually called its prosthetic group. The conjugate proteins are classified on the basis of the chemical nature of their prosthetic groups.
The major classes of proteins with their prosthetic group and examples are given below:
1. Lipoproteins, with lipids as prosthetic groups e.g., P, – lipoprotein of blood.
2. Glycoproteins, with carbohydrates (sugars) as prosthetic group, e.g., immunoglobulin G
3. Phosphoproteins, with phosphate group, e.g., casein of milk.
4. Haemoproteins, with haeme or iron porphyrin, prosthetic group, e.g., haemoglobin.
5. Flavoproteins, with flavin nucleotides as prosthetic group, e.g, succinate dehydrogenase.
6. Metalloproteins, with iron as prosthetic group, is ferritin, with zinc as alcohol dehydrogenase, with calcium is calmodulin, with molybdenum is dinitrogenase and with copper is plastocyanin.
Q.5. Give the techniques to study structure and function of proteins.
Ans: Study of individual proteins can be done by using following techniques:
(i) Separation and purification of proteins
(ii) Proteins can be characterized by electrophoresis using detergent sodium dodecyl sulphate (SDS).
(iii) Isoelectric focusing
(iv) The antibody-antigen interaction is also used to quantify and localize proteins, e.g. ELISA and RIA.
Q.6. What do you mean by structure of proteins? Elaborate.
Ans: There are four levels of structure or architecture in proteins, which are known as primary structure, secondary structure, tertiary structure and quaternary structure.The primary structure covers all the covalent bonds between amino acids and is characterized by the sequence of amino acids in the peptide. It also tells about locations of disulphide bonds. The secondary structure results from hydrogen bonding between side chains of amino acids to form α- helices and β- sheets.
The tertiary structure is because of folding of proteins as a result of interaction between the amino acid side chains usually controlled by chaperone proteins. The quaternary structures results when more than one polypeptide makes up the functional protein.
Q.7. Why do woolen clothes shrink when washed in hot water or dried in an electric dryer and the silk on the other hand does not shrink under similar conditions? Explain.
Ans: The major components of silk fibres and spider webs are fibroin, a type of structural protein and a member of fibrous proteins called |3- keratins. |3- conformation makes fibroin of silk soft and flexible in comparison to a- keratin of hair, feather and nails (which has a helix cross-linked by disulphide bonds) due to which a- keratin is tough, insoluble protective structure of varying hardness and flexibility.
Heating and steaming of wool increases the spacing of structural units. It is because of soft and flexible nature of β- conformation of β- keratins of silk. The Indian masses have been obtaining silk by boiling the cocoons in hot water since ancient times.