NEET Practice Questions (MCQs) with Answers & Solutions

(a) Coenzyme is an organic nonprotein molecule that associates with an enzyme molecule in catalsying biochemical reactions. It usually participates in the substrate-enzyme interaction by donating or accepting certain chemical groups.                                                                                   Holoenzyme is a complex comprising of enzyme molecule and its cofactor. The enzyme is catalytically active in this state.                                                                                                                         Apoenzyme is an inactive enzyme that must associate with a specific cofactor molecule in order to function.                                                                                                                                         Isoenzyme or isozyme is one of the several forms of an enzyme that catalyse the same reaction but differ from each other in such properties as substrate affinity and maximum rates of enzyme-substrate reaction.

(a) Amylases forms a class of enzymes that split or hydrolyse starch. Those found in animals are called $\mathrm{\alpha }$-amylases while those found in plants are termed as $\mathrm{\beta }$-amylases. $\mathrm{\alpha }$-amylase hydrolyses starch to maltose and other products and is found in vertebrate saliva and intestine. $\mathrm{\beta }$-amylase hydrolysis starch to maltose but by different process. Pancreatic amylase is amylopsin. Salivary amylase is ptyalin.

(c) Essential amino acids are amino acids that must be included in the diet of higher vertebrates because they cannot be synthesized by these organisms. The amino acids essential in the human diet are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.

(c) NAD and NADP are coenzymes for many oxidoreductase enzymes. The nicotinamide nucleotide play a widespread role as coenzymes to many dehydrogenase enzymes occurring both in the cystol and within the mitochondria. They are therfore key components of many metabolic pathways affecting carbohydrate, lipid and amino acid metabolism.

(b) The difference in the structure of pentose sugar of RNA and DNA is that, ribose sugar has hydroxyl group at C-2 position while deoxyribose sugar has a lone H atom in place of OH group.

(b) A phosphodiester bond is a group of strong covalent bonds between the phosphorus atom in a phosphate group and two other molecules over two ester bonds. Phosphodiester bonds are central to all life on Earth as they make up the backbone of the strands of DNA. In DNA and RNA, the phosphodiester bond is the linkage between the 3' carbon atom and the 5' carbon of the ribose sugar.

(d) Phosphodiester bond is the covalent bond that links a phosphate group and a sugar group by means of an oxygen bridge, in the sugar phosphate backbone of a nucleic acid molecule.

(c) Denaturation is to produce a structural change in a protein or nucleic acid that results in the reduction or loss of its biological properties. Denaturation involves unfolding of the double helix of nucleic acids, with loss of secondary and tertiary structure. It is caused by heat (thermal denaturation), chemicals and extremes of pH. The differences between raw and boiled eggs are largely a result of denaturation.

(a) Keratin is an insoluble, sulfur-containing fibrous protein that serves as the main structural molecule in hair and nails and is also present in the skin. Keratin is synthesized by cells known as keratinocytes and may be hard or soft. hard keratin is found in hair and nails, soft keratin is found in the epidermis of the skin in the form of flattened non-nucleated scales that slough continually. The main differences in various keratins arise from their sulfur content. If there are many cysteine disulfide cross-links, then there is very little flexibility as in horns, claws, hooves or nails. In wool, skin and muscle proteins, there are fewer disulfide cross-links which allows some stretching but returns to normal upon relaxation of tension.

(a) Vitamin B₆ (pyridoxine) in the form of pyridoxal phosphate coenzyme aids in the breakdown of amino acids and the metabolism of fats and carbohydrates. It also supports the central nervous system and maintains healthy skin. Deficiency of vitamin B₆ causes the epileptiform convulsions in human infants and a hypochromic microcytic anemia may develop.