Biomolecules and Enzymes

The folding of the polypeptide chain is stabilized by weak noncovalent interactions.

Kinetin (6-furfurylaminopurine) is a synthetic cytokinin, a class of plant growth regulators that promote cell division. The structure consists of a purine base (adenine) with a furfuryl amino group attached at the 6-position. It was the first cytokinin discovered and is used extensively in tissue culture.

 Gamma globulins are antibodies produced in blood, and not in liver.

Albumin is the most abundant plasma protein.

The sulphur content of proteins is approximately 0-4%. Only two amino acids contain sulphur: cysteine (~2.3% S) and methionine (~3.2% S). Since these constitute a small fraction of total amino acids, the overall sulphur percentage in proteins is very low, making option B correct.

Methionine, Alanine, and Tryptophan are all nonpolar (hydrophobic) amino acids based on their side chains. Methionine has a sulfur-containing alkyl chain, Alanine has a simple methyl group, and Tryptophan has a bulky indole ring - all are hydrophobic. Therefore, 'All of the above' is the correct answer.

Cystine contains a disulfide (-S-S-) bond formed by the oxidation of two cysteine thiol groups. This covalent linkage between two cysteine residues is crucial for protein structure stabilization. Arginine contains guanidino group, threonine has hydroxyl group, and asparagine contains amide group - none have disulfide bonds.

Proteins contain approximately 16% nitrogen by weight, which is used in Kjeldahl method for protein estimation. This constant nitrogen content is due to the peptide bonds linking amino acids. The nitrogen comes from the amino groups present in all amino acids that make up proteins.

Both D-serine and D-aspartate are naturally occurring D-amino acids found in mammalian brain tissue, functioning as neuromodulators and neurotransmitters. D-serine acts as a co-agonist at NMDA receptors, while D-aspartate regulates hormone release. D-alanine is not typically found in brain tissue.

Hopkins-Cole reaction (also called glyoxylic acid test) is specific for tryptophan, which contains an indole ring that reacts with glyoxylic acid in presence of concentrated sulfuric acid to form a purple-colored product. Glycine, serine, and histidine do not contain indole rings and do not give this reaction.

Homoserine, homocysteine, and ornithine are all non-protein amino acids, meaning they are not incorporated into proteins during translation but serve metabolic functions. Homoserine is a methionine biosynthesis intermediate, homocysteine is involved in methionine cycle, and ornithine participates in urea cycle.

Proline is unique among standard amino acids because its amino group is part of a pyrrolidine ring, making it a secondary amine (imino group) rather than a primary amine. This structural difference affects protein conformation and makes proline a helix-breaker. Other amino acids have primary amino groups.

Valine, leucine, and isoleucine are all branched-chain amino acids (BCAAs) characterized by aliphatic side chains with branching at the beta-carbon. These essential amino acids are important for protein synthesis, energy production, and muscle metabolism. The 'branched chain' refers to their structural similarity.

The one-letter amino acid code uses N for asparagine (asparagine - asparaginic acid + amine) and Q for glutamine (glutamine sounds like Q). D and E represent aspartate and glutamate respectively, while C and M are cysteine and methionine. The one-letter system was developed for efficient sequence notation.

Pyrrolysine was discovered in 2002 as the 22nd genetically encoded amino acid, found in certain methanogenic archaea and bacteria. It is encoded by the stop codon UAG in these organisms. Lysine, hydroxyproline, and alanine were discovered much earlier and are standard amino acids.