Biomolecules and Enzymes

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.

Isoleucine is a branched-chain amino acid with a hydrophobic side chain, which gives it a bitter taste. The bitter taste in amino acids is associated with their nonpolar, aliphatic characteristics.

Methionine contains a thioether group (-CH2-CH2-S-CH3) in its side chain, where a sulfur atom is bonded between two carbon atoms. This is a distinctive structural feature of methionine among the amino acids.

UGA is normally a stop codon, but in the presence of a SECIS element and specialized translation machinery, it codes for selenocysteine, the 21st proteinogenic amino acid. Selenocysteine contains selenium instead of sulfur.

UAG is typically a stop codon, but in certain archaea and bacteria, it codes for pyrrolysine, the 22nd proteinogenic amino acid. Pyrrolysine is used in methane-producing metabolism by some organisms.

Tryptophan, isoleucine, and phenylalanine are all essential amino acids that humans cannot synthesize and must obtain from their diet. Therefore, all of the listed options are correct.

IAA is the universal natural auxin synthesised in shoot apices, leaf primordia and developing seeds from tryptophan.

EC 6.3 includes ligases used to form carbon-nitrogen bonds.

The term abzyme is derived from two words: antibody and enzyme. An abzyme (also known as catmab) is a monoclonal antibody with catalytic activity.

Aspartic acid is an α-amino acid with the chemical formula HOOCCH(NH₂)CH₂COOH. The carboxylate anion, salt, or ester of aspartic acid is known as aspartate. The L-isomer of aspartate is one of the 20 proteinogenic amino acids, i.e., the building blocks of proteins. Its codons are GAU and GAC.