BIOMOLECULES AND ENZYMES

Detailed explanation-1: -The side chain imidazole of histidine is an aromatic motif; an ionizable group with the acidic ionization constant around pKa=6.5; a coordinating ligand of metallic cations (for example, Ca2+ and Zn2+); and a hydrogen bond donor and acceptor.

Detailed explanation-2: -The histidine has an imidazole side chain. The side chains of lysine and arginine are fully protonated at pH 7.0, but histidine is only partially protonated at pH 7.0.

Detailed explanation-3: -Like all amino acids, histidine has two functional groups: a carboxyl group (COOH) and an amine group (NH2). Its side chain has a basic group containing an imidazole ring.

Detailed explanation-4: -Histidine (His/H) is the only one of the proteinaceous amino acids to contain an imidazole functional group. It is an essential amino acid in humans and other mammals.

Detailed explanation-5: -Histidine’s R-group is an imidazole group; that is, a 5-membered pentagon ring containing two nitrogens. Due to those two nitrogens, histidine is a polar, hydrophilic amino acid. The nitrogens can also act as proton acceptors, making Histidine a basic, positively charged amino acid at physiological pH.