BIOMOLECULES AND ENZYMES

Detailed explanation-1: -Among the 20 common amino acids, five have a side chain which can be charged. At pH=7, two are negative charged: aspartic acid (Asp, D) and glutamic acid (Glu, E) (acidic side chains), and three are positive charged: lysine (Lys, K), arginine (Arg, R) and histidine (His, H) (basic side chains).

Detailed explanation-2: -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.

Detailed explanation-3: -Panel 1: The 3 positively charged amino acids are shown. These are: histidine (His, H), lysine (Lys, K), and arginine (Arg, R).

Detailed explanation-4: -Lysine and arginine are the two positively charged amino acids in proteins that have high aqueous pKa’s ( 10.5 for Lys1 and  13.8 for Arg2) indicating a strong propensity to carry charge at physiological pH.

Detailed explanation-5: -Lysine is an essential amino acid that has a net positive charge at physiological pH values making it one of the three basic (with respect to charge) amino acids. This polar amino acid is commonly found on the surfaces of proteins and enzymes, and sometimes appears in the active site.