What classification is given to the amino acid K?

The amino acid K, which refers to lysine, is classified as positively-charged. This charge arises from the presence of an amino group in its side chain, which is protonated at physiological pH, giving lysine a positive charge. The overall structure of lysine includes a long aliphatic chain with an amino group that can accept a proton (H⁺), thus leading to its basic properties.

Due to this positive charge, lysine often plays a significant role in protein interactions, especially in binding to negatively charged molecules, such as DNA, or in catalyzing enzyme reactions.

In the context of the other classifications, nonpolar refers to amino acids that do not have a charge and are hydrophobic, which does not apply to lysine. Negatively-charged amino acids, such as aspartate or glutamate, possess a carboxylate group that carries a negative charge, again contrasting with lysine's properties. Aromatic amino acids contain an aromatic ring structure, which is not present in lysine. This makes the classification of lysine as positively-charged the most accurate.