Which group is the amino acid H categorized under?

The amino acid histidine (commonly denoted as H) is categorized under positively-charged amino acids due to its side chain structure. Histidine contains an imidazole ring in its side chain, which can exist in both protonated (positively charged) and unprotonated (neutral) forms depending on the pH of the environment. At physiological pH (around 7.4), histidine often exists in its protonated form, giving it a positive charge, making it a crucial amino acid in various enzymatic reactions, especially in enzyme active sites where proton transfer is involved.

This unique characteristic of histidine allows it to play significant roles in protein structure and function, particularly in enzymes that require a buffering capacity or that need to facilitate the transfer of protons. Consequently, its classification as a positively-charged amino acid reflects its behavior in biological systems, particularly in terms of charge interactions and biochemical function.