Where would histidine generally be found in a protein structure?

Histidine is an amino acid with a unique side chain that contains an imidazole ring. This structure gives histidine the ability to participate in both hydrophobic and hydrophilic environments. The imidazole ring can exist in protonated or deprotonated forms depending on the pH, which allows histidine to engage in hydrogen bonding and interacts with water, making it capable of residing in hydrophilic regions of a protein.

Additionally, histidine's side chain can also be relatively nonpolar due to its contributions to hydrophobic interactions when in its uncharged state. This dual nature allows histidine to play versatile roles in protein structures, including stabilization of protein folding and in active sites of enzymes, which might be located in regions that are both hydrophobic and hydrophilic.

Amino acids with purely hydrophobic or hydrophilic properties tend to be found in either the core of the protein (to minimize their exposure to water) or on the surface (to interact with the aqueous environment), respectively. Histidine's unique properties enable it to occupy both types of environments effectively and thus be found throughout various regions of a protein structure.