Histidine often acts as a catalytic residue in enzyme active sites. It can facilitate various types of reactions such as hydrolysis, isomerization, and phosphorylation. In the active site, histidine can stabilize transition states, participate in substrate orientation, and act as a general acid or base. This versatility is why histidine is frequently found in the catalytic triads of serine proteases, where it works alongside serine and aspartate.
