Chemical tools for probing histidine modifications.

Histidine is a unique amino acid with critical roles in protein structure and function, ranging from metal ion binding to enzyme catalysis. Histidine residues in proteins also undergo diverse posttranslational modifications, including methylation, phosphorylation and hydroxylation, by various enzymes, some of them being only recently identified and characterised. In this review, we describe the development of chemical tools for understanding the role of histidine residues in chemical and biological systems. We spotlight the application of histidine analogues in probing biomedically important posttranslational modifications of histidine residues in proteins, and we highlight novel bioconjugation methods that enable chemoselective modifications of histidine residues in peptides and proteins.