Histidine phosphorylation in biological systems

Abstract

Histidine phosphorylation is important in prokaryotes and occurs to the extent of 6% of total phosphorylation in eukaryotes. Nevertheless phosphohistidine residues are not normally observed in proteins due to rapid hydrolysis of the phosphoryl group under acidic conditions. Many rapid processes employ phosphohistidines, including the bacterial phosphoenolpyruvate:sugar phosphotransferase system (PTS), the bacterial two-component systems and reactions catalyzed by enzymes such as nucleoside diphosphate kinase and succinyl–CoA synthetase. In the PTS, the NMR structure of the phosphohistidine moiety of the phosphohistidine-containing protein was determined but no X-ray structures of phosphohistidine forms of PTS proteins have been elucidated. There have been crystal structures of a few phosphohistidine-containing proteins determined: nucleoside diphosphate kinase, succinyl–CoA synthetase, a cofactor-dependent phosphoglycerate mutase and the protein PAE2307 from the hyperthermophilic archaeon Pyrobaculum aerophilum. A common theme for these stable phosphohistidines is the occurrence of ion-pair hydrogen bonds (salt bridges) involving the non-phosphorylated nitrogen atom of the histidine imidazole ring with an acidic amino acid side chain.

Introduction

Histidine in neutral and basic solution can adopt the two tautomeric forms shown in Fig. 1. In this solution, the preferred tautomer is in Fig. 1(b) with the proton located on the Nε2 atom of the imidazole ring (rather than on the Nδ1 atom) [1]. In proteins, however, hydrogen bonding to the Nδ1 atom of the side chain of a histidine residue can cause the Nδ1-H tautomer to be preferred, as occurs in serine proteases [2].

Histidine residues may be phosphorylated on either the Nδ1 or the Nε2 atom of the imidazole ring, as illustrated in Fig. 2. Prior to phosphorylation, whichever of these nitrogen atoms does not have the proton attached will be the one that becomes phosphorylated. Histidine phosphorylation is a critical component of cell signaling in prokaryotes and in lower eukaryotes [3], [4]. A total of 6% of the total phosphorylation in eukaryotes consists of phosphohistidine residues [5]. Thermodynamically, phosphohistidine in proteins has a large negative ΔG° of − 12 to − 14 kcal/mol [6], with the equilibrium in the direction of the dephosphorylated state. Phosphohistidines have a high-energy phosphoramidate bond and have a tendency to transfer the phosphoryl group to other molecules [7]. Free phosphohistidine is unstable in acid solution; in 1 M HCl, Nδ1-phosphohistidine and Nε2-phosphohistidine have half-lives of 18 s and 25 s, respectively [8]. At pH = 7 and pH = 8, Nδ1-phosphohistidine has a half-life of 1 min and 34 min, respectively; at pH > 7, Nε2-phosphohistidine has a half-life of 70 min [8].