Identification of a novel bond between a histidine and the essential tyrosine in catalase HPII of Escherichia coli

A bond between the Nδ of the imidazole ring of His 392 and the Cβ of the essential Tyr 415 has been found in the refined crystal structure at 1.9 Å resolution of catalase HPII of Escherichia coli. This novel type of covalent linkage is clearly defined in the electron density map of HPII and is confirmed by matrix‐assisted laser desorption/ionization mass spectrometry analysis of tryptic digest mixtures. The geometry of the bond is compatible with both the sp3 hybridization of the Cβ atom and the planarity of the imidazole ring. Two mutated variants of HPII active site residues, H128N and N201H, do not contain the His 392‐Tyr 415 bond, and their crystal structures show that the imidazole ring of His 392 was rotated, in both cases, by 80° relative to its position in HPII. These mutant forms of HPII are catalytically inactive and do not convert heme b to heme d, suggesting a relationship between the self‐catalyzed heme conversion reaction and the formation of the His‐Tyr linkage. A model coupling the two processes and involving the reaction of one molecule of H2O2 on the proximal side of the heme with compound I is proposed.