Probing the structure of catalase HPII of Escherichia coli--a review.

Escherichia coli produces two catalases or hydroperoxidases, HPI and HPII. HPI is a bifunctional catalase-peroxidase active as a tetramer of identical 80049-Da subunits encoded by katG. The expression of katG is controlled at the basal level by sigma s (KatF), and its induction by H2O2 is regulated by OxyR. HPII is a monofunctional catalase active as a tetramer of identical 84118-Da subunits encoded by katE. The induction of katE expression in the stationary phase is controlled by sigma s. The core of HPII is similar in sequence to other catalases including the conservation of several residues that have been implicated as playing a catalytic role, His128, Asn201, Ser167 and Tyr415. These residues have served as targets for site-directed mutagenesis in a study that has demonstrated their role in the catalytic mechanism of HPII. In addition, the two Cys residues in HPII have been targeted in a similar study revealing that they do not have a catalytic role, but that Cys438 is blocked by a novel modification. Despite many structural similarities to bovine liver catalase, the heme component of HPII has proved to be quite different. The presence of a cis heme d was determined spectrally and chromatographically, and the inability of certain mutants to generate the modified heme revealed that it was HPII itself that was catalysing the oxidation of heme b to heme d. The recent solution of the crystal structure of HPII and mass spectrometry have revealed that the heme d bound to HPII is a spirolactone structure with a cis orientation of the oxygens on the proximal side of the heme. This has created the problem of explaining how the oxidation of the heme can occur on the opposite side of the heme ring, remote from the catalytic residues.