Role of the lateral channel in catalase HPII of Escherichia coli

The heme-containing catalase HPII of Escherichia coli consists of a homotet ramer in which each subunit contains a core region with the highly conserve d catalase tertiary structure, to which are appended N- and C-terminal exte nsions making it the largest known catalase. HPII does not bind NADPH, a co factor often found in catalases. In HPII, residues 585-590 of the C-termina l extension protrude into the pocket corresponding to the NADPH binding sit e in the bovine liver catalase. Despite this difference, residues that defi ne the NADPH pocket in the bovine enzyme appear to be well preserved in HPI I. Only two residues that interact ionically with NADPH in the bovine enzym e (Asp212 and His304) differ in HPII (Glu270 and Glu362), but their mutatio n to the bovine sequence did not promote nucleotide binding. The active-sit e heme groups are deeply buried inside the molecular structure requiring th e movement of substrate and products through long channels. One potential c hannel is about 30 Angstrom in length, approaches the heme active site late rally, and is structurally related to the branched channel associated with the NADPH binding pocket in catalases that bind the dinucleotide. In HPII, the upper branch of this channel is interrupted by the presence of Arg260 i onically bound to Glu270. When Arg260 is replaced by alanine, there is a th reefold increase in the catalytic activity of the enzyme. Inhibitors of HPI I, including azide, cyanide, various sulfhydryl reagents, and alkylhydroxyl amine derivatives, are effective at lower concentration on the Ala260 mutan t enzyme compared to the wild-type enzyme. The crystal structure of the Ala 260 mutant variant of HPII, determined at 2.3 Angstrom resolution, revealed a number of local structural changes resulting in the opening of a second branch in the lateral channel, which appears to be used by inhibitors for a ccess to the active site, either as an inlet channel for substrate or an ex haust channel for reaction products.