Role of His63 in HutZ from Vibrio cholerae in the heme degradation reaction and heme binding†
HutZ from Vibrio cholerae is a dimeric enzyme that catalyzes oxygen-dependent degradation of heme via a similar catalytic mechanism to mammalian heme oxygenase. However, HutZ oxidizes the β- or δ-meso position of heme at a ∼1 : 1 ratio distinct from heme oxygenase, which initiates the degradation of heme solely at the α-meso position. His63 is a residue that potentially forms hydrogen bond with the heme 7-propionate group. To establish the role of His63 in regioselectivity of heme degradation by HutZ and heme binding, we constructed mutants of His63. Interestingly, the H63L mutant retained a comparable level of β- or δ-regioselectivity as wild-type HutZ. Ascorbic acid-assisted heme degradation by HutZ is pH-dependent, showing activity at pH 6.0 but not above pH 8.0. Compared to the wild-type protein, the H63L mutant was inactive, even at pH 6.0, and affinity for heme was significantly decreased in contrast with a comparable heme binding affinity at pH 8.0, as observed for the mutant of Asp132 to Val, which is located within hydrogen bonding distance of the heme axial ligand His170, but in a different protomer. In addition, the distance between heme and Trp109 increased from 16–18 Å for wild-type HutZ to 24–28 Å for the H63L mutant, indicating that protomer orientation is altered by the mutation, since Trp109 is in another subunit of the heme axial ligand. Our results collectively suggest that His63 positioned near heme does not contribute to regioselectivity of heme degradation but plays a key role in maintaining the orientation of subunits for HutZ to function of heme degradation.