Relation between the structure and the oxygen acitivation of heme oxygenase reaction

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(1)The hemin complex of HmuO, a 24-kDa soluble heme degradation enzyme in Corynebacterium diphtheriae, is coordinated axially to a neutral imidazole of a proximal histidine residue in HmuO. EPR of the NO-bound ferrous heme-HmuO mutant complexes reveals His20 as the proximal heme ligand in HmuO, and this is confirmed by resonance Raman result from the ligand free ferrous heme-H20A. When bound with exogenous imidazole, His20A mutant resumes full catalytic activity. Hence, it is the absence of the proximal His ligand, that is responsible for the inactivity of proximal His mutant.(2)We found that the mutation of R183 to E or D of rat HO-1 changes the a-regioselectivity of the HO catalysis. The result shows the importance of the hydrogen bonding interaction between the arginine at position 183 and the carboxylates of the heme propionate group, as well as steric effect of the distal helix, for the a-regioselectivity.(3)Wild-type enzyme of rat HO-1 degrades heme to verdoheme with hydrogen peroxide. However, D140A heme complex forms compound II with hydrogen oeroxide, and no heme degradation occurs. D140E mutant degrades heme normally, but D140N shows reactivity similar to that of D140A. These results indicate that the carboxylate at position 140 is essential to activate the iron-bound oxygen.(4)We determined the crystal structure of rat biliverdine reductase. The structure contains two domains: an N-terminal domain characteristic of a nucleotide binding fold and a C-terminal domain. We proposed modes of binding for NADPH and biliverdin.