The haem-distal pocket of nitric oxide reductase cyto-chrome P450 contains many Arg and Lys residues that are clustered to form a putative access channel for NADH. Asp88 is the sole negatively charged amino acid in this positive charge cluster, and thus it would be interesting to know its functional role. Here we found the intriguing phenomenon that mutation at this site of P450nor (D88A or D88V) considerably decreased the overall nitric oxide reductase activity without blocking the reducing half reac-tion in which the ferric enzyme–NO complex is reduced with NADH to yield a specific intermediate (I). .
