Role of the conserved distal heme asparagine of coral allene oxide synthase (Asn137) and human catalase (Asn148): mutations affect the rate but not the essential chemistry of the enzymatic transformations

A catalase-related allene oxide synthase (cAOS) and true catalases that metabolize hydrogen peroxide have similar structure around the heme. One of the distal heme residues considered to help control catalysis is a highly conserved asparagine. Here we addressed the role of this residue in metabolism of the natural substrate 8R-hydroperoxyeicosatetraenoic acid by cAOS and in H₂O₂ breakdown by catalase. In cAOS, the mutations N137A, N137Q, N137S, N137D, and N137H drastically reduced the rate of reaction (to 0.8–4% of wild-type), yet the mutants all formed the allene oxide as product. This is remarkable because there are many potential heme-catalyzed transformations of fatty acid hydroperoxides and special enzymatic control must be required. In human catalase, the N148A, N148S, or N148D mutations only reduced rates to 20% of wild-type. The distal heme Asn is not essential in either catalase or cAOS. Its conservation throughout evolution may relate to a role in optimizing catalysis.