Heme oxygenase (HO-1): His-132 stabilizes a distal water ligand and assists catalysis

The role of His-132 in heme oxygenase isozyme-1 (HO-1) and its catalytic activities in the enzyme were determined by absorption and resonance Raman spectroscopy. H132A, H132G and H132S, three mutants of HO-1, were prepared by mutation of His-132 to an alanine, glycine and serine. The results showed that the mutations did not block the loss of the water molecule located at the distal water ligand. However, they destabilized the ferrous-oxygen complex, weakened the affinity of the enzyme for heme and reduced the catalytic activity of the protein. These results suggest that His-132 is located close to the distal site of the iron atom of the heme pocket and that it assists catalysis of HO-1.