Bovine liver
catalase (BLC),
catalase-related
allene oxide synthase (cAOS) from Plexaura homomalla, and a recently isolated
protein from the cattle pathogen Mycobacterium avium ssp.
paratuberculosis (MAP-2744c (MAP)) are all tyrosinate-ligated
heme enzymes whose crystal structures have been reported. cAOS and MAP have low (<20%) sequence similarity to, and significantly different catalytic functions from, BLC. cAOS transforms 8R-hydroperoxy-eicosatetraenoic
acid to an
allene epoxide, whereas the MAP
protein is a putative organic
peroxide-dependent
peroxidase. To elucidate factors influencing the functions of these and related
heme proteins, we have investigated the
heme iron coordination properties of these tyrosinate-ligated
heme enzymes in their ferric and ferrous states using magnetic circular dichroism and UV-visible absorption spectroscopy. The MAP
protein shows remarkable spectral similarities to cAOS and BLC in its native Fe(III) state, but clear differences from ferric proximal
heme ligand His93Tyr Mb (
myoglobin) mutant, which may be attributed to the presence of an Arg(+)-N(ω)-H···¯O-Tyr (proximal
heme axial
ligand) hydrogen bond in the first three
heme proteins. Furthermore, the spectra of Fe(III)-CN¯, Fe(III)-NO, Fe(II)-NO (except for five-coordinate MAP), Fe(II)-CO, and Fe(II)-O(2) states of cAOS and MAP, but not H93Y Mb, are also similar to the corresponding six-coordinate complexes of BLC, suggesting that a tyrosinate (Tyr-O¯) is the
heme axial
ligand trans to the bound
ligands in these complexes. The Arg(+)-N(ω)-H to ¯O-Tyr hydrogen bond would be expected to modulate the donor properties of the proximal tyrosinate oxyanion and, combined with the subtle differences in the catalytic site structures, affect the activities of cAOS, MAP and BLC.