Heme O synthase

The synthesis of heme A (Fig. 2) involves the initial addition of the farnesyl moiety to the heme 2-vinyl group by heme O synthase, which generates heme O that only has this modification. In a second step, heme A synthase oxidizes the 8-methyl of heme O to an aldehyde, which generates heme A. An electron transfer mechanism (rather than double hydroxylation) has been proposed for this final biosynthetic step (28). 

CoxlO/CoxlS (Heme O Synthase/Heme A Synthase) and Related Heme A Insertion... 

Heme A is derived from the more generally utilized heme B. Heme O synthase (Cox 10) catalyzes the first reaction by the addition of the famesyl moiety to heme B to generate heme O, while heme A synthase (Cox 15) converts heme O to heme A by oxidizing the C-8 methyl substituent to an aldehyde (18-21) (Figure 3). 

Figure 3. Transformation of heme B to heme A catalyzed by the enzymes heme O synthase (Cox 10)...

Heme A is an obligatory cofactor in all eukaryotic and most bacterial cytochrome c oxidase enzymes (CcO). Because of its importance to CcO and aerobic metabolism, considerable effort has recently been invested in understanding the mechanism and regulation of heme A biosynthesis. The activity of heme A synthase is strictly dependent on O2, and yet there is no incorporation of O2 into the products. Heme A synthase is now known to utilize a unique electron-transfer mechanism when oxidizing heme O to heme A. Interestingly, the heme A biosynthetic pathway is regulated at least partly via a heme-dependent process in which heme A synthase is positively regulated by intracellular heme levels via Hapl. 

In order for the cyclooxygenase to function, a source of hydroperoxide (R—O—O—H) appears to be required. The hydroperoxide oxidizes a heme prosthetic group at the peroxidase active site of PGH synthase. This in turn leads to the oxidation of a tyrosine residue producing a tyrosine radical which is apparendy involved in the abstraction of the 13-pro-(5)-hydrogen of AA (25). The cyclooxygenase is inactivated during catalysis by the nonproductive breakdown of an active enzyme intermediate. This suicide inactivation occurs, on average, every 1400 catalytic turnovers. 

Other non-heme enzymes that use dioxygen are 4-methoxy-benzoate O-demethylase, extradiol catechol dioxygenases, the oxidoreductase isopenicillin N synthase, and a-keto acid-dependent enzymes (28). Moreover, the BH4-dependent glyceryl-ether monooxygenase (GEM) also appears to be dependent on nonheme iron for catalysis (see also Section I.E). 

Kabil O, Toaka S, LoBrutto R, Shoemaker R, and Banerjee R (2001) Pyridoxal phosphate binding sites are similar in human heme-dependent and yeast heme-independent cystathionine beta-synthases. Evidence from P NMR and pulsed EPR spectroscopy that heme and PLP cofactors are not proximal in the human enzyme. Journal of Biological Chemistry 276,19350-5. 

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