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. Author manuscript; available in PMC: 2013 Oct 30.
Published in final edited form as: Biochemistry. 2012 Oct 18;51(43):8554–8562. doi: 10.1021/bi301175x

Figure 1.

Figure 1

Reaction scheme of HO. The overall HO reaction proceeds via a multi-step mechanism. The first step is the oxidation of heme to α-hydroxyheme, requiring O2 and reducing equivalents supplied by NADPH cytochrome P450 reductase. O2 bound to the heme iron is activated to the hydrogen peroxy species, and α-hydroxyheme is then produced. The second step is the formation of verdoheme with the concomitant release of the hydroxylated α-meso carbon as CO. The third step is the conversion of verdoheme to biliverdin-iron chelate, also requiring electrons and consuming O2. In the final step, the iron of biliverdin-iron chelate is reduced, and ferrous iron and biliverdin are released from HO. The conversion of verdoheme to ferric biliverdin-iron chelate followed by the release of biliverdin is the rate-limiting step in the HO reaction.