Heme Degradation in Plants and Bacteria

Although there is very little data on the mechanistic aspects of the biosynthetic enzymes, it is thought that their reaction mechanisms are very similar to those of the mammalian heme oxygenases. Indirect evidence that the mechanism of action of the biosynthetic enzymes is similar to that of the mammalian heme oxygenases was provided by 0-labeling studies showing that the biosynthesis of phycocyanobilin occurred via a two-molecule mechanism (141, 142) comparable to that of the mammalian heme oxygenases (8). 

The molecular mechanisms by which Gram-positive bacteria such as C. diphtheriae acquire heme are not known. However, based on the similarities of heme transport with siderophore uptake in the Gram- 

However, in contrast to the human His25Ala HO-l heme complex, which has no detectable activity in the absence of imidazole (78), the His20Ala Hmu O rheme complex in the presence of NAD PH and NADPH-cytochrome P450 reductase was foimd to catalyze the initial meso-hydroxylation of the heme (151). The product of the reaction was Fe verdoheme, as judged by the electronic absorption spectrum and the detection of carbon monoxide as a product of the reaction. Hydrolytic conversion of the verdoheme product to biliverdin and subsequent HPLC analysis confirmed that the oxidative cleavage of the porphyrin macrocycle was specific for the a-meso-carbon. 

The work of the San Francisco group described in this article has only been possible because of the hard work and insights of Angela Wilks, Justin Torpey, and Yi Liu, and fruitful collaborations with the groups of Tom Poulos, Tom Loehr, Gerd LaMar, and Kevin Smith. The work in San Francisco and the preparation of this review were supported by National Institutes of Health grant DK30297. 

Maines, M. D. Heme Oxygenase Clinical Applications and Functions CRC Press Boca Raton, FL, 1992 p. 145. 

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