Escherichia coli heme biosynthesis

Here we limit ourselves to an example of a 4Fe-4S center from the radical 5 -adenosyl-L-methioiiine (SAM) enzyme oxygen-independent coproporphyrinogen III oxidase HemN. This enzyme catalyzes the oxidative decarboxylation of coproporphyrinogen III to protoporphyrinogen IX dming bacterial heme biosynthesis. The crystal structure of Escherichia coli HemN revealed the presence of an unusually coordinated iron-sulfur cluster and two molecules of SAM. 

The chemistry of the cofactors has provided a fertile area of overlap between organic chemistry and biochemistry, and the organic chemistry of the cofactors is now a thoroughly studied area. In contrast, the chemistry of cofactor biosynthesis is still relatively underdeveloped. In this review the biosynthesis of the cofactors shown in Fig. 1 will be described. Heme and cobalamin will be omitted as these are covered in the review by Allan Battersby and Finian Leeper in this volume. We will focus on cofactor biosynthesis in Escherichia coli because the relevant genetics and biochemistry have been most intensively studied in this organism and it is the easiest system in which to carry out molecular biology [1]. Enzymes from other sources will be described only if the corresponding enzyme from E. coli has not been isolated or subjected to mechanistic studies. 

Roy R, Knowles R (1995) Differential inhibition by allylsulfide of nitrification and methane oxidation in freshwater sediment. Appl Environ Microbiol 61 4278 1283 Saiki K, Mogi T, Anraku Y (1992) Heme O biosynthesis in Escherichia coli the cyoE gene in the cytochrome bo operon encodes a protoheme IX famesyl transferase. Biochem Biophys Res Commun 189 1491-1497... 

To ascertain the mechanism of heme A biosynthesis, we cloned HOS and HAS from Bacillus subtilis and heterologously expressed them in Escherichia coli. In addition to observing the production of both heme O and heme A, we also observed two additional previously unidentified heme products. Utilizing a number of techniques including optical spectroscopy, NMR spectroscopy, tandem mass spectroscopy, and chemical synthesis, we identified these two hemes as an alcohol intermediate and an overoxidized carboxylate product 19, 20). The carboxylate derivative, however, has only been identified when HAS is heterologously expressed in E. coli no carboxylate derivative has been observed when HAS is expressed under native conditions. 

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