Aklujkar, unpublished), form a branch adjacent to succinyl:acetate CoA-transferases of the genus Geobacter (data not shown). In a similar manner, the hypothetical 2-methylcitrate synthase Gmet_1124 www.selleckchem.com/products/BIBF1120.html and gene Geob_0514 of Geobacter FRC-32 form a branch adjacent
to citrate synthases of Geobacter species (data not shown), consistent with the notion that these two enzyme families could have recently evolved new members capable of converting propionate via propionyl-CoA to 2-methylcitrate. Figure 2 Growth of G. metallireducens on propionate. (a) The gene cluster predicted to encode enzymes of propionate metabolism. (b) The mTOR inhibitor proposed pathway of propionate metabolism. Gmet_0149 (GSU3448) is a homolog of acetate kinase that does not contribute sufficient acetate kinase activity to sustain growth of G. sulfurreducens [17] and has a closer BLAST hit to propionate kinase of E. coli (40% identical sequence) than to acetate kinase of E. coli. Although it does not cluster phylogenetically with either of the E. coli enzymes,
its divergence from acetate kinase (Gmet_1034 = GSU2707) is older than the last common ancestor of the Geobacteraceae (data not shown). This conserved gene product remains to be characterized as a propionate kinase or something else. The proposed pathway for growth of G. metallireducens on propionate (Figure 2) is contingent upon its Selleckchem MLN8237 experimentally established Orotic acid ability to grow on pyruvate [31]. G. sulfurreducens cannot utilize pyruvate as the carbon source unless hydrogen is provided as an electron donor [17]. Oxidation of acetyl-CoA derived from pyruvate in G. sulfurreducens may be prevented by a strict requirement for the succinyl:acetate CoA-transferase reaction (thermodynamically inhibited when acetyl-CoA exceeds acetate) to complete the TCA cycle in the absence of detectable activity of succinyl-CoA synthetase (GSU1058-GSU1059) [17]. With three sets of succinyl-CoA synthetase genes
(Gmet_0729-Gmet_0730, Gmet_2068-Gmet_2069, and Gmet_2260-Gmet_2261), G. metallireducens may produce enough activity to complete the TCA cycle. G. sulfurreducens and G. metallireducens may interconvert malate and pyruvate through a malate oxidoreductase fused to a phosphotransacetylase-like putative regulatory domain (maeB; Gmet_1637 = GSU1700), which is 51% identical to the NADP+-dependent malic enzyme of E. coli [32]. G. sulfurreducens has an additional malate oxidoreductase without this fusion (mleA; GSU2308) that is 53% identical to an NAD+-dependent malic enzyme of B. subtilis [33], but G. metallireducens does not. G. metallireducens possesses orthologous genes for all three pathways that activate pyruvate or oxaloacetate to phosphoenolpyruvate in G. sulfurreducens (Figure 3a): phosphoenolpyruvate synthase (Gmet_0770 = GSU0803), pyruvate phosphate dikinase (Gmet_2940 = GSU0580) and GTP-dependent phosphoenolpyruvate carboxykinase Gmet_2638 = GSU3385) [17].
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