First Biochemical Characterization of a Methylcitric Acid Cycle from Bacillus subtilis Strain 168.

The genome of Bacillus subtilis strain 168 contains the mother cell metabolic gene (mmg) operon that encodes homologues from the methylcitric acid cycle. We showed that the three genes, mmgDE and yqiQ(mmgF), provide three of the five steps of the methylcitric acid cycle. We also showed that the fourth step can be supplied by citB (aconitase), and we suggest that the fifth missing step, the propionyl-CoA synthetase, is probably skipped because the ß-oxidation of methyl-branched fatty acids by the enzymes encoded by mmgABC should produce propionyl-CoA. We also noted interesting enzymology for MmgD and MmgE. First, MmgD is a bifunctional citrate synthase/2-methylcitrate synthase with 2.3-fold higher activity as a 2-methylcitrate synthase. This enzyme catalyzes the formation of either (2S,3R)- or (2R,3S)-2-methylcitrate, but reports of 2-methylcitrate synthases from other species indicated that they produced the (2S,3S) isomer. However, we showed that MmgD and PrpC (from Escherichia coli) in fact produce the same stereoisomer. Second, the MmgE enzyme is not a stereospecific 2-methylcitrate dehydratase because it can dehydrate at least two of the four diastereomers of 2-methylcitrate to yield either (E)-2-methylaconitate or (Z)-2-methylaconitate. We also showed for the first time that the E. coli homologue PrpD exhibited the same lack of stereospecificity. However, the physiological pathways proceed via (Z)-2-methylaconitate, which served as the substrate for the citB enzyme in the synthesis of 2-methylisocitrate. We completed our characterization of this pathway by showing that the 2-methylisocitrate produced by CitB is converted to pyruvate and succinate by the enzyme YqiQ(MmgF).

The mmgA gene from Bacillus subtilis encodes a degradative acetoacetyl-CoA thiolase.

Early in sporulation, the mother cell compartment of Bacillus subtilis transcribes the mother cell metabolic gene (mmg) operon. The gene mmgA was assigned by other workers using sequence homology as an acetyl-CoA acetyltransferase [E.C. 2.3.1.9]. The gene was overexpressed in Escherichia coli, and the protein was purified by Ni(2+)-affinity chromatography. However, the expected MmgA-catalyzed biosynthesis of acetoacetyl-CoA from acetyl-CoA was undetectable by a standard UV assay, HPLC, and mass spectrometry. These methods indicated a preference for the reverse degradative thiolytic reaction, with a k(cat) of 80 s(-1), and a K(m) of 70 and 50 microM for CoA and acetoacetyl-CoA, respectively.

PksS from Bacillus subtilis is a cytochrome P450 involved in bacillaene metabolism.

As part of the pksX gene cluster of Bacillus subtilis strain 168, pksS has been preliminarily annotated as a cytochrome P450 homolog that hydroxylates the polyketide product of this cluster, which was recently shown to be involved in the biosynthesis of bacillaene and dihydrobacillaene. Here we report that there is a frame-shift error in the reported sequence for pksS, and that we have successfully cloned, overexpressed, and purified the protein encoded by the corrected sequence. By utilizing electronic absorption spectrophotometry, we have observed that the ferrous CO complex of PksS absorbs maximally near 450 nm, which confirms the annotation that this protein is a cytochrome P450. We have also established a cell-free system derived from crude cytosolic B. subtilis protein extracts which provides reductase activity essential to sustaining the putative catalytic cycle of PksS. Using LC-MS analysis we have collected data which suggests that the substrate for PksS is dihydrobacillaene.

An efficient enzymatic synthesis of thiamin pyrophosphate.

Thiamin pyrophosphate was synthesized in 71% yield, on a multi-milligram scale, using overexpressed thiazole kinase, pyrimidine kinase, thiamin phosphate synthase, and thiamin phosphate kinase. This provides a facile route to isotopically labeled thiamin pyrophosphate from its readily available pyrimidine and thiazole precursors.

Relative rates of Michael reactions of 2'-(phenethyl)thiol with vinyl sulfones, vinyl sulfonate esters, and vinyl sulfonamides relevant to vinyl sulfonyl cysteine protease inhibitors.

[reaction: see text] The relative rates of Michael additions of 2'-(phenethyl)thiol to representative vinyl sulfonyl Michael acceptors were measured. The dependence of the reactivity of the Michael acceptor on the nature of the sulfonyl R substituent was determined in order to evaluate the effect of these substituents on the inactivation kinetics of comparably substituted vinyl sulfonyl cysteine protease inhibitors. The rates of these Michael additions vary over 3 orders of magnitude, with phenyl vinyl sulfonate esters (R = OPh) being ca. 3000-fold more reactive than N-benzyl vinyl sulfonamides (R = NHBn).