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Incorporation of methionine-derived methyl groups into sirohaem by Escherichia coli.

ABSTRACT: Sirohaem is a new type of haem that has been detected as a prosthetic group of several bacterial and plant enzymes that catalyse the six-electron reductions of sulphite to sulphide or of nitrite to NH(3). When a methionine-requiring mutant of Escherichia coli K12 was grown on a minimal medium supplemented with d-glucose and l-[Me-(3)H]methionine, 2.4 methyl groups per spectrophotometrically detectable haem group were incorporated into the sirohaem prosthetic group of the NADPH-sulphite reductase isolated from the organism. When the same strain of cells was grown on minimal medium supplemented with d-[U-(14)C]glucose and l-[Me-(3)H]methionine, the sirohaem isolated was found to contain a ratio of glucose-derived carbon/methionine-derived methyl of 19.8. This ratio is in excellent agreement with the value of 20 predicted by the iron-dimethyl-urotetrahydroporphyrin structure for sirohaem proposed by Murphy, Siegel, Kamin & Rosenthal [(1973) J. Biol. Chem.248, 2801-2814]. It can be concluded that sirohaem is indeed methylated, with the methyl groups derived from methionine (rather than by modification of existing side chains, as in protohaem). The structure proposed by Murphy et al. (1973) is therefore probably correct in its essential features. A possible relationship between the pathway for biosynthesis of sirohaem and that for synthesis of vitamin B(12) is discussed.

SUBMITTER: Siegel LM

PROVIDER: S-EPMC1183713 | biostudies-other | 1977 Dec

REPOSITORIES: biostudies-other

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Project description:In nature, Escherichia coli are exposed to harsh and non-ideal growth environments-nutrients may be limiting, and cells are often challenged by oxidative stress. For E. coli cells confronting these realities, there appears to be a link between oxidative stress, methionine availability, and the enzyme that catalyzes the final step of methionine biosynthesis, cobalamin-independent methionine synthase (MetE). We found that E. coli cells subjected to transient oxidative stress during growth in minimal medium develop a methionine auxotrophy, which can be traced to an effect on MetE. Further experiments demonstrated that the purified enzyme is inactivated by oxidized glutathione (GSSG) at a rate that correlates with protein oxidation. The unique site of oxidation was identified by selectively cleaving N-terminally to each reduced cysteine and analyzing the results by liquid chromatography mass spectrometry. Stoichiometric glutathionylation of MetE by GSSG occurs at cysteine 645, which is strategically located at the entrance to the active site. Direct evidence of MetE oxidation in vivo was obtained from thiol-trapping experiments in two different E. coli strains that contain highly oxidizing cytoplasmic environments. Moreover, MetE is completely oxidized in wild-type E. coli treated with the thiol-oxidizing agent diamide; reduced enzyme reappears just prior to the cells resuming normal growth. We argue that for E. coli experiencing oxidizing conditions in minimal medium, MetE is readily inactivated, resulting in cellular methionine limitation. Glutathionylation of the protein provides a strategy to modulate in vivo activity of the enzyme while protecting the active site from further damage, in an easily reversible manner. While glutathionylation of proteins is a fairly common mode of redox regulation in eukaryotes, very few proteins in E. coli are known to be modified in this manner. Our results are complementary to the independent findings of Leichert and Jakob presented in the accompanying paper (Leichert and Jakob 2004), which provide evidence that MetE is one of the proteins in E. coli most susceptible to oxidation. In eukaryotes, glutathionylation of key proteins involved in protein synthesis leads to inhibition of translation. Our studies suggest a simpler mechanism is employed by E. coli to achieve the same effect.

Project description:BackgroundA broad diversity of natural and non-natural esters have now been made in bacteria, and in other microorganisms, as a result of original metabolic engineering approaches. However, the fact that the properties of these molecules, and therefore their applications, are largely defined by the structural features of the fatty acid and alcohol moieties, has driven a persistent interest in generating novel structures of these chemicals.ResultsIn this research, we engineered Escherichia coli to synthesize de novo esters composed of multi-methyl-branched-chain fatty acids and short branched-chain alcohols (BCA), from glucose and propionate. A coculture engineering strategy was developed to avoid metabolic burden generated by the reconstitution of long heterologous biosynthetic pathways. The cocultures were composed of two independently optimized E. coli strains, one dedicated to efficiently achieve the biosynthesis and release of the BCA, and the other to synthesize the multi methyl-branched fatty acid and the corresponding multi-methyl-branched esters (MBE) as the final products. Response surface methodology, a cost-efficient multivariate statistical technique, was used to empirical model the BCA-derived MBE production landscape of the coculture and to optimize its productivity. Compared with the monoculture strategy, the utilization of the designed coculture improved the BCA-derived MBE production in 45%. Finally, the coculture was scaled up in a high-cell density fed-batch fermentation in a 2 L bioreactor by fine-tuning the inoculation ratio between the two engineered E. coli strains.ConclusionPrevious work revealed that esters containing multiple methyl branches in their molecule present favorable physicochemical properties which are superior to those of linear esters. Here, we have successfully engineered an E. coli strain to broaden the diversity of these molecules by incorporating methyl branches also in the alcohol moiety. The limited production of these esters by a monoculture was considerable improved by a design of a coculture system and its optimization using response surface methodology. The possibility to scale-up this process was confirmed in high-cell density fed-batch fermentations.

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Incorporation of methionine-derived methyl groups into sirohaem by Escherichia coli.

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