Site-directed mutagenesis of cysteine-195 in isocitrate lyase from Escherichia coli ML308

Allicin Induces Thiol Stress in Bacteria through S-Allylmercapto Modification of Protein Cysteines

Allicin (diallyl thiosulfinate) from garlic is a highly potent natural antimicrobial substance. It inhibits growth of a variety of microorganisms, among them antibiotic-resistant strains. However, the precise mode of action of allicin is unknown. Here, we show that growth inhibition of Escherichia coli during allicin exposure coincides with a depletion of the glutathione pool and S-allylmercapto modification of proteins, resulting in overall decreased total sulfhydryl levels. This is accompanied by the induction of the oxidative and heat stress response. We identified and quantified the allicin-induced modification S-allylmercaptocysteine for a set of cytoplasmic proteins by using a combination of label-free mass spectrometry and differential isotope-coded affinity tag labeling of reduced and oxidized thiol residues. Activity of isocitrate lyase AceA, an S-allylmercapto-modified candidate protein, is largely inhibited by allicin treatment in vivo. Allicin-induced protein modifications trigger protein aggregation, which largely stabilizes RpoH and thereby induces the heat stress response. At sublethal concentrations, the heat stress response is crucial to overcome allicin stress. Our results indicate that the mode of action of allicin is a combination of a decrease of glutathione levels, unfolding stress, and inactivation of crucial metabolic enzymes through S-allylmercapto modification of cysteines.

Regulation by glutathionylation of isocitrate lyase from Chlamydomonas reinhardtii

Post-translational modification of protein cysteine residues is emerging as an important regulatory and signaling mechanism. We have identified numerous putative targets of redox regulation in the unicellular green alga Chlamydomonas reinhardtii. One enzyme, isocitrate lyase (ICL), was identified both as a putative thioredoxin target and as an S-thiolated protein in vivo. ICL is a key enzyme of the glyoxylate cycle that allows growth on acetate as a sole source of carbon. The aim of the present study was to clarify the molecular mechanism of the redox regulation of Chlamydomonas ICL using a combination of biochemical and biophysical methods. The results clearly show that purified C. reinhardtii ICL can be inactivated by glutathionylation and reactivated by glutaredoxin, whereas thioredoxin does not appear to regulate ICL activity, and no inter- or intramolecular disulfide bond could be formed under any of the conditions tested. Glutathionylation of the protein was investigated by mass spectrometry analysis, Western blotting, and site-directed mutagenesis. The enzyme was found to be protected from irreversible oxidative inactivation by glutathionylation of its catalytic Cys(178), whereas a second residue, Cys(247), becomes artifactually glutathionylated after prolonged incubation with GSSG. The possible functional significance of this post-translational modification of ICL in Chlamydomonas and other organisms is discussed.

Dual role of isocitrate lyase 1 in the glyoxylate and methylcitrate cycles in Mycobacterium tuberculosis

The role of isocitrate lyase (ICL) in the glyoxylate cycle and its necessity for persistence and virulence of Mycobacterium tuberculosis has been well described. Recent reports have alluded to an additional role for this enzyme in M. tuberculosis metabolism, specifically for growth on propionate. A product of beta-oxidation of odd-chain fatty acids is propionyl-CoA. Clearance of propionyl-CoA and the by-products of its metabolism via the methylcitrate cycle is vital due to their potentially toxic effects. Although the genome of M. tuberculosis encodes orthologues of two of the three enzymes of the methylcitrate cycle, methylcitrate synthase and methylcitrate dehydratase, it does not appear to contain a distinct 2-methylisocitrate lyase (MCL). Detailed structural analysis of the MCL from Escherichia coli suggested that the differences in substrate specificity between MCLs and ICLs could be attributed to three conserved amino acid substitutions in the active site, suggesting an MCL signature. However, here we provide enzymatic evidence that shows that despite the absence of the MCL signature, ICL1 from M. tuberculosis can clearly function as a MCL. Furthermore, the crystal structure of ICL1 with pyruvate and succinate bound demonstrates that the active site can accommodate the additional methyl group without significant changes to the structure.

ALG2, the Hansenula polymorpha isocitrate lyase gene

To set the basis for molecular and cellular studies of the glyoxylate cycle in methylotrophic yeasts, we isolated and characterized ALG2, the Hansenula polymorpha isocitrate lyase gene. Complementation work and sequence analysis revealed an ORF of 1458 nucleotides, encoding a 486 amino acid protein with a predicted molecular mass of 54.9 kDa. This protein is shorter than the Saccharomyces cerevisiae and Candida tropicalis ICLs, lacks a PST1 signal and possesses a PTS2-like signal. The transcriptional regulation of ALG2 mRNA levels by carbon source is mainly achieved by glucose repression-derepression, whereas ethanol induction plays only a minor role. We present evidence indicating that, in H. polymorpha, neither isocitrate lyase activity nor the ALG2 gene product are necessary for C(1)-peroxisome degradation triggered by ethanol. Therefore, the involvement of glyoxylate in degradation, as described by Kulachkovsky et al. (1997) for Pichia methanolica, does not necessarily apply to all methylotrophic yeasts. The relevant nucleotide sequence has been deposited at GenBank (Accession No. AF373067.1).