Biosynthesis and Functions of Mycothiol, the Unique Protective Thiol of Actinobacteria

Abstract

SUMMARY Mycothiol (MSH; AcCys-GlcN-Ins) is the major thiol found in Actinobacteria and has many of the functions of glutathione, which is the dominant thiol in other bacteria and eukaryotes but is absent in Actinobacteria. MSH functions as a protected reserve of cysteine and in the detoxification of alkylating agents, reactive oxygen and nitrogen species, and antibiotics. MSH also acts as a thiol buffer which is important in maintaining the highly reducing environment within the cell and protecting against disulfide stress. The pathway of MSH biosynthesis involves production of GlcNAc-Ins-P by MSH glycosyltransferase (MshA), dephosphorylation by the MSH phosphatase MshA2 (not yet identified), deacetylation by MshB to produce GlcN-Ins, linkage to Cys by the MSH ligase MshC, and acetylation by MSH synthase (MshD), yielding MSH. Studies of MSH mutants have shown that the MSH glycosyltransferase MshA and the MSH ligase MshC are required for MSH production, whereas mutants in the MSH deacetylase MshB and the acetyltransferase (MSH synthase) MshD produce some MSH and/or a closely related thiol. Current evidence indicates that MSH biosynthesis is controlled by transcriptional regulation mediated by σB and σR in Streptomyces coelicolor. Identified enzymes of MSH metabolism include mycothione reductase (disulfide reductase; Mtr), the S-nitrosomycothiol reductase MscR, the MSH S-conjugate amidase Mca, and an MSH-dependent maleylpyruvate isomerase. Mca cleaves MSH S-conjugates to generate mercapturic acids (AcCySR), excreted from the cell, and GlcN-Ins, used for resynthesis of MSH. The phenotypes of MSH-deficient mutants indicate the occurrence of one or more MSH-dependent S-transferases, peroxidases, and mycoredoxins, which are important targets for future studies. Current evidence suggests that several MSH biosynthetic and metabolic enzymes are potential targets for drugs against tuberculosis. The functions of MSH in antibiotic-producing streptomycetes and in bioremediation are areas for future study.

The Activity and Cofactor Preferences of N-Acetyl-1-d-myo-inosityl-2-amino-2-deoxy-α-d-glucopyranoside Deacetylase (MshB) Change Depending on Environmental Conditions

“…Instead, these organisms use the small molecule mycothiol (MSH) 2 as their primary reducing agent and in xenobiotic metabolism for the detoxification of drugs and other toxins (1)(2)(3)(4). MSH is likely to be critical for the survival of mycobacteria inside activated macrophages, where the mycobacteria are subjected to oxidative bursts.…”

Metabolism of the Antituberculosis Drug Ethionamide

“…Also, enhanced resistance to ETH is another characteristic of the MshA-, MshB-, and MshDdeficient mutants, with complete resistance at 50 μg/mL [52,53]. The high levels of resistance of the MSH mutants to 3 and ETH suggest that MSH is involved in the activation of these drugs, because both 3 and ETH are prodrugs which need to be activated intracellularly and whose reactive groups must be unmasked before inhibiting their target [54]. Recently, it was demonstrated that mutations in MSH biosynthesis genes may contribute to INH or ETH resistance across mycobacterial species, because these mutants showed low-level resistance to INH but were highly resistant to ETH [55].…”

Metabolism of the Antituberculosis Drug Ethionamide

“…Also, enhanced resistance to ETH is another characteristic of the MshA-, MshB-, and MshDdeficient mutants, with complete resistance at 50 μg/mL [52,53]. The high levels of resistance of the MSH mutants to 3 and ETH suggest that MSH is involved in the activation of these drugs, because both 3 and ETH are prodrugs which need to be activated intracellularly and whose reactive groups must be unmasked before inhibiting their target [54]. Recently, it was demonstrated that mutations in MSH biosynthesis genes may contribute to INH or ETH resistance across mycobacterial species, because these mutants showed low-level resistance to INH but were highly resistant to ETH [55].…”