Anti Mycobacterial Activity of Actinomycetes Producing Mycothiol
M. Mohamed Mahroop Raja,
M. Mohamed Imran
E. Vani Ugin
Mycothiol (MSH or AcCys-GlcN-Ins) is an unusual thiol compound found in the Actinobacteria including Actinomycetes and Mycobacterium. The enzymes involved in mycothiol biosynthesis are of interest as potential targets for new drugs directed against Mycobacterium tuberculosis. In this review, it is focused on MSH-dependent enzymes that utilize MSH for their activity either as a cofactor or as a substrate indicate that MSH plays a key role in protecting cells against a variety of challenges and suggest that enzymes of MSH metabolism could be possible targets for development of new drugs and vaccines directed against Mycobacterium tuberculosis and other infectious Actinomycetes.
November 24, 2010; Accepted: December 02, 2010;
Published: February 26, 2011
(MSH) has recently have been identified as a major thiol in a number of Actinomycetes
(Aharonowitz et al., 1995). Low molecular-weight
thiols play a key role in maintaining a reducing environment in the cell, which
is necessary for regular metabolic activities to occur. These thiols thus represent
a major biological adaptation that is important for the survival of organisms
under various endogenous and exogenous stresses. In eukaryotes and gram-negative
bacteria, the much-studied tripeptide Glutathione (GSH) is the dominant thiol.
GSH plays a major role in protecting the cell against oxygen toxicity (Anderson,
1998) by removing reactive oxygen species that may result from atmospheric
oxygen and basal metabolic activities in aerobic organisms.
Another thiol present in Actinomycetes, Ergothioneine (ESH), is a betaine of
2-thiol-L-histidine. Unlike MSH, ESH has been detected in plants, fungi, animals
and bacteria. However, only fungi and actinomycetes are able to synthesize this
thiol (Genghof, 1970). The amount of ESH present in
actinomycetes is ten-fold lower than that of MSH. Like GSH, MSH has a functional
cysteine moiety but instead of the two amino acids, glycine and glutamic acid,
present in GSH, there are two sugar moieties, inositol and N-glucosamine. The
glycosidic link between glucosamine and inositol is comparable in strength to
the amide bond connecting the acetyl group to the cysteine and the second amide
bond connecting the cysteine to the glucosamine (Fig. 1).
This stability in the bonds may have played a role in the evolution of MSH as
the major intracellular thiol in actinomycetes (Fahey, 2001).
Moreover, certain actinomycetes such as Mycobacteria are known for their complex
cell walls consisting of polysaccharides, lipopolysaccharides and complex fatty
acids. The preponderance of glucosamine and in particular inositol, in the Actinomycete
cell wall may have favoured the use of a sugar thiol (MSH) for redox
control over the peptide (GSH) thiol.
|| Structure of mycothiol
Actinomycetes in MSH are potential targets for drugs directed against Mycobacteria.
New antibiotics that are active against resistant bacteria (Raja
et al., 2010) have lived on earth for several billion years. During
this time, they encountered in nature a wide range naturally occurring antibiotics.
To survive, bacteria developed antibiotic resistance mechanism (Hoskeri
et al., 2010).
FUNCTIONS OF THIOL
One of the major functions of thiols is to serve as a storage form of cysteine,
because cysteine tends to autooxidize in a matter of minutes, readily forming
toxic peroxy radicals and hydrogen peroxide. MSH undergoes copper catalyzed
auto-oxidation 30-fold more slowly than cysteine and 7-fold more slowly than
GSH. This important difference implies that the ability to cope with oxidative
stress is much higher in actinomycetes. In terms of chemical reactivity, MSH
is an inherently poor nucleophile compared with ovothiol and GSH (Spies
and Steenkamp, 1994). The physical parameters of mycothiol such as the thiol
pKa and the stability of the free radical form have not been elucidated, although
the redox potential of mycothiol is expected to be similar to that of glutathione.
In this review which focus on MSH-dependent enzymes that utilize MSH for their activity either as a cofactor or as a substrate which summarizes the current knowledge about MSH metabolism. As most studies on MSH metabolism have been performed on Mycobacteria, the majority of the information is drawn from Mycobacterial research.
IN VIVO AND IN VITRO REACTION OF MYCOTHIOL ON MYCOBACTERIAL CELL
All organisms growing in an aerobic environment produce one or more low molecular
mass thiol (s) as a major metabolite. In the majority of eukaryotes, Gluthathione
(GSH) is the principal antioxidant thiol in gram negative bacteria. In prokaryotes
and certain eukaryotes, Gluthathione (GSH), Trypanothione (TSH) and Thioredoxin
(Trx), maintains the cellular redox homeostasis. Most Actinomycetes produce
mycothiol (MSH) as their principal low molecular mass thiol (1D-myo-inosityl-2-(N-acetyl-L-cysteinyl)-amido-2-deoxy-alpha-D-glucopyranoside).
Present knowledge indicates (Eq. 1 and 2)
that the redox cycling of Mycothiol (MSH) to mycothiol disulfide (MSSM) is very
critical for the in vivo and in vitro survival of mycobacteria
(Newton and Fahey, 2002) Since MSH is only present in
Actinomycetes and plays an important protective role, all the enzymes involved
in its maintenance are potential drug targets (Newton et
Eq. 1 and 2: The formaldehyde dehydrogenase reaction of MscR. The nitrosothiol reductase reaction of MscR, indicating that MSH sulphinamide is formed in vitro and that MSNO is eventually converted to nitrate and MSH in vivo through unknown reactions.
METABOLIC REACTION OF MYCOTHIOL
The biosynthetic pathway of MSH has been elucidated in the formation of Nacetylglucosaminylinositol
is catalyzed by the gene product of mshA. N-acetylglucosaminylinositol is then
deacetylated by MSH deacetylase, encoded by MshB, to yield glucosaminylinositol
(Buchmeier et al., 2003). The coupling of cysteine
to the 20 amine of this pseudodisaccharide is catalyzed by an MSH ligase, encoded
by mshC. The final step is the N-acetylation of cysteinylglucosaminylinositol
to yield MSH, catalyzed by mycothiol synthase, the product of mshD (Koledin
et al., 2002). Streptomyces coelicolor mutants in the four genes
involved in MSH biosynthesis have been isolated and recently an mshD mutant
of Amycolatopsis mediterranei has been reported (Chen et
al., 2005). S1 mapping analysis has demonstrated that three of these
genes, mshA, mshC, mshD, are induced under osmotic challenge in Streptomyces
coelicolor (Park et al., 2006).
Biosynthesis of mycothiol catalyzed by MshA, MshB, MshC, MshD. Degradation
reactions to scavenge mycothiol for cysteine in times of nutrient starvation.
Protective reactions catalyzed by: Mycothiol Amidase (Mca) and putative mycothiol-S-tranferases
involved in the detoxification of xenobiotic agents; putative thiol transferases,
nitrosothiol reductase (MscR) and thiol peroxidases or peroxiredoxins involved
in oxidative and nitrosative stress protection and Mycothiol Reductase (Mtr),
which maintains mycothiol in a reduced form within the Mycobacterial cell. Metabolic
reactions catalyzed by enzymes such as maleylpyruvate isomerase requiring mycothiol
as a cofactor for growth on diverse carbon sources. More recently demonstrated
that the GSH-independent gentisate pathway in actinomycetes requires MSH as
a cofactor for the catalysis of maleylpyruvate to fumarylpyruvate by a maleylpyruvate
isomerase, encoded by ncg l2918 (Feng et al., 2006).
In this review, it is focused conclusively to require MSH for activity. These
enzymes play important roles in the Actinomycetes, as they maintain the redox
balance within the cell and protect the cell against nitrosative stress and
toxins (Rawat and Av-Gay, 2007). In addition, the role
of MSH in growth-supporting biodegradative metabolism is the beginning to be
elucidated with the identification of MSH-dependent enzymes such as maleylpyruvate
isomerase, whether MSH participates in the novel enzymatic activities, such
as the MSH dependent amidase, that require MSH as a cofactor. Because MSH or
enzymes involved in MSH biosynthesis and metabolism are not present in mammals
and given the emerging importance of MSH in Mycobacteria, MSH and reactions
involving MSH are potential targets for drugs directed against Mycobacteria.
We thank the Department of Microbiology, Jamal Mohamed College (Autonomous), Tiruchirappalli-620020, for supporting and fulfilling all the needs to carry out this study.
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