Mycolactone
Encyclopedia
Mycolactone is a polyketide
-derived macrolide
produced and secreted by a group of very closely related pathogenic mycobacteria that have been assigned a variety of species names including, M. ulcerans, M. liflandii (an unofficial designation), M. pseudoshottsii, and some strains of M. marinum. These mycobacteria are collectively referred to as mycolactone-producing mycobacteria or MPM.
Mycolactone is produced by MPM through condensation of two polyketide chains (known as the core and acyl side chain). Different MPM produce characteristic mixtures of mycolactone congeners. The structural heterogeneity of mycolactones is due to variations in the acyl side chain. The structure of the mycolactone core is invariant.
The genes required for mycolactone biosynthesis form a contiguous 110-kb cluster on a large plasmid
. The lactone core is produced by two polyketide synthases (PKS) that are encoded by the genes, mlsA1 and mlsA2, and a third polyketide synthase, encoded by the mlsB gene, produces the fatty acid
-acyl side chain. Three putative accessory genes are found in the mycolactone cluster. One of these, MUP053, encodes a p450 monooxygenase that is thought to produce the hydroxyl at C′-12 on the fatty acid side chain. The gene encoding a FabH-like, type III ketosynthase (KS), located upstream of mlsA1, encodes a putative “joinase” (MUP045), and a type II thioesterase (TE II) gene (MUP038) is located between mlsA2 and mlsB.
Five distinct, naturally-occurring mycolactone structural variants have been described so far:
core with an ester
-linked polyketide
chain. Three plasmid
-encoded polyketide synthase
(PKS) enzymes are responsible for its production: MLSA 1 and MLSA 2 which generate the core, and MLSB is responsible for the synthesis of the polyketide chain. As shown in Figure 1, MLSB (1.2 MDa) contains seven consecutive extension modules and MLSA 1 (1.8 MDa) consists of eight. The remaining PKS enzyme, MLSA 2, contains the ninth module of MLSA . The C-terminal domains of both MLSA2 and MLSB includes a thioesterase
(TE) that was thought to catalyze the formation of the mycolactone core but appears inactive. Each module consists of either malonyl-CoA
or methylmalonyl-CoA
Acyltransferase
(AT) that allows for chain extension, a ketosynthase (KS), which catalyzes chain elongation, and an Acyl carrier protein
(ACP) where the growing polyketide chain is attached. Modules may also consist of any of the following modifying domains: a dehydratase (DH), an enoyl reductase (ER) and one of two types of ketoreductase (KR) domains. Type A and B KRs refer to the two directions of ketoreduction that are correlated with specific amino acids in the active site
. Four of the DH domains are predicted to be inactive based on a point mutation
found in the active site sequence.
Figure 1. Domain Organization of Mycolactone.
Polyketide
Polyketides are secondary metabolites from bacteria, fungi, plants, and animals. Polyketides are usually biosynthesized through the decarboxylative condensation of malonyl-CoA derived extender units in a similar process to fatty acid synthesis...
-derived macrolide
Macrolide
The macrolides are a group of drugs whose activity stems from the presence of a macrolide ring, a large macrocyclic lactone ring to which one or more deoxy sugars, usually cladinose and desosamine, may be attached. The lactone rings are usually 14-, 15-, or 16-membered...
produced and secreted by a group of very closely related pathogenic mycobacteria that have been assigned a variety of species names including, M. ulcerans, M. liflandii (an unofficial designation), M. pseudoshottsii, and some strains of M. marinum. These mycobacteria are collectively referred to as mycolactone-producing mycobacteria or MPM.
Mycolactone is produced by MPM through condensation of two polyketide chains (known as the core and acyl side chain). Different MPM produce characteristic mixtures of mycolactone congeners. The structural heterogeneity of mycolactones is due to variations in the acyl side chain. The structure of the mycolactone core is invariant.
The genes required for mycolactone biosynthesis form a contiguous 110-kb cluster on a large plasmid
Plasmid
In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...
. The lactone core is produced by two polyketide synthases (PKS) that are encoded by the genes, mlsA1 and mlsA2, and a third polyketide synthase, encoded by the mlsB gene, produces the fatty acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
-acyl side chain. Three putative accessory genes are found in the mycolactone cluster. One of these, MUP053, encodes a p450 monooxygenase that is thought to produce the hydroxyl at C′-12 on the fatty acid side chain. The gene encoding a FabH-like, type III ketosynthase (KS), located upstream of mlsA1, encodes a putative “joinase” (MUP045), and a type II thioesterase (TE II) gene (MUP038) is located between mlsA2 and mlsB.
Five distinct, naturally-occurring mycolactone structural variants have been described so far:
- Mycolactone A/B (M. ulcerans from Africa, Malaysia, Japan)
- Mycolactone C (M. ulcerans from Australia)
- Mycolactone D (M. ulcerans from China)
- Mycolactone E (M. liflandii)
- Mycolactone F (M. pseudoshottsii and M. marinum from around the world)
Biosynthesis
Mycolactone consists of a 12-membered macrolideMacrolide
The macrolides are a group of drugs whose activity stems from the presence of a macrolide ring, a large macrocyclic lactone ring to which one or more deoxy sugars, usually cladinose and desosamine, may be attached. The lactone rings are usually 14-, 15-, or 16-membered...
core with an ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
-linked polyketide
Polyketide
Polyketides are secondary metabolites from bacteria, fungi, plants, and animals. Polyketides are usually biosynthesized through the decarboxylative condensation of malonyl-CoA derived extender units in a similar process to fatty acid synthesis...
chain. Three plasmid
Plasmid
In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...
-encoded polyketide synthase
Polyketide synthase
Polyketide synthases are a family of multi-domain enzymes or enzyme complexes that produce polyketides, a large class of secondary metabolites, in bacteria, fungi, plants, and a few animal lineages...
(PKS) enzymes are responsible for its production: MLSA 1 and MLSA 2 which generate the core, and MLSB is responsible for the synthesis of the polyketide chain. As shown in Figure 1, MLSB (1.2 MDa) contains seven consecutive extension modules and MLSA 1 (1.8 MDa) consists of eight. The remaining PKS enzyme, MLSA 2, contains the ninth module of MLSA . The C-terminal domains of both MLSA2 and MLSB includes a thioesterase
Thioesterase
Thioesterases are enzymes which belong to the Esterase family. Esterases, in turn, are one type of the several hydrolases known.Thioesterases exhibit Esterase activity specifically at a thiol group.Thioesterases or thiolester hydrolases are identified as members of E.C.3.1.2.-Examples:Acetyl-coA...
(TE) that was thought to catalyze the formation of the mycolactone core but appears inactive. Each module consists of either malonyl-CoA
Malonyl-CoA
Malonyl-CoA is a coenzyme A derivative.-Functions:It plays a key role in chain elongation in fatty acid biosynthesis and polyketide biosynthesis....
or methylmalonyl-CoA
Methylmalonyl-CoA
Methylmalonyl-CoA is the coenzyme A linked form of methylmalonic acid. Methylmalonyl-CoA is formed from propionyl-CoA by propionyl-CoA carboxylase by help of biotin . It is converted into succinyl-CoA by methylmalonyl-CoA mutase, in a reaction that requires vitamin B12 as a cofactor...
Acyltransferase
Acyltransferase
Acyltransferase is a type of transferase enzyme that acts upon acyl groups.Examples include:* Glyceronephosphate O-acyltransferase* Lecithin-cholesterol acyltransferase...
(AT) that allows for chain extension, a ketosynthase (KS), which catalyzes chain elongation, and an Acyl carrier protein
Acyl carrier protein
The acyl carrier protein is an important component in both fatty acid and polyketide biosynthesis with the growing chain bound during synthesis as a thiol ester at the distal thiol of a 4'-phosphopantethiene moiety...
(ACP) where the growing polyketide chain is attached. Modules may also consist of any of the following modifying domains: a dehydratase (DH), an enoyl reductase (ER) and one of two types of ketoreductase (KR) domains. Type A and B KRs refer to the two directions of ketoreduction that are correlated with specific amino acids in the active site
Active site
In biology the active site is part of an enzyme where substrates bind and undergo a chemical reaction. The majority of enzymes are proteins but RNA enzymes called ribozymes also exist. The active site of an enzyme is usually found in a cleft or pocket that is lined by amino acid residues that...
. Four of the DH domains are predicted to be inactive based on a point mutation
Point mutation
A point mutation, or single base substitution, is a type of mutation that causes the replacement of a single base nucleotide with another nucleotide of the genetic material, DNA or RNA. Often the term point mutation also includes insertions or deletions of a single base pair...
found in the active site sequence.