Interaction of mycobacterial polymethylpolysaccharides with paranaric acid and palmitoyl-coenzyme A: structural specificity and monomeric dissociation constants

Abstract

The long-chain polyenoic fatty acids alpha- and beta-paranaric acid form complexes with the 6-O-methylglucose polysaccharide from Mycobacterium smegmatis as demonstrated by an enhanced fluorescence emission of the paranaric acid. This interaction is eliminated by digestion of the methylglucose polysaccharide with alpha-amylase and glucoamylase, which removes four hexose units from the nonreducing end of the chain. Titration of the methylglucose polysaccharide with either paranaric acid isomer suggests formation of a 1:1 complex with a dissociation constant (K(d)) of 0.4 muM. The fluorescence emission of this complex is quenched by palmitoyl-CoA, which indicates that the paranaric acid can be displaced by the acoyl-CoA, a conclusion confirmed by gel filtration. The presumed polysaccharide/palmitoyl-CoA complex has a K(d) of about 0.1 muM. Acoyl-CoA derivatives with shorter fatty acid chains and free palmitic acid complete less effectively, indicating that they form weaker complexes with the polysaccharide. The methylmannose polysaccharides with 12 or 13 sugar units also complex paranaric acid strongly (K(d) approximately 0.4 muM), whereas the isomer with 11 sugar units complexes weakly.The methylglucose polysaccharide has been coupled to L-tryptophan methyl ester. The fluorescence emission spectrum of the attached tryptophan group is shifted to a shorter wavelength relative to N-acetyl-L-tryptophan methyl ester, and this effect is enhanced in the corresponding derivative made with the amylase-digested polysaccharide. The circular dichroism spectrum of the polysaccharide-tryptophan derivative shows three bands with negative ellipticity, in the 270-300 nm region, not observed in the amylase-digested derivative. These results imply that the tryptophan is in a more structured environment in the former than in the latter derivative. alpha-Paranaric acid binds to the polysaccharide-tryptophan conjugate and shows an enhanced fluorescence emission with partial quenching of the tryptophan fluorescence emission, suggestive of Förster energy transfer from tryptophan to paranaric acid.