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14 Metabolic Pathways Relevant to Predation, Signaling, and Development
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The first portion of this chapter, entitled “Catabolic Pathways,” deals with the catabolism of amino acids and lipids, as they are the principal carbon and energy sources derived from prey bacteria. The second portion of the chapter, entitled “Anabolic Pathways,” highlights the synthesis of lipids because of their unusual chemical structures in myxobacteria, and also the spore-specific components trehalose and ether lipids. Most myxobacteria, including Myxococcus xanthus, can catabolize prey microorganisms. M. xanthus utilizes amino acids and lipids as carbon and energy sources, incorporates purines and pyrimidines via salvage pathways, and fails to utilize sugars. In most cases there is excellent agreement between the presence of a particular amino acid catabolic pathway and the ability of that amino acid to stimulate growth in defined and minimal media. Lipid oxidation has been demonstrated by 14C-labeling experiments in Myxococcus virescens and methyloleate feeding in M. xanthus. Fatty acids are usually degraded by β oxidation, where two carbon acetate units are sequentially removed from the carboxyl end of the fatty acid, also known as the Δ terminus, as opposed to the methyl end or ω terminus. Monosaccharides are used for exopolysaccharide, peptidoglycan, and lipopolysaccharide biosynthesis. In Escherichia coli, trehalose is degraded to glucose by a periplasmic trehalase; no homolog exists in M. xanthus.
Amino acid catabolic reactions 1 through 5 (alanine to cysteine). See the text.
Amino acid catabolic reactions 6 through 9 (glutamine to glycine). See the text.
Amino acid catabolic reactions 10 through 12 (histidine to methionine). See the text.
Amino acid catabolic reactions 13 through 16 (phenylalanine to threonine). See the text.
Fatty acid biosynthesis is a cyclic process. Condensation of malonyl-ACP with the growing chain by FabB, FabF, or FabH results in chain extension by two carbons. The β-keto group is reduced to a β-hydroxyl group by FabG, which is then dehydrated to a trans-∆2 unsaturated bond by FabZ or FabA. This bond is reduced to full saturation by FabI or isomerized to cis-∆3 and preserved in later chain extension to create unsaturated fatty acids.
A putative fatty acid biosynthesis operon in M. xanthus. The operon begins with plsX (MXAN4772), fabD (MXAN4771), fabG (MXAN4770), acpP (MXAN4769), and fabF (MXAN4768). This operon closely resembles the fatty acid operon found in E. coli. They differ in that the E. coli operon contains fabH after plsX and the rpmF gene upstream of the operon is transcribed in the same direction as opposed to the opposite orientation found in M. xanthus.
A second putative fatty acid biosynthesis cluster operon in M. xanthus. The operon contains 10 genes (MXAN6392 through 6401): acpP, fabZ, a β-lactamase gene, fabF, fabF, fabZ, fabG, fabG, fabF, and fabF.
Acyl chains can be linked to the glycerol backbone of certain lipids in three ways. First is the ester linkage; this is the most common linkage type. Second is the ether linkage, requiring a fatty alcohol instead of a fatty acid. Third is the alk-1-enyl linkage, where an unsaturation immediately follows the ether bond.