Incomplete citric acid cycle obliges aminolevulinic acid synthesis via the C5 pathway in a methylotroph

Abstract

The enzymic activities of the citric acid cycle and the connected pathway of 5-aminolevulinic acid (ALA) formation in the methylotroph Methylophilus methylotrophus (strain AS1) have been studied. The organism has the enzymes required for conversion of pyruvate to 2-oxoglutarate. Of these, isocitrate dehydrogenase is unusual because of its preference of NAD as coenzyme over NADP. In addition, the segment of the cycle that oxidizes 2-oxoglutarate to oxaloacetate is incomplete, lacking 2-oxoglutarate and succinate and malate dehydrogenase activities. Furthermore, alternative routes of 2-oxoglutarate oxidation to succinate are undetectable. The enzymes of the glyoxylate cycle are also absent. This suggests that the cycle in M. methylotrophus has no catabolic role and is purely biosynthetic. We also show that M. methylotrophus uses the C5 pathway of ALA formation. Cell-free extracts can convert glutamate to ALA in an ATP-, NADPH- and tRNA-dependent manner via the intermediate formation of Glu-tRNAGlu and glutamate 1-semialdehyde. Consistent with the absence of a detectable route by which it could synthesize succinate, M. methylotrophus cannot generate ALA from succinyl-CoA and glycine, the pathway found in mammalian cells and yeast.