Succinyl-Coenzyme A Synthetase and its Role in δ-Aminolevulinic Acid Biosynthesis in Euglena gracilis

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Abstract
Euglena gracilis cells synthesize the key tetrapyrrole precursor, δ-aminolevulinic acid (ALA), by two routes: plastid ALA is formed from glutamate via the transfer RNA-dependent five-carbon route, and ALA that serves as the precursor to mitochondrial hemes is formed by ALA synthase-catalyzed condensation of succinyl-coenzyme A and glycine. The biosynthetic source of succinyl-coenzyme A in Euglena is of interest because this species has been reported not to contain α-ketoglutarate dehydrogenase and not to use succinyl-coenzyme A as a tricarboxylic acid cycle intermediate. Instead, α-ketoglutarate is decarboxylated to form succinic semialdehyde, which is subsequently oxidized to form succinate. Desalted extract of Euglena cells catalyzed ALA formation in a reaction that required coenzyme A and GTP but did not require exogenous succinyl-coenzyme A synthetase. GTP could be replaced with ATP. Cell extract also catalyzed glycine-and α-ketoglutarate-dependent ALA formation in a reaction that required coenzyme A and GTP, was stimulated by NADP+, and was inhibited by NAD+. Succinyl-coenzyme A synthetase activity was detected in extracts of dark- and light-grown wild-type and nongreening mutant cells. In vitro succinyl-coenzyme A synthetase activity was at least 10-fold greater than ALA synthase activity. These results indicate that succinyl-coenzyme A synthetase is present in Euglena cells. Even though the enzyme may play no role in the transformation of α-ketoglutarate to succinate in the atypical tricarboxylic acid cycle, it catalyzes succinyl-coenzyme A formation from succinate for use in the biosynthesis of ALA and possibly other products.