Succinate dehydrogenase Succinyl-coenzyme

In addition to succinate, malonate, fumarate, and phosphate, it has been reported in recent years that succinate dehydrogenase is activated by ATP, ITP, IDP (195), reduced ubiquinone-10 (195-197), succinyl coenzyme-A (198), formate, ClOr, I", Br-, C1 , NOa, SO4 ", acid pH... 

Two turns of the TCA cycle, with NADH produced at the pyruvate dehydrogenase, isocitrate dehydrogenase, alpha-ketoglutarate, and malate dehydrogenase steps and FADH2 produced at the succinate dehydrogenase step and GTP (equivalent to ATP) produced at the succinyl-coenzyme A synthetase step. 

Aconitase catalyzes the isomerization of citrate to isodtrate, isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate to a-ketoglutarate, and a-ketoglutarate dehydrogenase catalyzes the oxidative decarboxylation of a-keto-glutarate to succinyl-CoA. Succinyl-CoA and the remaining intermediates are the 4-carbon intermediates of the Krebs cycle. Succinyl thiokinase catalyzes the release of coenzyme A from succinyl-CoA and the production of GTP. Succinate dehydro-... 

So, the biosynthesis of methionine (Met, M), the first of the essential amino adds to be considered (Scheme 12.13), begins by the conversion of aspartate (Asp, D) to aspartate semialdehyde in the same way glutamate (Glu, E) was converted to glutamate semialdehyde (vide supra. Scheme 12.6). Phosphorylation on the terminal carboxylate of aspartate (Asp, D) by ATP in the presence of aspartate kinase (EC 2.7.2.4) and subsequent reduction of the aspart-4 yl phosphate by NADPH in the presence of aspartate semialdehyde dehydrogenase (EC 1.2.1.11) yields the aspartate semialdehyde. The aspartate semialdehyde is further reduced to homoserine (homoserine oxoreductase, EC 1.1.1.3) and the latter is succinylated by succinyl-CoA with the liberation of coenzyme A (CoA-SH) in the presence of homoserine O-succinyl-transferase (EC 2.3.1.46). Then, reaction with cysteine (Cys, C) in the presence of cystathionine y-synthase (EC 2.5.1.48) produces cystathionine and succinate. In the presence of the pyridoxal phosphate protein cystathionine P-lyase (EC 4.4.1.8), both ammonia and pyruvate are lost from cystathionine and homocysteine is produced. Finally, methylation on sulfur to generate methionine (Met, M) occurs by the donation of the methyl from 5-methyltetrahydrofolate in the presence of methonine synthase (EC 2.1.1.13). 

---