Succinylation and inactivation of 3-hydroxy-3-methylglutaryl-CoA synthase by succinyl-CoA and its possible relevance to the control of ketogenesis

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Abstract
Succinyl-CoA(3-carboxypropionyl-CoA) inactivates ox liver mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (EC 4.1.3.5) in a time-dependent manner, which is partially prevented by the presence of substrates of the enzyme. The inactivation is due to the enzyme catalysing its own succinylation. Complete inactivation corresponds to about 0.5 mol of succinyl group bound/mol of enzyme dimer. The succinyl-enzyme linkage appears to be a thioester bond and is probably formed with the active-site cysteine residue that is normally acetylated by acetyl-CoA. Succinyl-CoA binds to 3-hydroxy-3-methylglutaryl-CoA synthase with a binding constant of 340 .mu.M and succinylation occurs with a rate constant of 0.57 min-1. Succinyl-enzyme breaks down with a half-life of about 40 min (k = 0.017 min-1) at 30.degree. C and pH 7 and is destablilized by the presence of acetyl-CoA and succinyl-CoA. A control mechanism is postulated in which flux though the 3-hydroxy-3-methylglutaryl-CoA cycle of ketogenesis is regulated according to the extent of succinylation of 3-hydroxy-3-methylglutaryl-CoA synthase.