New site-directed reversible inhibitors of 3-hydroxy-3-methylglutaryl-CoA reductase

Further insights into the mechanism of action of methylmalonyl-CoA mutase by electron paramagnetic resonance studies

Novel analogues of methylmalonyl-CoA and succinyl-CoA have been prepared and used for mechanistic investigations on the coenzyme-B12-dependent methylmalonyl-CoA mutase. 1-Carboxyethyl-CoA (1) and 2-carboxyethyl-CoA (2) as well as their sulphoxides (3 and 4) were moderately good inhibitors with Ki values 4-20 times higher than the Km for succinyl-CoA. 2-Carboxyethyl-CoA (2) and its sulphoxide 4 induced EPR signals when bound to the enzyme-coenzyme-B12 complex. The EPR spectrum of 2 and its sulphoxide 4 differed very much from those induced by the other substrates. In the case of 2 the EPR spectrum of the holoenzyme/inhibitor complex showed the presence of an organic radical coupled to cobal(II)amin. The same experiment with 4 leads to the formation of enzyme-bound cobal(II)amin with no detectable organic radical. The analogues 1 and 3 exhibited higher Ki values and did not induce EPR signals binding to the enzyme-coenzyme-B12 complex. Formyl-CoA and acrylate inhibited the enzyme synergistically but were unable to induce EPR signals and to form the product. Ethylmalonyl-CoA, known as a poor substrate, induced a similar but less intense EPR signal than the natural substrate methylmalonyl-CoA. The results are discussed in terms of the mechanism of the methylmalonyl-CoA mutase reaction.

Inactivation of 3-hydroxy-3-methylglutaryl-CoA synthase and other Acyl-CoA-utilizing enzymes by 3-Oxobutylsulfoxyl-CoA

3-Oxobutylsulfoxyl-CoA has been produced by oxidation of S-3-oxobutyl-CoA, the thioether analog of acetoacetyl-CoA. Avian hydroxymethylglutaryl-CoA (HMG-CoA) synthase is inactivated by oxobutylsulfoxyl-CoA in a time-dependent fashion. Protection against inactivation is afforded by the substrate, acetyl-CoA, suggesting that inactivation involves modification of the enzyme's active site. Pretreatment of HMG-CoA synthase with the inactivator blocks the enzyme's ability to form Michaelis and acetyl-S-enzyme intermediates, supporting the hypothesis that modification is active-site directed. Incubation of enzyme with oxobutylsulfoxyl-[32P]CoA followed by precipitation with trichloroacetic acid indicates that inactivation correlates with stoichiometric formation of a covalent adduct between enzyme and a portion of the inactivator that includes the CoA nucleotide. The observation of reagent partitioning suggests that HMG-CoA synthase catalyzes conversion of oxobutylsulfoxyl-CoA into a reactive species that modifies the protein. Treatment of inactivated enzyme with DTT or other mercaptans restores enzyme activity and reverses the covalent modification with release of CoASH. Oxobutylsulfoxyl-CoA inactivates beta-ketothiolase and HMG-CoA lyase in a process that is also reversed by DTT. These three enzymes all contain active site cysteines, suggesting that inactivation results from disulfide formation between a cysteine and the CoA moiety of the inhibitor. The data are consistent with the hypothesis that enzymatic cleavage of oxobutylsulfoxyl-CoA results in the transient formation of a sulfenic acid derivative of CoA which subsequently reacts to form a stable disulfide linkage to protein.

Inhibition of 3-hydroxy-3-methylglutaryl-CoA reductase and citrate synthase by sulfoxides and sulfones of substrate-analogue CoA-thioether derivatives

Sulfoxides and sulfones were prepared by specific oxidation of 3-hydroxy-3-methylglutaryl-CoA-analogue CoA-thioether derivatives and their kinetic properties were determined with 3-hydroxy-3-methylglutaryl-CoA reductase. The oxidized CoA-thioether derivatives with a hydroxyl group at C3 were powerful competitive inhibitors, their Ki values being much smaller than the Km for 3-hydroxy-3-methylglutaryl-CoA. Sulfoxides and sulfones of substrate analogues of citrate synthase were also prepared. When tested in the appropriate reaction with citrate synthase, the sulfoxide and sulfone derivatives were competitive inhibitors, but their Ki values were greater than the Km values of the corresponding unmodified substrates.