Mechanistic study on the inactivation of general acyl-CoA dehydrogenase by a metabolite of hypoglycin A
摘要:
General acyl-CoA dehydrogenase (GAD) is a flavin-dependent (FAD) enzyme that catalyzes the oxidation of a fatty acyl-CoA to the corresponding alpha,beta-enolyl-CoA. When GAD is exposed to (methylenecyclopropyl)acetyl-CoA (MCPA-CoA), a metabolite of hypoglycin A that is the causative agent of Jamaican vomiting sickness, time-dependent inhibition occurs with concomitant bleaching of the active-site FAD. The inactivation mechanism is generally believed to be initiated by C-alpha anion formation followed by ring fragmentation and the covalent modification of FAD. However, formation of a cyclopropyl radical intermediate through one-electron oxidation followed by ring opening and then radical recombination to yield a modified FAD is an appealing alternative. As described herein, studies of the inactivation of GAD by (1S)- and (1R)-MCPA-CoA bearing a stereospecific tritium label at C-alpha have provided direct evidence suggesting that C-alpha proton abstraction occurs during inactivation and the two diastereomers of MCPA-CoA bind to the same locus in the active site of GAD. Despite the fact that the inactivations mediated by (1R)- and (1S)-MCPA-CoA proceed at different rates, the observed partition ratios are almost identical. Using [alpha,alpha-H-2(2)]MCPA-CoA as inhibitors, we have found that the sluggish inactivation observed for (1S)-MCPA-CoA is not due to mechanistic rerouting, but is instead a result of the retardation of the initial deprotonation step. Thus, the equivalent partition ratios found in these studies clearly indicate that inactivation by either (1R)- or (1S)-MCPA-CoA follows the same chemical course. Such a lack of stereospecificity for the bond rupture at C-beta of MCPA-CoA in the enzyme active site suggests that the ring-opening step leading to inactivation is likely a spontaneous event. Since the rearrangement of alpha-cyclopropyl radicals to ring-opened alkyl radicals is extremely rapid, the ring cleavage induced by an alpha-cyclopropyl radical may bypass the chiral discrimination normally imposed by the enzyme. Thus, the mechanistic insights deduced from this study support our early notion that inactivation of GAD by MCPA-CoA is likely to proceed through a radical mechanism.
Asymmetric total synthesis of the individual diastereoisomers of hypoglycin A
作者:Jack E. Baldwin、Robert M. Adlington、David Bebbington、Andrew T. Russell
DOI:10.1039/c39920001249
日期:——
The individual diastereoisomers that constitute the unusual methylenecyclopropane containing α-amino acidhypoglycin A have been synthesised utilising the Sharpless epoxidation to permit an asymmetric methylene cyclopropane synthesis.
Asymmetric total synthesis of the individual diastereoisomers of hypoglycin A.
作者:Jack E. Baldwin、Robert M. Adlington、David Bebbington、Andrew T. Russell
DOI:10.1016/s0040-4020(01)89313-3
日期:1994.1
The individual diastereoisomers that constitute the unusual methylenecyclopropane containing α-amino acid hypoglycin A have been synthesised utilising the Sharpless epoxidation to permit an asymmetric methylenecyclopropane synthesis.
Revision of Absolute Configuration of Enantiomeric (Methylenecyclopropyl)carbinols Obtained from (<i>R</i>)-(−)- and (<i>S</i>)-(+)-Epichlorohydrin and Methylenetriphenylphosphorane. Implications for Reaction Mechanism and Improved Synthesis of Antiviral Methylenecyclopropane Analogues of Nucleosides
作者:Xinchao Chen、Jiri Zemlicka
DOI:10.1021/jo010511j
日期:2002.1.1
reaction of (R)- and (S)-epichlorohydrin 5 with methylenetriphenylphosphorane or resolution of the corresponding oxaphospholane 6 via a salt with L-(+)-tartaric acid and subsequent Wittig transformation with formaldehyde were revised. The (-)-oxaphospholane 6 has the S,S and (-)-(methylenecyclopropyl)carbinol (4) the R configuration. The configurations of (+)-6 and (+)-4 are then R,R and S, respectively