- Studies of acyl-CoA dehydrogenase catalyzed allylic isomerization: A one-base or two-base mechanism?
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Acyl-CoA dehydrogenases are flavoproteins that catalyze the conversion of a fatty acyl thioester substrate to the corresponding α,β-enoyl-CoA product. It has been well established that a glutamate residue in the active site [e.g., E367 in short-chain acyl-CoA dehydrogenase (SCAD) of Megasphaera elsdenii] is responsible for the initial α-proton abstraction. Early studies have also shown that this class of enzymes is capable of catalyzing γ-H abstraction to afford the allylic isomerization between α,β- and β,γ-enone thioesters and/or inactivation by 2- or 3-acetylenic acyl-CoA derivatives. Although a dual role has been proposed for the glutamate residue in both α- and γ-deprotonation, the existence of a second active-site basic group to mediate the observed reactions occurring at γ-C remains a feasible mechanism. In an attempt to discern between these two possibilities, we have prepared a few oxirane-containing acyl-CoA derivatives aimed at trapping active-site bases in the vicinity of the α- and/or γ-C. It was found that 2,3-epoxybutyryl-CoA is a new class-selective irreversible inactivator against SCAD; however, the inability of other oxirane-containing probes to react with these enzymes prompted us to tackle this mechanistic problem by directly studying the role of Glu-367 in SCAD-catalyzed 1,3-isomerization. The effect of E367Q mutation on the proficiency of SCAD to mediate the γ-H exchange of crotonoyl-CoA was examined. The capabilities of the wild-type SCAD and its E367Q mutant to catalyze the γ-H abstraction during the inactivation by 2-butynoyl-CoA was also compared. The fact that the mutant protein fails to promote γ-H exchange/abstraction provides strong evidence supporting a one-base mechanism of this enzyme-catalyzed allylic isomerization. Since the catalysis of acyl-CoA dehydrogenases is closely related, such a one-base mechanism is expected to be conserved within this family of enzymes.
- Dakoji, Srikanth,Shin, Injae,Becker, Donald F.,Stankovich, Marian T.,Liu, Hung-Wen
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- UNSATURATED CARBOXYLIC ACID POLYENOLATES. LITHIUM TRIENOLATE OF SORBIC ACID AS d6 SYNTHON: ADDITION TO KETONES AND UNSATURATED KETONES
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Deprotonation of sorbic acid by two equivalents of lithium diethylamide, and aldol-type reaction of the resulting trienolate dianion (2) with carbonylic compounds affords the pentadienyl hydroxy acids (6) or 7-hydroxyhexa-2,4-dienoic acids (7) according to the duration of the reaction.The reaction of the dianion (2) with α,β-unsaturated ketones gives 1,4-ε-adducts for di- or tri-substituted enones, but either 1,4-γ- or 1,4-α-adducts for monosubstituted enones.
- Ballester, Pablo,Costa, Antonio,Garcia-Raso, Angel,Mestres, Ramon
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p. 2797 - 2804
(2007/10/02)
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- LITHIUM TRIENOLATE OF SORBIC ACID AS A D6 SYNTHON. SYNTHESIS OF 7-HYDROXY 2,4-DIENOIC ACIDS.
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The trienolate 2 of sorbic acid is an easily available d6 synthon.Preparation of 7-hydroxy dienoic acids 5 from ketones is here described.
- Ballester, P.,Costa, A.,Garcia-Raso, A.,Gomez-Solivellas, A.,Mestres, R.
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p. 3625 - 3628
(2007/10/02)
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