43] Butyryl-CoA dehydrogenase from Megasphaera elsdenii: EC 1.3.99.2 butyryl-CoA:(acceptor) oxidoreductase

Abstract

The absorption coefficient of butyryl-CoA dehydrogenase from Megasphaera elsdenii at 450nm is determined as 14.4mM-1 cm-1 in the CoA-free form and 14.2mM-1 cm-1 in the CoA-liganded form (both yellow). The latter value is considerably higher than the earlier published estimate. Phenazine ethosulphate offers great advantages over phenazine methosulphate as a coupling dye in the catalytic assay despite giving lower Vmax. values (506min-1 as compared with 1250min 1 under the conditions used). The phenazine ethosulphate assay is used to establish a pH optimum of 8.05 for oxidation of 100 pM-butyryl-CoA. The rates of oxidation of a range of straight-chain, branched-chain and alicyclic acyl thioesters are used to provide the following information. (1) Only straight-chain acyl groups containing 4-6 carbon atoms are easily accommodated by the postulated hydrophobic pocket of the enzyme. (2) C-3-substituted acyl-CoA thioesters are not oxidized at a significant rate, suggesting that the C-3 pro-S-hydrogen atom of straight-chain substrates is partially exposed to the solvent. (3) Acyl-CoA thioesters with substitutions at C-2 are oxidized, though at a lower rate than their straight-chain counterparts. This implies that the C-2 pro-S-hydrogen atom of straight-chain substrates is partially exposed to the solvent. (4) Saturated alicyclic carboxylic acyl-CoA thioesters with 4-7 carbon atoms in the ring are oxidized, with maximal activity for the cyclohexane derivative. This implies that optimal oxidation requires a true trans orientation of the two departing hydrogen atoms. (5) The strain imposed by bound unsaturated alicyclic acyl thioesters strikingly perturbs the flavin visible-absorption spectrum, with the exception ofthe cyclohex-2-ene derivative, which forms a complex with similar spectral properties to those of the crotonyl-CoA complex. (6) In the thiol moiety of thioester substrates the amide bond of N-acetylcysteamine is essential for both binding and catalysis. The adenosine structure contributes substantially to strong binding, but is less important in determining the catalytic rate.