title: Butyryl-CoA dehydrogenase from Megasphaera elsdenii. Specificity of the catalytic reaction.
abstract: The absorption coefficient of butyryl-CoA dehydrogenase from Megasphaera elsdenii at 450 nm is determined as 14.4 mM-1 X cm-1 in the CoA-free form and 14.2 mM-1 X 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 (506 min-1 as compared with 1250 min-1 under the conditions used). The phenazine ethosulphate assay is used to establish a pH optimum of 8.05 for oxidation of 100 microM-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. Only straight-chain acyl groups containing 4-6 carbon atoms are easily accommodated by the postulated hydrophobic pocket of the enzyme. 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. 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. 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. The strain imposed by bound unsaturated alicyclic acyl thioesters strikingly perturbs the flavin visible-absorption spectrum, with the exception of the cyclohex-2-ene derivative, which forms a complex with similar spectral properties to those of the crotonyl-CoA complex. 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.
mesh_terms: D000214:Acyl Coenzyme A / Q000031:analogs & derivatives / Q000378:metabolism; D001665:Binding Sites; D044926:Butyryl-CoA Dehydrogenase; D004579:Electron Transport; D044943:Fatty Acid Desaturases / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D046911:Macromolecular Substances; D013053:Spectrophotometry; D013329:Structure-Activity Relationship; D013379:Substrate Specificity; D045854:Veillonellaceae / Q000201:enzymology*
chemical_list: D000214:Acyl Coenzyme A; D046911:Macromolecular Substances; D044943:Fatty Acid Desaturases; D044926:Butyryl-CoA Dehydrogenase