title: Kinetic mechanism of acetyl-CoA synthase: steady-state synthesis at variable Co/Co2 pressures.
abstract: Steady-state initial rates of acetyl-CoA synthesis (upsilon/[E(tot)]) catalyzed by acetyl-CoA synthase from Clostridium thermoaceticum (ACS) were determined at various partial pressures of CO and CO2. When [CO] was varied from 0 to 100 microM in a balance of Ar, rates increased sharply from 0.3 to 100 min(-1). At [CO] > 100 microM, rates declined sharply and eventually stabilized at 10 min(-1) at 980 microM CO. Equivalent experiments carried out in CO2 revealed similar inhibitory behavior and residual activity under saturating [CO]. Plots of upsilon/[E(tot)] vs [CO2] at different fixed inhibitory [CO] revealed that Vmax/[E(tot)] (kcat) decreased with increasing [CO]. Plots of upsilon/[E(tot)] vs [CO2] at different fixed noninhibitory [CO] showed that Vmax/[E(tot)] was insensitive to changes in [CO]. Of eleven candidate mechanisms, the simplest one that fit the data best had the following key features: (a) either CO or CO2 (at a designated reductant level and pH) activate the enzyme (E' + CO right arrow over left arrow E, E' + CO2/2e-/2H+ right arrow over left arrow E); (b) CO and CO2 are both substrates that compete for the same enzyme form (E + CO right arrow over left arrow ECO, E + CO2/2e-/2H+ right arrow over left arrow ECO, and ECO --> E + P); (c) between 3 and 5 molecules of CO bind cooperatively to an enzyme form different from that to which CO2 and substrate CO bind (nCO + ECO right arrow over left arrow (CO)nECO), and this inhibits catalysis; and (d) the residual activity arises from either the (CO)nECO state or a heterogeneous form of the enzyme. Implications of these results, focusing on the roles of CO and CO2 in catalysis, are discussed.
authors.affiliation: Department of Chemistry, Texas A&M University, College Station, Texas 77843, USA.
authors.forename: E L
authors.initials: EL
authors.lastname: Maynard
authors.forename: C
authors.initials: C
authors.lastname: Sewell
authors.forename: P A
authors.initials: PA
authors.lastname: Lindahl
mesh_terms: D000106:Acetate-CoA Ligase / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*; D000465:Algorithms; D001665:Binding Sites; D002245:Carbon Dioxide / Q000737:chemistry* / Q000494:pharmacology; D002248:Carbon Monoxide / Q000737:chemistry* / Q000494:pharmacology; D003013:Clostridium / Q000201:enzymology; D003198:Computer Simulation; D004789:Enzyme Activation / Q000187:drug effects; D007700:Kinetics; D008956:Models, Chemical; D011312:Pressure
chemical_list: D002245:Carbon Dioxide; D002248:Carbon Monoxide; D000106:Acetate-CoA Ligase