Reaction Mechanism and Structural Model of ADP-forming Acetyl-CoA Synthetase from the Hyperthermophilic Archaeon Pyrococcus furiosus: EVIDENCE FOR A SECOND ACTIVE SITE HISTIDINE RESIDUE*S⃞

摘要

In Archaea, acetate formation and ATP synthesis from acetyl-CoA is catalyzed by an unusual ADP-forming acetyl-CoA synthetase (ACD) (acetyl-CoA + ADP + Pi ⇆ acetate + ATP + HS-CoA) catalyzing the formation of acetate from acetyl-CoA and concomitant ATP synthesis by the mechanism of substrate level phosphorylation. ACD belongs to the protein superfamily of nucleoside diphosphate-forming acyl-CoA synthetases, which also include succinyl-CoA synthetases (SCSs). ACD differs from SCS in domain organization of subunits and in the presence of a second highly conserved histidine residue in the β-subunit, which is absent in SCS. The influence of these differences on structure and reaction mechanism of ACD was studied with heterotetrameric ACD (α2β2) from the hyperthermophilic archaeon Pyrococcus furiosus in comparison with heterotetrameric SCS. A structural model of P. furiosus ACD was constructed suggesting a novel spatial arrangement of the subunits different from SCS, however, maintaining a similar catalytic site. Furthermore, kinetic and molecular properties and enzyme phosphorylation as well as the ability to catalyze arsenolysis of acetyl-CoA were studied in wild type ACD and several mutant enzymes. The data indicate that the formation of enzyme-bound acetyl phosphate and enzyme phosphorylation at His-257α, respectively, proceed in analogy to SCS. In contrast to SCS, in ACD the phosphoryl group is transferred from the His-257α to ADP via transient phosphorylation of a second conserved histidine residue in theβ-subunit, His-71β. It is proposed that ACD reaction follows a novel four-step mechanism including transient phosphorylation of two active site histidine residues: