| Abstrakt: |
Acetyl-coenzyme A synthases (ACS) are Ni–Fe–S containingenzymes found in archaea and bacteria. They are divisible into 4 classes. Class I ACS's catalyze the synthesis of acetyl-CoAfrom CO2 + 2e-, CoA, and a methyl group, and contain5 types of subunits (α, β, γ, δ, and ε). Class II enzymes catalyze essentially the reversereaction and have similar subunit composition. Class III ACS'scatalyze the same reaction as Class I enzymes, but use pyruvateas a source of CO2 and 2e-, and are composed of 2 autonomous proteins, an α2β2 tetramerand a γδ heterodimer. Class IV enzymes catabolize CO to CO2 and are α-subunit monomers. Phylogeneticanalyses were performed on all five subunits. ACS α sequences divided into 2 major groups, including Class I/II sequences and Class III/IV-like sequences. Conserved residuesthat may function as ligands to the B- and C-clusters wereidentified. Other residues exclusively conserved in Class I/IIsequences may be ligands to additional metal centers in Class I and II enzymes. ACS β sequences also separated into twogroups, but they were less divergent than the α's, and the separation was not as distinct. Class III-like β sequences contained ∼300 residues at their N-termini absent in Class I/II sequences. Conserved residues identifiedin β sequences may function as ligands to active siteresidues used for acetyl-CoA synthesis. ACS γ-sequencesseparated into 3 groups (Classes I, II, and III), while δ-sequences separated into 2 groups (Class I/II and III). These groups are less divergent than those of α sequences. ACS ε-sequence topology showed greaterdivergence and less consistency vis-à-vis the other subunits, possibly reflecting reduced evolutionary constraintsdue to the absence of metal centers. The α subunit phylogeny may best reflect the functional diversity of ACS enzymes. Scenarios of how ACS and ACS-containing organisms mayhave evolved are discussed. [ABSTRACT FROM AUTHOR] |
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