PARTIAL REACTIONS CATALYZED BY PROTEIN-COMPONENTS OF THE ACETYL-COA DECARBONYLASE SYNTHASE ENZYME COMPLEX FROM METHANOSARCINA-BARKERI

In methanogens, the acetyl-CoA decarbonylase synthase (ACDS) complex, which has five different subunits, catalyzes synthesis and cleavage of acetyl-CoA according to the reaction: CO2 + 2H(+) + 2e(-) + CH3-H4SPt + CoA reversible arrow acetyl-CoA + H4SPt + H2O, where H4SPt and CH3- H4SPt are tetrahydrosarcinapterin and N-5-methyl-tetrahydrosarcinapter in, respectively. We have dissociated the ACDS complex into three prot ein components by limited proteolytic digestion, Catalysis of acetyl-C oA synthesis was lost in parallel with the loss of the intact beta sub unit; however, no decrease in activity was detected in any of three pa rtial reactions found to be catalyzed by distinct protein components o f the proteolyzed ACDS complex: (a) CO dehydrogenase, catalyzed by the alpha epsilon component, (b) CH3-H(4)pteridine:cob(I)amide-protein me thyltransferase, catalyzed by the intact gamma subunit and fragments o f the delta subunit, and (c) acetyltransferase, catalyzed by a truncat ed form of the beta subunit. The results indicated that the beta subun it is responsible for binding CoA and acetyl-CoA and suggested that ac etylenzyme formation occurs on the beta subunit. A value of 5.5 x [H+] (-1) M(-1) was determined for the equilibrium constant of the followin g reaction at pH 7.5 and 25 degrees C: CH3-H4SPt + cob(I)amide-protein + H+ reversible arrow H4SPt + CH3-cob(III)amide-protein.