Magnetic resonance studies of the proximity and spatial arrangement of propionyl coenzyme A and pyruvate on a biotin-metalloenzyme, transcarboxylase.

A spin-labeled ester of CoA, R-CoA (3-carboxy-2,2,5,5-tetramethyl-1-pyrolidinyl-1-oxy CoA thioester), has been shown by competition studies using electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) to bind specifically to the propionyl-CoA binding sites of transcarboxylase. Titrations indicate 0.7 +/- 0.2 binding site for R-CoA per enzyme-bound biotin with a dissociation constant of 0.33 +/- 0.12 mM. Propionyl-CoA binds to this site with a 1.3-fold lower disonable agreement with kinetically determined inhibitor constants of CoA and propionyl-CoA and propionyl-CoA (D. B. Northrop (1969), J. Biol. Chem. 244, 5808). The bit of this spin-label on 1/T1 of water protons. The formation of a ternary transcarboxylase-R-CoA-pyruvate complex is suggested by the failure of pyruvate to displace R-CoA from the tight site and is established by the paramagnetic effects of enzyme-bound R-CoA on the relaxation rates of the protons and 13C atoms of enzyme-bound pyruvate. From the paramagnetic effects of R-CoA on the relaxation rates of the methyl protons of pyruvate at 40.5 and 100 MHz, and on the 13C-enriched carbonyl and carboxyl carbon atoms of pyruvate at 25.1 MHz, a correlation time of 7 nsec and distances from the bound nitroxide radical to the methyl protons, the carbonyl, and carboxyl carbon atoms of bound pyruvate of 7.9 +/- 0.7, 10.3 +/- 0.8, and 12.1 +/- 0.9 A, respectively, are calculated. These distances establish the close proximity of the CoA ester and keto acid sites on transcarboxylase. Together with the previously determined distances from the enzyme-bound (Co(II) to the methyl protons and 2 carbon atoms of bound pyruvate and to 12 protons and 3 phosphorus atoms of bound propionyl-CoA, the present distances are used to derive a composite model of the bound substrates in the overall transcarboxylation reaction. In this model the distance from the methyl carbon of pyruvate and the methylene carbon of propionyl-CoA, between which the carboxyl transfer takes place is only approximately 7 A. Depending on the detailed mechanism of the carboxyl transfer, the distance through which the carboxybiotin must migrate is therefore between 0 and 7 A. Hence the major role of the 14-A arm of carboxybiotin is not to permit a large carboxyl migration but, rather to permit carboxybiotin to traverse the gap which occurs at the interface of three subunits and to insinuate itself between the CoA and keto acid sites.