Pyruvate formate-lyase (acetyl-CoA:formate C-acetyltransferase, EC 2.3.1.54) from anaerobic Escherchia coli cells converts pyruvate to acetyl-CoA and formate by a unique homolytic mechanism that involves a free radical harbored in the protein structure. By EPR spectroscopy of selectively 13C-labeled enzyme, the radical (g = 2.0037) has been assigned to carbon-2 of a glycine residue. Estimated hyperfine coupling constants to the central 13C nucleus (Au = 4.9 mT and A, = 0.1 mT) and to 13C nuclei in a and 13 positions agree with literature data for glycine radical models. N-coupling was verified through uniform I'N-labeling. The large IH hyperfine splitting (1.5 mT) dominating the EPR spectrum was asignd to the a proton, which in the enzyme radical is readily solvent-exchangeable. 996The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

The location of energy minima on the conformational energy surface of molecules by computational methods (conformational searching) continues to play a key role in computer-assisted molecular modeling. Although a number of conformational search procedures have been devised over the past several years, new more efficient methods are urgently needed if molecules with increased complexity are to be treated in a quantitative manner. In this paper we describe a method, termed low-mode search (LMOD), which is based on eigenvector following (or mode following), for the exhaustive exploration of the potential energy hypersurface of molecules. It is particularly efficient at searching the conformational space of both cyclic and acyclic molecules, and we describe its effectiveness for a number of conformational search problems including acyclic, monocyclic, and bicyclic hydrocarbons and cyclic pentapeptides. No special treatment of rings in cyclic molecules is necessary, nor is it necessary to define rotatable bonds. LMOD generates structures “automatically” with minimum input from the user. We demonstrate that LMOD is one of the most efficient procedures yet devised for conformational searching of small- to medium-sized molecules.

Pyruvate formate-lyase (formate acetyltransferase; EC 2.3.1.54) of Escherichia coli cells is post-translationally interconverted between inactive and active forms. Conversion of the inactive to the active form is catalyzed by an Fe2 -dependent activating enzyme and requires adenosylmethionine and dihydroflavodoxin. This process is shown here to introduce a paramagnetic moiety into the structure of pyruvate formate-lyase. It displays an EPR signal at g = 2 with a doublet splitting of 1.5 mT and could comprise an organic free radical located on an amino acid residue of the polypeptide chain. Hypophosphite was discovered as a specific reagent that destroys both the enzyme radical and the enzyme activity; it becomes covalently bound to the protein. (Fl). The latter is generated from NADPH-and pyruvate-dependent oxidoreductases (4). Studies of the reconstituted system have suggested that the conversion reaction proceeds as follows (5) 1332The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.