Pyruvate formate-lyase mechanism involving the protein-based glycyl radical.

Pyruvate formate-lyase (also called formate acetyltransferase; EC 2.3.1.54; PFI .) catalyses the thiolytic cleavage of pyruvate by CoA, yielding acetyl-CoA and formate. This reaction is the key step in the glucose-fermentation route in Escherichziz coli and various other bacteria. Operationally, it resembles the (B-keto)thiolase reaction of the fatty-acid degradation cycle. The mechanism of pyruvate formate-lyase, however, is fundamentally different, since the carbon-carbon bond of its substrate is cleaved homolytically rather than heterolytically. This property emerged with the discovery of a protein-based radical in the active enzyme form [ 11. The unpaired spin has recently been assigned to C-2 of (;lyi” [ 2 ] . The radical is produced by a postribosomal hydrogen-atom abstraction that is catalysed by PFI, activase using adenosylmethionine (AdoMet) and reduced flavodoxin as co-substrates [ 11. A separate reaction that quenches the protein radical in PFI, is catalysed by the multifunctional AdhE protein and is initiated when anaerobic cells are shifted to positive redox potentials [ 3 ] . Metabolic aspects of PFI, interconversion between inactive (E) and active (Em) forms and the genetic/transcriptional background of the system have already been reviewed 141. This review will focus on enzyme-catalytic structure/function properties. Pyruvate formate-lyase mechanism involving the protein-based glycyl radical J. Knappe. S. Elbert, M. Frey and A. F. V. Wagner lnstitut fur Biologische Chemie, Universitat Heidelberg, D-6900 Heidelberg, Germany