A detailed structural description of Escherichia coli succinyl-CoA synthetase

Succinyl-CoA synthetase (SCS) carries out the substrate-level phosphorylati on of GDP or ADP in the citric acid cycle. A molecular model of the enzyme from Escherichia coli, crystallized in the presence of CoA, has been refine d against data collected to 2.3 Angstrom resolution. The crystals are of sp ace group P4(3)22, having unit cell dimensions a=b=98.68 Angstrom, c = 403. 76 Angstrom and the data set includes the data measured from 23 crystals. E . coli SCS is an (alpha beta)(2)-tetramer; there are two copies of each sub unit in the asymmetric unit of the crystals. The crystal packing leaves two choices for which pair of alpha beta-dimers form the physiologically relev ant tetramer. The copies of the alpha beta-dimer are similar, each having o ne active site where the phosphorylated histidine residue and the thiol gro up of CoA are found. CoA is bound in an extended conformation to the nucleo tide-binding motif in the N-terminal domain of the alpha-subunit. The phosp horyl group of the phosphorylated histidine residue is positioned at the am ino termini of two alpha-helices, one from the C-terminal domain of the alp ha-subunit and the other from the C-terminal domain of the beta-subunit. Th ese two domains have similar topologies, despite only 14% sequence identity . By analogy to other nucleotide-binding proteins, the binding site for the nucleotide may reside in the N-terminal domain of the beta-subunit. If thi s is so, the catalytic histidine residue would have to move about 35 Angstr om to react with the nucleotide. (C) 1999 Academic Press.