A detailed structural description of Escherichia coli succinyl-CoA synthetase

Succinyl-CoA synthetase (SCS) carries out the substrate-level phosphorylation 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 refined against data collected to 2.3 Angstrom resolution. The crystals are of space 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 subunit in the asymmetric unit of the crystals. The crystal packing leaves two choices for which pair of alpha beta-dimers form the physiologically relevant tetramer. The copies of the alpha beta-dimer are similar, each having one active site where the phosphorylated histidine residue and the thiol group of CoA are found. CoA is bound in an extended conformation to the nucleotide-binding motif in the N-terminal domain of the alpha-subunit. The phosphoryl group of the phosphorylated histidine residue is positioned at the amino termini of two alpha-helices, one from the C-terminal domain of the alpha-subunit and the other from the C-terminal domain of the beta-subunit. These 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 this is so, the catalytic histidine residue would have to move about 35 Angstrom to react with the nucleotide. (C) 1999 Academic Press.