The crystal structure of succinyl-CoA synthetase from Escherichia coli at 2.5-A resolution.

Abstract

The x-ray crystal structure of succinyl-CoA synthetase (SCS) from Escherichia coli has been determined by the method of mu!tiple isomorphous replacement to a resolution of 2.5 A. Crystals of SCS are tetragonal with a space group of P4,22 oand unit cell dimensions of a = b = 98.47 A and c = 400.6 A. One molecule of SCS (142 kDa) is contained in the asymmetric unit. The current model has been refined to a conventional R factor of 21.6% with root meqn square deviations from ideal stereochemistry of 0.022 A for bond lengths and 3.25" for bond angles. The quaternary organization of the E. coli enzyme is an cyz& heterotetramer. In this tetramer, the cy-subunits interact only with the P-subunits, whereas the P-subunits interact to form the dimer of aP-dimers. The two active site pockets are located at regions of contact between a-and p-subunits. One molecule of coenzyme Ais bound to each cy-subunit at a typical nucleotide-binding motif, and His-246 of each cy-subunit is phosphorylated. This phosphohistidine, a catalytic intermediate, is stabilized by two helix dipoles (the "power" helices), one from each of the two subunit types. A short segment of the P-subunit from one @-dimer is in close proximity to the CoA-binding site of the other @-dimer, providing a possible rationale for the overall tetrameric structure.