THE CRYSTAL-STRUCTURE OF SUCCINYL-COA SYNTHETASE FROM ESCHERICHIA-COLI AT 2.5-ANGSTROM RESOLUTION

The x-ray crystal structure of succinyl-CoA synthetase (SCS) from Esch erichia coli has been determined by the method of multiple isomorphous replacement to a resolution of 2.5 Angstrom. Crystals of SCS are tetr agonal with a space group of P4(3)22 and unit cell dimensions of a = b = 98.47 Angstrom and c = 400.6 Angstrom. One molecule of SCS (142 kDa ) is contained in the asymmetric unit. The current model has been refi ned to a conventional R factor of 21.6% with root mean square deviatio ns from ideal stereochemistry of 0.022 Angstrom for bond lengths and 3 .25 degrees for bond angles. The quaternary organization of the E. col i enzyme is an alpha(2) beta(2) heterotetramer. In this tetramer, the alpha-subunits interact only with the beta-subunits, whereas the beta- subunits interact to form the dimer of alpha beta-dimers. The two acti ve site pockets are located at regions of contact between alpha- and b eta subunits. One molecule of coenzyme A is bound to each alpha-subuni t at a typical nucleotide-binding motif, and His-246 of each alpha-sub unit is phosphorylated. This phosphohistidine, a catalytic intermediat e, is stabilized by two helix dipoles (the ''power'' helices), one fro m each of the two subunit types. A short segment of the beta-subunit f rom one cup-dimer is in close proximity to the CoA-binding site of the other cup-dimer, providing a possible rationale for the overall tetra meric structure.