The structure of unliganded E. coli phosphofructokinase
The allosteric phosphofructokinase from E. coli crystallized in the absence of ligands. X-ray diffraction data were collected to 2.4 aa, AA resolution and the structure was solved by the method of molecular replacement, using the model of the liganded R-state of the enzyme. The atomic model was refined by rigid body refinement and by the least-squares method of Hendrickson and Konnert. The final structure is compared to the high resolution model of the liganded, active form of the enzyme and to the low resolution structure of the inhibited phosphofructokinase from B. stearothermophilus. It is found that the quaternary structure of the unliganded model is more similar to the liganded, active form than to the inhibited T-state of B. stearothermophilus pfk. There are however considerable differences in the tertiary and quaternary structures, apparently resulting from ligand binding. These changes are not localised to the binding sites. The overall change, though to result mainly from the binding of the activators, is consistent with the closing of the active site observed in the structure of the active state. The ways in which the ligand binding could bring about these changes are considered. The possible effect of the changes on the enzyme activity are discussed. It is possible that the structure represents an inactive T-state, different from that in the presence of inhibitors. Whatever the exact interpretation it is clear that the structure shows considerable flexibility suggesting that the two-state allosteric model is an oversimplification.