Phosphofructokinase 1 - Mechanism

Mechanism

PFK1 is an allosteric enzyme whose activity can be described using the symmetry model of allosterism whereby there is a concerted transition from an enzymatically inactive T-state to the active R-state. F6P binds with a high affinity to the R state but not the T state enzyme. For every molecule of F6P that binds to PFK1, the enzyme progressively shifts from T state to the R state. Thus a graph plotting PFK1 activity against increasing F6P concentrations would adopt the sigmoidal curve shape traditionally associated with allosteric enzymes.

PFK1 belongs to the family of phosphotransferases and it catalyzes the transfer of γ-phosphate from ATP to fructose-6-phosphate. The PFK1 active site comprises both the ATP-Mg2+ and the F6P binding sites. Some proposed residues involved with substrate binding in E. coli FPK1 include Asp127 and Arg171. In B. stearothermophilus PFK1, the positively charged side chain of Arg162 residue forms a hydrogen-bonded salt bridge with the negatively charged phosphate group of F6P, an interaction which stabilizes the R state relative to the T state and is partly responsible for the homotropic effect of F6P binding. In the T state, enzyme conformation shifts slightly such that the space previously taken up by the Arg162 is replaced with Glu161. This swap in positions between adjacent amino acid residues inhibits the ability of F6P to bind the enzyme.

Allosteric activators such as AMP and ADP bind to the allosteric site as to facilitate the formation of the R state by inducing structural changes in the enzyme. Similarly, inhibitors such as ATP and PEP bind to the same allosteric site and facilitate the formation of the T state, thereby inhibiting enzyme activity.