A Structural and Kinetic Study into the Role of the Quaternary Shift in Bacillus stearothermophilus Phosphofructokinase
Mosser, Rockann Elizabeth
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Bacillus stearothermophilus phosphofructokinase (BsPFK) is a homotetramer that is allosterically inhibited by phosphoenolpyruvate (PEP), which binds along one dimer-dimer interface. The substrate, fructose-6-phosphate (F6P), binds along the other dimer-dimer interface. The different functional forms BsPFK can take when in the presence of F6P and PEP can be described by the following diproportionation equilibrium: XE + EA <--> XEA + E where XE is the enzyme bound to PEP, EA is the enzyme bound to F6P, E represents the apo enzyme, and XEA is the ternary complex formed when both substrate and inhibitor are bound. Currently in the Protein Data Bank (PDB) there are two relevant forms of wild-type BsPFK, the EA form and the X'E form, which represents the enzyme bound to the PEP analog, phosphoglycolate (PGA). When comparing the EA and the X'E structures, a 7? rotation about the substrate-binding interface is observed and is termed the quaternary shift. The current study uses methyl TROSY NMR to examine the different liganded states of BsPFK, and for the first time structural data for the XEA species is shown. In addition, crystallography was used to obtain the first apo structure of BsPFK. To distinguish between changes associated with the quaternary shift and those associated with the intra-subunit tertiary changes, the variant D12A BsPFK was studied using kinetics, crystallography, and NMR. Crystal structures of apo and PEP bound forms of D12A BsPFK both indicate a shifted structure similar to the X'E form of wild-type. Kinetic studies of D12A BsPFK, when compared to wild-type, show a 50-fold diminished F6P binding affinity, 100-fold enhanced binding affinity, and a similar coupling constant. A conserved hydrogen bond between D12 and T156 takes place across the substrate binding interface in the EA form of BsPFK. The variant T156A BsPFK shows similar binding, coupling, and structural characteristics to D12A BsPFK. PEP still inhibits these variants of BsPFK despite the fact that the enzymes are in the quaternary shifted position prior to PEP binding. Therefore the quaternary shift of BsPFK primarily perturbs ligand binding but does not directly contribute to heterotropic allosteric inhibition.