Re: why is the phosphoglycerate kinase rxn in glycolysis reversible when

Mauricio,

The reason for the reversibility (or lack of it) in the kinase reactions
you mention have to do with LeChatelier’s Principle.  In the cell the
concentration of ATP is typically much higher than that of ADP.  The kinase
reactions that are essentially irreversible in vivo are those that convert
ATP into ADP.  Phosphoglycerate kinase converts 1,3-diphosphoglycerate and
ADP into 3-phosphoglycerate and ATP.  Since the product ATP is present in
high concentrations, this favors the reverse reaction.  By contrast, the
hexokinase reaction converts glucose and ATP into glucose-6-phosphate and
ADP.  Here, the high ATP concentration pushes the equilibrium so far to the
right that in vivo the reverse reaction is essentially shut down.  However
the reverse reaction can be made to occur in vitro by supplying the enzyme
with high concentrations of glucose-6-phosphate and ADP.