pH kinetic studies of XXXX brain hexokinase.

@entity3775 @entity1792 @entity291 @entity413 @entity855
The variation of kinetic parameters with pH was examined for @entity291 brain hexokinase with @entity413 and @entity855 as substrates. The -log V1 and -log (V1/Km) profiles for both substrates were examined and seen to decrease below pH 6.5. All profiles asymptotically approached slopes of -1, indicating that the loss of activity in each instance was due to the protonation of a single group on the enzyme. Analysis of the data indicated two ionizable groups were involved in the reaction. One functions in the binding of @entity855 and in catalysis while the other participates in the binding of @entity413 . The -log V1 profiles both showed a "hump" attributed to a loss of activity in the pH region 7.5-5.5. Addition of @entity3775 ions to the reaction mixture increased the magnitude of the hump, but the inhibition was abolished by the addition of @entity1792 . Kinetic studies carried out at pH 7 indicated that @entity3775 was a competitive inhibitor with respect to @entity855 and noncompetitive with respect to @entity413 . However, secondary plots of the kinetic data were nonlinear, concave downward, indicating that the inhibition is not of a simple type. Possible explanations for this phenomenon are presented.
