May 4, 1976

Glucose-6-phosphate dehydrogenase from brewers' yeast. The effects of pH and temperature on the steady-state kinetic parameters of the two-chain protein species

Biochemistry
S A Kuby, R N Roy

Abstract

A systematic study has been made of the pH- and temperature-dependency of the steady-state kinetic parameters of the stabilized two-subunit enzyme species of glucose-6-phosphate dehydrogenase, in the absence of superimposed association-dissociation reactions. The Vmax(app) data obtained in several buffers between pH 5 and 10 and at 18-32 degrees C lead to the postulate that at least two sets of protonic equilibria may govern the catalysis (one near pH 5.7 AT 25 DEGREES C and another near pH 9.2); furthermore, two pathways for product formation (i.e., two Vmax's) appear to be required to explain the biphasic nature of the log Vmax(app) vs. pH curves, with Vmax(basic) greater than Vmax(acidic + neutral). Of the several buffers explored, either a uniform degree of interaction or a minimal degree of buffer species interaction could be assessed from the enthalpy changes associated with the derived values for ionization constants attributed to the protonic equilibria in the enzyme-substrates ternary complexes for the case of Tris-acetate-EDTA buffers, at constant ionic strength. With the selection of this buffer at 0.1 (T/2) and at 25 and 32 degrees C, a self-consistent kinetic mechanism has emerged which allows for the random bindin...Continue Reading

Mentioned in this Paper

Buffers
Thermodynamics
Macromolecular Compounds
Biochemical Pathway
Complex (molecular entity)
Tromethamine
Plasma Protein Binding Capacity
Glucosephosphate Dehydrogenase
Glucose-6-phosphate Dehydrogenase Measurement, Quantitative
Calorimetry

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