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Hexokinase III from Rana catesbeiana
1Department of Biochemistry and Biophysics, Iowa State University, Ames 50011.
Summary
Hexokinase III from bullfrog liver shows pH-dependent glucose inhibition and Michaelis-Menten kinetics for ATP. Its activity increases with pH, and it cannot use galactose as a substrate.
Area of Science:
- Biochemistry
- Enzymology
Background:
- Hexokinases are crucial enzymes in glucose metabolism.
- Understanding hexokinase III from ectothermic vertebrates like the American bullfrog (Rana catesbeiana) provides insights into enzyme adaptation.
Purpose of the Study:
- To partially purify and characterize Hexokinase III from American bullfrog liver.
- To investigate the kinetic properties and substrate specificity of bullfrog Hexokinase III.
Main Methods:
- Partial purification of Hexokinase III using DEAE-cellulose column chromatography.
- Enzyme activity assays were performed at various pH levels and substrate concentrations.
Main Results:
- Hexokinase III exhibited significant glucose inhibition at lower pH values (5.9-7.5), with minimal inhibition at pH 8.7.
- Enzyme kinetics followed Michaelis-Menten behavior with respect to ATP, showing an apparent Michaelis constant of 0.23 mM at pH 7.5.
- Maximum reaction velocity increased with rising pH, and galactose was not utilized as a substrate.
Conclusions:
- Bullfrog liver Hexokinase III displays unique regulatory properties, including pH-dependent glucose inhibition.
- The enzyme's kinetic characteristics suggest specific roles in amphibian carbohydrate metabolism.