Yeast mitochondrial F1-ATPase: the unusual kinetics with excess magnesium ions at low salt concentrations
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1Department of Chemistry, Indiana University, Bloomington 47405.
Abstract:
Studies with the yeast mitochondrial F1-ATPase showed that non-Michaelis-Menten kinetics occurred around pH 8 when there was an excess of magnesium over ATP and a low salt concentration. Increasing the salt concentration activated the enzyme and resulted in Michaelis-Menten kinetics. In contrast, increasing the salt concentration had no effect when ATP was in excess of magnesium. The data were interpreted to mean that magnesium ions inhibit by limiting the amount of free ATP and that the salt anions are able to overcome this effect by replacing the ATP. Mechanistically this is consistent with a ligand substitution reaction in which either free ATP or salt anions are required to displace a product from the enzyme.
Insights
Magnesium ions inhibit yeast mitochondrial F1-ATPase activity at high pH by limiting ATP availability. Increased salt concentration reverses this inhibition, restoring Michaelis-Menten kinetics by facilitating ligand substitution.
Area of Science:
- Biochemistry
- Enzymology
- Mitochondrial Function
Background:
- Yeast mitochondrial F1-ATPase is crucial for cellular energy production.
- Enzyme kinetics can be influenced by substrate concentrations, pH, and ionic strength.
- Understanding regulatory mechanisms of F1-ATPase is key to comprehending energy metabolism.
Purpose of the Study:
- To investigate the non-Michaelis-Menten kinetics of yeast mitochondrial F1-ATPase under specific conditions.
- To elucidate the roles of magnesium, ATP, and salt concentration in modulating enzyme activity.
- To explore the underlying inhibitory and activating mechanisms.
Main Methods:
- Enzyme kinetic assays were performed on purified yeast mitochondrial F1-ATPase.
- Varying concentrations of magnesium, ATP, and salt were used.
- Kinetic parameters were analyzed under different pH conditions, particularly around pH 8.
Main Results:
- Non-Michaelis-Menten kinetics were observed at pH 8 with excess magnesium over ATP and low salt.
- Increased salt concentration activated the enzyme, leading to Michaelis-Menten kinetics.
- Salt's effect was diminished when ATP was in excess of magnesium.
Conclusions:
- Magnesium ions inhibit F1-ATPase by reducing free ATP availability.
- Salt anions counteract magnesium inhibition by displacing ATP, suggesting a ligand substitution mechanism.
- This mechanism involves either free ATP or salt anions displacing a product from the enzyme active site.