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Association of chicken mitochondrial creatine kinase with the inner mitochondrial membrane
Abstract:
The stoichiometry and dissociation constant for the binding of homogeneous chicken heart mitochondrial creatine kinase (MiMi-CK) to mitoplasts was examined under a variety of conditions. Salts and substrates release MiMi-CK from mitoplasts in a manner that suggests an ionic interaction. The binding of MiMi-CK to mitoplasts is competitively inhibited by Adriamycin, suggesting that they compete for the same binding site. Fluorescence measurements also show that Adriamycin binds to MiMi-CK so that the effect of Adriamycin on the binding of MiMi-CK to mitoplasts is not simple. Titrating mitoplasts with homogeneous MiMi-CK at different pH values shows a pH-dependent equilibrium involving a group(s) on either the membrane or the enzyme with a pKa = 6. Extrapolating these titrations to infinite MiMi-CK concentration gives 14.6 IU bound/nmol cytochrome aa3 corresponding to 1.12 mol MiMi-CK/mol cytochrome aa3. Chicken heart mitochondria contain, after isolation, 2.86 +/- 0.42 IU/nmol cytochrome aa3. Titrating respiring mitoplasts with carboxyatractyloside gives at saturation 3.3 mol ADP/ATP translocase/mol cytochrome aa3. Therefore, chicken heart mitoplasts can maximally bind about 1 mol of MiMi-CK per 3 mol translocase; in normal chicken heart mitochondria about 1 mol of MiMi-CK is present per 13 mol translocase.
Insights
Chicken heart mitochondrial creatine kinase (MiMi-CK) binds to mitoplasts via ionic interactions. Adriamycin competes for the same binding site, and binding is pH-dependent, with a pKa of 6.
Area of Science:
- Biochemistry
- Cell Biology
- Enzymology
Background:
- Mitochondrial creatine kinase (MiMi-CK) plays a crucial role in cellular energy buffering.
- Understanding the binding of MiMi-CK to the inner mitochondrial membrane (mitoplasts) is key to elucidating its function.
- Previous studies have suggested interactions between MiMi-CK and mitochondrial components.
Purpose of the Study:
- To determine the stoichiometry and dissociation constant for MiMi-CK binding to chicken heart mitoplasts.
- To investigate the nature of the interaction between MiMi-CK and mitoplasts.
- To explore the influence of pH, substrates, salts, and Adriamycin on this binding.
Main Methods:
- Isolation of homogeneous chicken heart mitochondrial creatine kinase (MiMi-CK).
- Binding assays using mitoplasts under varying conditions (pH, salts, substrates).
- Competitive inhibition studies with Adriamycin.
- Fluorescence spectroscopy.
- Enzyme activity measurements (IU) and quantification of cytochrome aa3 and ADP/ATP translocase.
Main Results:
- MiMi-CK binding to mitoplasts is ionic and competitively inhibited by Adriamycin, indicating a shared binding site.
- Binding is pH-dependent with a pKa of 6, suggesting involvement of titratable groups.
- Stoichiometry indicates a maximal binding capacity of approximately 1 mol MiMi-CK per 3 mol translocase on chicken heart mitoplasts.
- Isolated chicken heart mitochondria contain MiMi-CK in a ratio of approximately 1 mol per 13 mol translocase.
Conclusions:
- Chicken heart mitochondrial creatine kinase binds to mitoplasts through ionic interactions at a site also recognized by Adriamycin.
- The binding equilibrium is sensitive to pH, with a pKa of 6.
- The observed stoichiometry suggests that mitoplasts have a higher capacity for MiMi-CK binding than is typically occupied in native mitochondria, implying potential for dynamic regulation.