Destabilization of the Quaternary Structure of Bovine Heart Cytochrome c Oxidase upon Removal of Tightly Bound Cardiolipin.
The quaternary structural stability of cardiolipin-containing (CcO(CL+)) versus CL-free cytochrome c oxidase (CcO(CL-)) was compared using structural perturbants as probes. Exposure to increasing concentrations of urea or guanidinium chloride causes sequential dissociation of five subunits from CcO(CL+) in the order VIa and VIb, followed by III and VIIa, and ultimately Vb. Removal of CL from CcO destabilizes the association of each of these five subunits with the core of CcO. Subunits VIa and VIb spontaneously dissociate from CcO(CL-) even in the absence of denaturant and are no longer present after purification of the CL-free 11-subunit complex by ion exchange chromatography. The other 11 subunits remain associated in a partially active complex, but the association of subunits III, VIIa, and Vb is weakened; i.e., the midpoints for the subunit dissociation curves are each shifted to a lower perturbant concentration (lower by 1.1-1.7 M urea; lower by 0.3-0.4 M GdmCl). This corresponds to a decrease of ∼9 kJ in the Gibbs free association energy for each of these subunits when CL is removed from CcO. With either CcO(CL+) or CcO(CL-), loss of enzymatic activity occurs coincident with dissociation of subunits III and VIIa. The loss of activity is irreversible, and reactivation of CcO(CL-) by exogenous CL occurs only if both subunits remain associated with the core of CcO. Inclusion of sulfate anions stabilizes the association of VIIa more than III, resulting in a slight separation of the urea-induced dissociation curves. In this case, activity loss correlates much better with dissociation of subunit VIIa than III. We conclude that (1) bound cardiolipin is an important stabilizing factor in the quaternary structure of CcO and (2) association of subunit VIIa (possibly together with subunit III) is critical for functional CL binding and full electron-transfer activity of CcO.
Department of Biochemistry, The University of Texas Health Science Center , San Antonio, Texas 78229-3900, United States.
Erik
E
Sedlák
Department of Biochemistry, The University of Texas Health Science Center , San Antonio, Texas 78229-3900, United States.
Neal C
NC
Robinson
D000818:Animals; D002308:Cardiolipins / Q000737:chemistry*; D002417:Cattle; D003576:Electron Transport Complex IV / Q000737:chemistry*; D019791:Guanidine / Q000737:chemistry; D008958:Models, Molecular; D009206:Myocardium / Q000201:enzymology*; D011485:Protein Binding; D011489:Protein Denaturation; D020836:Protein Structure, Quaternary; D021122:Protein Subunits / Q000737:chemistry; D013816:Thermodynamics; D014508:Urea / Q000737:chemistry
D002308:Cardiolipins; D021122:Protein Subunits; D014508:Urea; D003576:Electron Transport Complex IV; D019791:Guanidine