Functional binding of cardiolipin to cytochrome c oxidase

Bovine cytochrome c oxidase usually contains 3-4 mol of tightly bound cardiolipin per cytochrome aa₃ complex. At least two of these cardiolipins are required for full electron transport activity. Without the tightly bound cardiolipin, cytochrome c oxidase has only 40-50% of its original activity when assayed in detergents that support activity, e.g., dodecyl maltoside. By measuring the restoration of electron transport activity, functional binding constants for cardiolipin and a number of cardiolipin analogues have been evaluated (K_d,app=1 μM for cardiolipin). These binding constants agree reasonably well with direct measurement of the binding using [¹⁴C]-acetyl-cardiolipin (K_d<0.1 μM) when the enzyme is solubilized with Triton X-100. These data are discussed in relationship to the wealth of data that is known about the association of cardiolipin with cytochrome c oxidase and the other mitochrondrial electron transport complexes and transporters.