Functional binding of cardiolipin to cytochromec oxidase

Bovine cytochromec oxidase usually contains 3–4 mol of tightly bound cardiolipin per cytochromeaa₃ complex. At least two of these cardiolipins are required for full electron transport activity. Without the tightly bound cardiolipin, cytochromec 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 cytochromec oxidase and the other mitochrondrial electron transport complexes and transporters.