An active cytochrome c oxidase that has no tightly bound cardiolipin

Role of cardiolipin in stability of integral membrane proteins

Cardiolipin (CL) is a unique phospholipid with a dimeric structure having four acyl chains and two phosphate groups found almost exclusively in certain membranes of bacteria and of mitochondria of eukaryotes. CL interacts with numerous proteins and has been implicated in function and stabilization of several integral membrane proteins (IMPs). While both functional and stabilization roles of CL in IMPs has been generally acknowledged, there are, in fact, only limited number of quantitative analysis that support this function of CL. This is likely caused by relatively complex determination of parameters characterizing stability of IMPs and particularly intricate assessment of role of specific phospholipids such as CL in IMPs stability. This review aims to summarize quantitative findings regarding stabilization role of CL in IMPs reported up to now.

How lipids affect the activities of integral membrane proteins

The activities of integral membrane proteins are often affected by the structures of the lipid molecules that surround them in the membrane. One important parameter is the hydrophobic thickness of the lipid bilayer, defined by the lengths of the lipid fatty acyl chains. Membrane proteins are not rigid entities, and deform to ensure good hydrophobic matching to the surrounding lipid bilayer. The structure of the lipid headgroup region is likely to be important in defining the structures of those parts of a membrane protein that are located in the lipid headgroup region. A number of examples are given where the conformation of the headgroup-embedded region of a membrane protein changes during the reaction cycle of the protein; activities of such proteins might be expected to be particularly sensitive to lipid headgroup structure. Differences in hydrogen bonding potential and hydration between the headgroups of phosphatidycholines and phosphatidylethanolamines could be important factors in determining the effects of these lipids on protein activities, as well as any effects related to the tendency of the phosphatidylethanolamines to form a curved, hexagonal H(II) phase. Effects of lipid structure on protein aggregation and helix-helix interactions are also discussed, as well as the effects of charged lipids on ion concentrations close to the surface of the bilayer. Interpretations of lipid effects in terms of changes in protein volume, lipid free volume, and curvature frustration are also described. Finally, the role of non-annular, or 'co-factor' lipids, tightly bound to membrane proteins, is described.

Lipid-protein interactions in biological membranes: a structural perspective

Lipid molecules bound to membrane proteins are resolved in some high-resolution structures of membrane proteins. An analysis of these structures provides a framework within which to analyse the nature of lipid-protein interactions within membranes. Membrane proteins are surrounded by a shell or annulus of lipid molecules, equivalent to the solvent layer surrounding a water-soluble protein. The lipid bilayer extends right up to the membrane protein, with a uniform thickness around the protein. The surface of a membrane protein contains many shallow grooves and protrusions to which the fatty acyl chains of the surrounding lipids conform to provide tight packing into the membrane. An individual lipid molecule will remain in the annular shell around a protein for only a short period of time. Binding to the annular shell shows relatively little structural specificity. As well as the annular lipid, there is evidence for other lipid molecules bound between the transmembrane alpha-helices of the protein; these lipids are referred to as non-annular lipids. The average thickness of the hydrophobic domain of a membrane protein is about 29 A, with a few proteins having significantly smaller or greater thicknesses than the average. Hydrophobic mismatch between a membrane protein and the surrounding lipid bilayer generally leads to only small changes in membrane thickness. Possible adaptations in the protein to minimise mismatch include tilting of the helices and rotation of side chains at the ends of the helices. Packing of transmembrane alpha-helices is dependent on the chain length of the surrounding phospholipids. The function of membrane proteins is dependent on the thickness of the surrounding lipid bilayer, sometimes on the presence of specific, usually anionic, phospholipids, and sometimes on the phase of the phospholipid.

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 (Kd,app = 1 microM for cardiolipin). These binding constants agree reasonably well with direct measurement of the binding using [14C]-acetyl-cardiolipin (Kd < 0.1 microM) 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 mitochondrial electron transport complexes and transporters.

Cardiolipin-depleted bovine heart cytochrome c oxidase: binding stoichiometry and affinity for cardiolipin derivatives

Detergent-solubilized bovine heart cytochrome c oxidase requires 2 mol of tightly bound cardiolipin (CL) per mole of monomeric complex for functional activity. Four lines of evidence support this conclusion: (1) Phospholipid depletion shows that two tightly bound CL's must remain associated with cytochrome c oxidase in order to maintain full electron transport activity. (2) Removal of the two tightly bound CL's correlates with decreased activity that is restored by reassociation of 2 mol of exogenous CL. (3) CL-depleted cytochrome c oxidase has two high-affinity binding sites for 2-[14C]acetylcardiolipin (AcCL), Kd,app less than 0.1 microM, that are not present in enzyme containing endogenous CL. An additional 2-3 lower affinity AcCL binding sites, Kd,app = 4 microM, are present in the CL-depleted complex, but these sites are also present in enzyme containing endogenous CL. (4) CL, monolysocardiolipin (MLCL), and dilysocardiolipin (DLCL) compete for AcCL binding with approximately the same relative affinities as those measured by the restoration of electron transport activity (MLCL competes much better than DLCL). However, MLCL and DLCL are only 60% and 15% as effective as CL in restoring maximum activity when they are bound to the high-affinity sites. The binding specificity of CL, MLCL, DLCL, and some of their acylated derivatives indicates that the apolar tails are most important for binding, not the polar head group. The presence or absence of hydroxyl groups in CL, MLCL, or DLCL also has little effect upon binding affinities. Binding specificity clearly favors CL since phosphatidylglycerol, phosphatidic acid, and phosphatidylcholine each have very low affinity for the CL binding sites (Kd,app greater than 20 microM). We, therefore, conclude that restoration of activity to CL-depleted cytochrome c oxidase is highly specific and requires the reassociation of CL, or structurally similar compounds, with two high-affinity binding sites.

Calorimetric studies of cytochrome oxidase-phospholipid interactions

Thermotropic phase transitions in phospholipid vesicles reconstituted with mitochondrial cytochrome oxidase (EC 1.9.3.1) were studied using differential scanning calorimetry. Both dimyristoylphosphatidylcholine (DMPC) and mixtures of DMPC and cardiolipin were used at different lipid-to-protein ratios. The incorporated protein reduces the energy absorbed during phase transitions of DMPC vesicles, and causes a small decrease in the transition temperature (tm). delta H depends on the amount of protein in the vesicles. This dependence indicates that about 72 DMPC molecules are influenced per cytochrome alpha alpha 3 monomer. The transition parameters remain unaffected by changes in ionic strength or by reduction of the enzyme. Incorporation of cytochrome oxidase depleted of subunit III into DMPC liposomes resulted in a larger decrease of tm, but the amount of perturbed phospholipids remains similar to that in the case of the intact enzyme. Incorporation of cytochrome oxidase into DMPC/cardiolipin vesicles counteracts the effect of cardiolipin in decreasing the enthalpy of the DMPC transition. Thus cytochrome oxidase segregates the phospholipids by attracting cardiolipin from the bulk lipid. Cytochrome c does not significantly affect this apparent cardiolipin 'shell' around membranous cytochrome oxidase.

The phospholipids associated with cytochrome c oxidase isolated from beef, dogfish and cod heart

Beef (Bos taurus), dogfish (Squalus acanthias) and cod (Gadus morhua) heart submitochondrial particles and cytochrome c oxidase (EC 1.9.3.1) were prepared. The head groups and side chains of the phospholipids associated with these samples were analysed quantitatively. The fish phospholipids contained a higher proportion of long chain poly-unsaturated fatty acids than was found in the beef samples. The enzyme fraction showed no head group or fatty acyl chain preference when compared with the composition of the whole tissue, implying no special lipid requirement for enzyme activity other than membrane fluidity. No cardiolipin was associated with the dogfish oxidase. The cod oxidase was inseparable from a CO-binding "b-type" cytochrome.