Cardiolipin - Structure

Structure

Cardiolipin (CL) is a kind of diphosphatidylglycerol lipid. Two phosphatidylglycerols connect with a glycerol backbone in the center to form a dimeric structure. So it has four alkyl groups and potentially carries two negative charges. As there are four distinct alkyl chains in cardiolipin, the potential for complexity of this molecule species is enormous. However, in most animal tissues, cardiolipin contains 18-carbon fatty alkyl chains with 2 unsaturated bonds on each of them. It has been proposed that the (18:2)4 acyl chain configuration is an important structural requirement for the high affinity of CL to inner membrane proteins in mammalian mitochondria. However, studies with isolated enzyme preparations indicate that its importance may vary depending on the protein examined.

Since there are two phosphates in the molecule, each of them can catch one proton. Although it has a symmetric structure, Ionizing one phosphate happens at a very different levels of acidity than ionizing both: pK₁ =3 and pK₂ > 7.5. So, under normal physiological conditions (wherein pH is around 7), the molecule may carry only one negative charge. The hydroxyl groups (–OH and –O-) on phosphate would form a stable intramolecular hydrogen bond with the centered glycerol’s hydroxyl group, thus forming a bicyclic resonance structure. This structure traps one proton, which is quite helpful for oxidative phosphorylation.

As the head group forms such compact bicycle structure, the head group area is quite small relative to the big tail region consist of 4 acyl chains. Based on this special structure, the fluorescent mitochondrial indicator, nonyl acridine orange (NAO) was introduced in 1982, and was later found to target mitochondria by binding to CL. NAO has a very large head and small tail structure which can compensate with cardiolipin’s small head large tail structure, and arrange in a highly ordered way. Several studies were published utilizing NAO both as a quantitative mitochondrial indicator and an indicator of CL content in mitochondria. However, it is found that NAO was influenced by membrane potential and/or the spatial arrangement of CL. so it's not proper to use NAO for CL or mitochondria quantitative studies of intact respiring mitochondria. But NAO still represents a simple method of assessing CL content.