Cardiolipin, also known as diphosphatidylglycerol, is a unique phospholipid found predominantly in the inner mitochondrial membrane of eukaryotic cells and in the plasma membrane of some prokaryotes. It is composed of four fatty acid chains and two glycerol molecules, making it a dimeric phospholipid.
The chemical composition of cardiolipin can vary depending on the specific organism or tissue in which it is found. In general, the fatty acid chains of cardiolipin are composed of a mixture of saturated and unsaturated fatty acids, with a preference for long-chain fatty acids such as palmitic acid (C16:0) and linoleic acid (C18:2). The glycerol molecules in cardiolipin are linked by two phosphodiester bonds, giving it its characteristic dimeric structure.
The unique structure of cardiolipin allows it to play a critical role in mitochondrial function, including the regulation of membrane protein activity and the maintenance of membrane integrity. Cardiolipin also plays a role in programmed cell death, or apoptosis, by facilitating the release of cytochrome c from the mitochondria. Deficiencies in cardiolipin have been linked to a range of diseases, including Barth syndrome, a rare genetic disorder characterized by cardiomyopathy and skeletal myopathy
Biosynthesis and Metabolism
The biosynthetic pathway to cardiolipin is like that of some other phospholipids in that it passes through the common intermediate phosphatidic acid, which is imported from the endoplasmic reticulum and transported to the inner mitochondrial membrane by specific protein complexes. Then, cytidine diphosphate diacylglycerol is produced mainly by a distinctive synthase in mitochondria (TAM41 in yeast or TAMM41 in animals) as a key intermediate for the biosynthesis of phosphatidylglycerol. Subsequent steps in cardiolipin biosynthesis are unique reactions, which are very different in prokaryotes and eukaryotes.
1. In prokaryotes such as bacteria, cardiolipin (diphosphatidylglycerol) synthase (CLS) catalyses a transfer of the phosphatidyl moiety of one phosphatidylglycerol to the free 3'‑hydroxyl group of another, with the elimination of one molecule of glycerol, via the action of one of two structurally related enzymes (depending on species), which are part of the phospholipase D superfamily. In effect, transphosphatidylation occurs with one phosphatidylglycerol acting as a donor and the other an acceptor of a phosphatidyl moiety. The reaction is energy independent, and the enzymes can operate in reverse under some physiological conditions to convert cardiolipin back to phosphatidylglycerol, so the biosynthesis of cardiolipin is regulated via that of phosphatidylglycerol.
There are in fact three distinct cardiolipin synthases (ClsA/B/C) in the bacterium E. coli, with ClsA as the primary source during exponential growth. A second minor mechanism has been found in this organism in which cardiolipin is formed by condensation of phosphatidylglycerol and phosphatidylethanolamine with elimination of ethanolamine via the action of ClsB/C. There is a very different bifunctional cardiolipin/phosphatidylethanolamine synthase in Xanthomonas campestris, which is related to the phospholipase D superfamily and can synthesise cardiolipin from phosphatidylglycerol and CDP-diacylglycerol, but it also catalyses ethanolamine-dependent phosphatidylethanolamine formation. The Archaea have their own unique cardiolipin synthase, which utilizes archaetidylglycerol, a stereochemically distinct diether analogue of phosphatidylglycerol, as precursor
2. With eukaryotes (yeasts, plants and animals), the first committed step in the biosynthesis of cardiolipin is the formation of phosphatidylglycerolphosphate, a key intermediate in the biosynthesis of phosphatidylglycerol (as described in the web page on phosphatidylglycerol). The cardiolipin (or diphosphatidylglycerol) synthase, a phosphatidyl transferase, then links phosphatidylglycerol to diacylglycerol phosphate from the activated phosphatidyl moiety cytidine diphosphate diacylglycerol, with elimination of cytidine monophosphate (CMP). The reaction requires a source of energy and enzymes from all species examined in detail need certain divalent cations (Mg2+, Mn2+ or Co2+) together with a high pH (8 to 9). In rat liver and in higher plants, the cardiolipin synthase resides in the inner mitochondrial membrane, while in yeast it is part of a large protein complex in mitochondria. The enzymes involved in the synthesis of the precursors and of cardiolipin per se are located on the inner leaflet (matrix side) of the inner membrane, presumably close to each other and perhaps part of a single protein complex in yeast at least.
As eukaryotic cardiolipin synthase is a mitochondrial enzyme and mitochondria are believed to be phylogenetic derivatives of ancient prokaryotes, it may appear strange that there has been such a change in mechanism, but protein domain analyses indicate that both pathways evolved convergently. Surprisingly, Streptomyces coelicolor and other Actinomycetes use the eukaryote biosynthetic system, while the protozoan parasite, Trypanosoma brucei, utilizes the prokaryotic pathway. It is noteworthy that a key enzyme involved in the biosynthesis of phosphatidylglycerol and cytidine diphosphate diacylglycerol in mitochondria, i.e., a cytidine diphosphate diacylglycerol synthase ('Tam41' in yeast, 'Tamm41' in mammals), is structurally distinct from the corresponding enzyme in the endoplasmic reticulum.
Recommended Reading
- Ball, W.B., Neff, J.K. and Gohil, V.M. The role of nonbilayer phospholipids in mitochondrial structure and function. FEBS Letts, 592, 1273-1290 (2018); DOI.
- Bautista, J.S., Falabella, M., Flannery, P.J., Hanna, M.G., Heales, S.J.R., Pope, S.A.S. and Pitceathly, R.D.S. Advances in methods to analyse cardiolipin and their clinical applications. Trends Anal. Chem., 157, 116808 (2022); DOI.
- Christie, W.W. and Han, X. Lipid Analysis - Isolation, Separation, Identification and Lipidomic Analysis (4th edition), 446 pages (Oily Press, Woodhead Publishing and now Elsevier) (2010) - see Science Direct.
- Dowhan, W. and Bogdanov, M. Eugene P. Kennedy's legacy: defining bacterial phospholipid pathways and function. Front. Mol. Biosci., 8, 666203 (2021); DOI.
- Duncan, A.L. Monolysocardiolipin (MLCL) interactions with mitochondrial membrane proteins. Biochem. Soc. Trans., 48, 993-1004 (2020); DOI.
- Fox, C.A. and Ryan, R.O. Studies of the cardiolipin interactome. Prog. Lipid Res., 88, 101195 (2022); DOI.
- Jiang, Z.T., Shen, T., Huynh, H., Fang, X., Han, Z. and Ouyang, K.F. Cardiolipin regulates mitochondrial ultrastructure and function in mammalian cells. Genes, 13, 1889 (2022); DOI.
- Luévano-Martínez, L.A. and Duncan, A.L. Origin and diversification of the cardiolipin biosynthetic pathway in the Eukarya domain. Biochem. Soc. Trans., 48, 1035-1046 (2020); DOI.
- Maguire, J.J., Tyurina, Y.Y., Mohammadyani, D., Kapralov, A.A., Anthonymuthu, T.S., Qu, F., Amoscato, A.A., Sparvero, L.J., Tyurin, V.A., Planas-Iglesias, J., He, R.-R., Klein-Seetharaman, J., Bayir, H. and Kagan, V.E. Known unknowns of cardiolipin signaling: The best is yet to come. Biochim. Biophys. Acta, Lipids, 1862, 8-24 (2017); DOI - and other articles in this special journal issue on "Lipids of Mitochondria".
