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Ionization constants pKa of cardiolipin.

ABSTRACT: Cardiolipin is a phospholipid found in the inner mitochondrial membrane and in bacteria, and it is associated with many physiological functions. Cardiolipin has a dimeric structure consisting of two phosphatidyl residues connected by a glycerol bridge and four acyl chains, and therefore it can carry two negative charges. The pKa values of the phosphate groups have previously been reported to differ widely with pKa1 = 2.8 and pKa2 = 7.5-9.5. Still, there are several examples of experimental observations from cardiolipin-containing systems that do not fit with this dissociation behavior. Therefore, we have carried out pH-titration and titration calorimetric experiments on two synthetic cardiolipins, 1,1',2,2'-tetradecanoyl cardiolipin, CL (C14:0), and 1,1',2,2'-tetraoctadecenoyl cardiolipin, CL (C18:1). Our results show that both behave as strong dibasic acids with pKa1 about the same as the first pKa of phosphoric acid, 2.15, and pKa2 about one unit larger. The characterization of the acidic properties of cardiolipin is crucial for the understanding of the molecular organization in self-assembled systems that contain cardiolipin, and for their biological function.

SUBMITTER: Olofsson G

PROVIDER: S-EPMC3772843 | biostudies-literature | 2013

REPOSITORIES: biostudies-literature

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Ionization constants pKa of cardiolipin.

Olofsson Gerd G Sparr Emma E

PloS one 20130913 9

Cardiolipin is a phospholipid found in the inner mitochondrial membrane and in bacteria, and it is associated with many physiological functions. Cardiolipin has a dimeric structure consisting of two phosphatidyl residues connected by a glycerol bridge and four acyl chains, and therefore it can carry two negative charges. The pKa values of the phosphate groups have previously been reported to differ widely with pKa1 = 2.8 and pKa2 = 7.5-9.5. Still, there are several examples of experimental obser ...[more]

PMID: 24058458

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Project description:The anionic phospholipid cardiolipin has an unusual dimeric structure with a two-phosphate headgroup and four acyl chains. Cardiolipin is present in energy-transducing membranes that maintain electrochemical gradients, including most bacterial plasma membranes and the mitochondrial inner membrane, where it mediates respiratory complex assembly and activation, among many other roles. Dysfunctional biogenesis of cardiolipin is implicated in the pathogenesis of several diseases including Barth syndrome. Because cardiolipin is a dominant anionic lipid in energy-conserving membranes, its headgroup is a major contributor to surface charge density and the bilayer electrostatic profile. However, the proton dissociation behavior of its headgroup remains controversial. In one model, the pKa values of the phosphates differ by several units and the headgroup exists as a monoanion at physiological pH. In another model, both phosphates ionize as strong acids with low pKa values and the headgroup exists in dianionic form at physiological pH. Using independent electrokinetic and spectroscopic approaches, coupled with analysis using Gouy-Chapman-Stern formalism, we have analyzed the ionization properties of cardiolipin within biologically relevant lipid bilayer model systems. We show that both phosphates of the cardiolipin headgroup show strong ionization behavior with low pKa values. Moreover, cardiolipin variants lacking structural features proposed to be required to maintain disparate pKa values--namely the secondary hydroxyl on the central glycerol or a full complement of four acyl chains--were shown to have ionization behavior identical to intact cardiolipin. Hence, these results indicate that within the physiological pH range, the cardiolipin headgroup is fully ionized as a dianion. We discuss the implications of these results with respect to the role of cardiolipin in defining membrane surface potential, activating respiratory complexes, and modulating membrane curvature.

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Ionization constants pKa of cardiolipin.

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Ionization constants pKa of cardiolipin.

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