Anionic lipid headgroups as a proton-conducting pathway along the surface of membranes: a hypothesis.

Abstract

Evidence has been gathering from several laboratories that protons in proton-pumping membranes move along or within the bilayer rather than exchange with the bulk phase. These experiments are typically conducted on the natural membrane in vivo or in vitro or on fragments of natural membrane. Anionic lipids are present in all proton-pumping membranes. Model studies on the protonation state of the fatty acids of liposomes containing entrapped water show that the bilayers always contain mixtures of protonated and deprotonated carboxylates. Protonated fatty acids form stable acid-anion pairs with deprotonatedfatty acids through unusually strong hydrogen bonds. Such acid-anion dimers have a single negative charge, which is shared by the four negative oxygens of both headgroups. The two pK values of the resulting dimer will be significantly different from the pK of the monomeric species, so that the dimer will be stable over a wide pH range. It is proposed that anionic lipid headgroups in biological membranes share protons as acid-anion dimers and that anionic lipids thus trap and conduct protons along the headgroup domain of bilayers that contain such anionic lipids-. Protons pumped from the other side ofthe membrane may enter and move within the headgroup sheet because the protonation rate of negatively charged proton acceptors is 5 orders of magnitude faster than that of water. Protons trapped in the acidic headgroup sheet need not leave this region in order to be utilized by a responsive proton-translocating pore (a transport protein using the proton gradient). Experiments suggest the proton concentration in the headgroup domain may vary widely and the anionic lipid headgroup sheet may therefore function as a proton buffer. -Due to the Gouy-Chapman-Stern layer at polyanionic surfaces, anionic lipids will also sequester protons from the bulk solution at low and moderate ionic strengths. At high ionic strength metal cations may replace protons sequestered near the headgroups, but these cations cannot substitute for protons in the "proton-conducting pathway," which is based on hydrogen bonding.Recent experiments in several laboratories have suggested that protons that are translocated by proton-pumping membranes, such as those of mitochondria and halobacteria, need not enter the external bulk water phase in order to be utilized by protontransporting proteins ofthe membrane, such as ATP synthetase. Notable among these experiments are -those of Michel and Oesterhelt (1), who made careful measurements of proton-dependent ATP synthesis and pH changes in and near illuminated, Halobacterium halobium cells. They found, that the pH is not lowered in the bulk aqueous phase as the protons pumped.by the purple patches of bacteriorhodopsin are utilized by ATP synthetase, which is not present in the purple patches but is found at some distance in red patches ofthe same plasma membrane. They concluded that.the protons do not pass through the bulk aqueous phase but move very close to the membrane surface. This system is un...

From the Gla domain to a novel small-molecule detector of apoptosis

“…However, capture of a single proton by a malonate moiety creates an acid-anion dimer, with an overall charge of -1, and with an unusual short and symmetrical internal hydrogen bond [27] (Figure 8A). Vibration of the captured proton leads to distribution of the single remaining anionic charge over the four carboxylate oxygen, thus markedly increasing ionic radius with charge dispersion [27] (Figure 8A), which in turn may act to promote interaction with the membrane interface ( Figure 8B). Acidification, which is one of the hallmarks of apoptosis [19], will obviously enhance this protonation.…”

“…While the exact mechanism remains to be elucidated in future research, one potential mechanism, based on analogy to trans-membrane translocation of other hydrophobic anions [23,24], is outlined in Figure 8B. The di-anion form prevails in aqueous solution due to the acid dissociation constants of alkyl-malonic acid, [26] entailing high water solubility; (2 and 3) capture of a single proton will result in mono-anionic species, with a symmetrical and short internal hydrogen bond, with practical sharing of the hydrogen atom between the two carboxylates, and dispersion of the remaining negative charge over the four carboxyl oxygen atoms (4) as previously described [27]. (B) Proposed mechanism for the translocation of ML-10 through the intact, but depolarized, acidic and PS-exposing plasma membrane of the apoptotic cell, as observed in this study: (1) acidification of the external membrane leaflet due to PS externalization favors proton capture by the malonate moiety, with resultant increased hydrophobicity and charge dispersion, acting to facilitate penetration into the membrane interface; (2) trans-membrane passage facilitated by the depolarization, similar to that of other hydrophobic anions [23][24][25], and potentially assisted by activation of the membrane scramblase system; (3) sequestration in the cytoplasm, facilitated by the cytosolic acidification characteristic of apoptosis.…”

“…At physiological pH, the di-anionic form of malonate predominates, respective of the dissociation constants of the carboxylic groups of alkyl-malonic acid (pKa values of 5.8 and 2.8 in physiological conditions) [26]. However, capture of a single proton by a malonate moiety creates an acid-anion dimer, with an overall charge of -1, and with an unusual short and symmetrical internal hydrogen bond [27] (Figure 8A). Vibration of the captured proton leads to distribution of the single remaining anionic charge over the four carboxylate oxygen, thus markedly increasing ionic radius with charge dispersion [27] (Figure 8A), which in turn may act to promote interaction with the membrane interface ( Figure 8B).…”

Thermostability of model protocell membranes

“…1). We examined four pure fatty acids at pH 8.5, which is approximately the pH at which half of the fatty acid is ionized and half is protonated, and which corresponds to the pH of maximum stability because every protonated carboxylate can act as a hydrogen bond donor to an adjacent ionized carboxylate (4,8). Pure myristoleic acid (C14:1) vesicles released small amounts of DNA after 1 h at 50°C, with increasing amounts released at higher temperatures (Ϸ10-20% released after 1 h at 60-80°C).…”

Effect of Polymer Composition and pH on Membrane Solubilization by Styrene-Maleic Acid Copolymers

“…After ionization of the first acid group, the presence of a negative charge will cause the other acid group to be more resistant toward releasing its proton. In addition, the singly protonated state may be stabilized by an internal hydrogen bridge in the maleic acid group (22).…”