Functional significance of the lipid-protein interface in photosynthetic membranes

Abstract

The functional significance of the lipid-protein interface in photosynthetic membranes, mainly in thylakoids, is reviewed with emphasis on membrane structure and dynamics. The lipid-protein interface is identified primarily by the restricted molecular dynamics of its lipids as compared with the dynamics in the bulk lipid phase of the membrane. In a broad sense, lipid-protein interfaces comprise solvation shell lipids that are weakly associated with the hydrophobic surface of transmembrane proteins but also include lipids that are strongly and specifically bound to membrane proteins or protein assemblies. The relation between protein-associated lipids and the overall fluidity of the thylakoid membrane is discussed. Spin label electron paramagnetic resonance spectroscopy has been identified as the technique of choice to characterize the protein solvation shell in its highly dynamic nature; biochemical and direct structural methods have revealed an increasing number of protein-bound lipids. The structural and functional roles of these protein-bound lipids are mustered, but in most cases they remain to be determined. As suggested by recent data, the interaction of the non-bilayer-forming lipid, monogalactosyldyacilglycerol (MGDG), with the main light-harvesting chlorophyll a/b-binding protein complexes of photosystem-II (LHCII), the most abundant lipid and membrane protein components on earth, play multiple structural and functional roles in developing and mature thylakoid membranes. A brief outlook to future directions concludes this review.

Biomimetic monolayer films of monogalactosyldiacylglycerol incorporating ubiquinone

“…These differences have been explained by the presence of unsaturations in the acyl chains of polar lipids which prevented them from close packing at the interface. It is known that in natural membranes variations in the saturation grade of the acyl chains and the 18 ratio MGDG/DGDG are used to maintain membrane fluidity to adapt to environment changes [2]. by Roche et al [48], being this discrepancy probably caused by the different subphase pH value, but the collapse pressure (11 mN·m -1 ) coincides with that one reported by Roche et al…”

“…The thylakoid membrane also contains several proteins and molecules, having the main function of transporting and releasing electrons to produce chemical energy for dark reactions. MGDG and DGDG contents affect the function of thylakoid membrane proteins [2], especially the light harvesting complexes of the photosystem. One of the main electron and proton shuttle molecules in green plants is plastoquinone (PQ) [3,4] which is mainly found in chloroplasts.…”

Monogalactosyldiacylglycerol and digalactosyldiacylglycerol role, physical states, applications and biomimetic monolayer films

“…These last results confirm that the presence of hydrated ions of the electrolyte also produce an expansion of the monolayer compared with the pure water subphase, whereas the presence of electrolytes does not affect the collapse pressure [13]. 16 There is strong consensus in the collapse pressure for unsaturated DGDG regardless the conditions employed. In water subphase and similar temperature the collapse pressure was observed at 46-47 mN·m -1 with 0.6-1.3 unsaturations per DGDG molecule [31,45,49].…”

“…The cone shape of MGDG, with a galactose at the tip and the two fatty acyl chains oriented towards the base of the cone, leads to hexagonal type-II (HII) regions, whereas the cylindrically shaped DGDG, SQDG and PG forms convex structures in aqueous environments [12,15]. Thylakoid membranes of photosynthetic organisms increases the ratio of non-bilayer/bilayer forming lipids and the unsaturation index to adapt the fluidity and the related functions to changes in external temperature or to an increase in the protein:lipid ratio that can occur during greening [16][17][18]. 6 The lipids present in thylakoid membranes can be classified in three groups depending on their function in the membrane: i) lipids that forms the bilayer matrix; ii) lipids that surround the protein-cofactor supercomplexes and interact with the outer surface of the complexes, and iii)…”

“…Gounaris et al [32] found that 0.5 double bonds per molecule of MGDG induce the conversion of lamellar phase to HII structure and increasing the unsaturation index, the transition temperature of the lamellar phase to HII is decreased [16].…”