Polar Glycerolipids and Membrane Lipid Rafts

Abstract

Current understanding of the structure and functioning of biomembranes is impossible without determining the mechanism of formation of membrane lipid rafts. The formation of liquid-ordered and disordered phases (Lo and Ld) and lipid rafts in membranes and their simplified models is discussed. A new consideration of the processes of formation of lipid phases Lo and Ld and lipid rafts is proposed, taking into account the division of each of the glycerophospholipids into several groups. Generally accepted three-component schemes for modeling the membrane structure are critically considered. A four-component scheme is proposed, which is designed to more accurately assume the composition of lipids in the resulting Lo and Ld phases. The role of the polar head groups of phospholipids and, in particular, phosphatidylethanolamine is considered. The structure of membrane rafts and the possible absence of a clear boundary between the Lo and Ld phases are discussed.

Keywords: computer modeling; glycerophospholipid groups with different unsaturation; lateral membrane heterogeneity; lipid phases Lo and Ld; lipid-lipid H-bonds; mechanisms of nanodomain formation; model biomembranes; molecular dynamics; physicochemical properties of lipid bilayers.

Figure 1

Models of the possible organization of lipid rafts in eukaryotic cells. Some of the rafts in 1–3 in the membranes can take place even without the proteins included in them. (A)—location of the raft in one of the membrane monomers. (B)—location of the raft including the GPU in one of the membrane monomers. (C)—a raft occupying part of both monolayers of the membrane. (D)—possible intermediate zones between the phases of Ld and Lo. (E)—α- and β-surfaces of Chol, reversed by SM and POPC, respectively [28]. (F)–location of receptor, effector and biotopic proteins in the membrane. Ld—liquid disordered lipid phase, Lo—liquid ordered lipid phase, DOPC—dioleoyl phosphatidylcholines, DPPC—dipalmitoyl phosphatidylcholines, SM—sphingomyelin, POPC—palmitoleoyl phosphatidylcholines, α-spiral—a protein molecule with an α-spiral structure, Cer—silversides, Chol—cholesterol, α- and β-surface—two surfaces of the Chol molecule.

Figure 2

Chemical forms of certain types of phospholipids and sphingophospholipids involved in the construction of lipid rafts. (A)—phosphatidylcholine (PC), (B)—sphingomyelin (SM), (C)—phosphatidylethanolamine (PE), (D)—phosphatidylserine (PS).

Figure 3

Chemical formulas of sterols involved in the construction of rafts, their synthesis precursors, and products of further transformation. (A)—lanosterol, (B)—Chol, (C)—esters of Chol and FA, (D)—β-sitosterol, (E)—ergosterol.

Figure 4

Chemical formulas of sphingosine, ceramides, and certain types of sphingoglycolipids involved in the construction of lipid rafts. (A)—sphingosine, (B)—ceramides, (C)—cerebrosides, (D)—dihexosides of ceramides.