Glycerol-based lipids, structure

Fig. 8.1 Structure of glycerol-based lipids. Stick drawing represents carbon backbone of headgroup bold) and fatty acid chains. R represents hydrocarbon chains ranging from 10 to 22 in length with and without double bonds. Headgroups are attached to the ot-3 position of glycerol and the nature chirality is noted for the sn-2 position...

The property of the glycerol-based lipids to form micelles even without saponification is especially pronounced in phospholipids. The basic structural unit of the phospholipid molecule is phosphatidic acid shown in the scheme below. The substituents R and R are long-chained alkane or alkene groups. The phosphate group can be bound in different structures such as choline or serine, molecules which play important roles in metabolism. 

One of the principal constituents of membranes, water, is often ignored. Luckily, because of its fast convergence rates, the structure and dynamics of water at the bilayer interface have been studied extensively. For some time, based on the known slower bilayer permeation rate of hydrophobic cations relative to hydrophobic anions, it has been speculated that there is a membrane dipole potential . The genesis of this positive potential has been the subject of much speculation. Experimental studies have shown it to be largely due to the orientation of ester carbonyl groups, which are common in glycerol based lipids. However, a substantial component could only be explained by a contribution due to oriented water molecules at the bilayer surface. A simulation conducted in such a way as to remove artifacts due to electrostatics showed this clearly to be the case. At the headgroup/water... 

Figure 20.1. General structure of glycerol-based lipids. Three building blocks (X, Y, and Z) are linked to the hydroxyl groups of a glycerol backbone. Potential candidates of the building blocks are listed.

Fig. 2. Structure of glycerophosphate-based lipids. The complete lipid structure shown is l,2-distearoyl-s -glycerol-3-phosphocholine or phosphatidylcholine (PC). Substitution of eholine in the box with the head groups listed below results in the other phospholipid structures. CDP-diacylglycerol has a CMP and phosphatidic acid (PA) has a hydroxyl group in place of choline (not shown). Cardiolipin (CL) is also referred to as diphos-phatidyIglycerol since it contains two PAs joined by a glycerol.

Glycerol (propane-1,2,3-triol) is a very important structural part of the phospholipids, the major components of natural membrane bilayers. Based on this natural scaffold, diesters of glycerol and of related derivatives have been extensively used for the construction of synthetic building blocks carrying vicinal lipid chains (for examples, see Scheme 14). 

Liu D, Qiao D, Li Z et al (2008) Structure-function relationship research of glycerol backbone-based cationic lipids for gene delivery. Chem Biol Drug Des 71 336—344... 

Methanogen lipids have been intensively studied and characterized due to their structures being one of the most remarkable features that distinguish the Archaea from all other organisms (Woese et al., 1990). The polar lipids of methanogens comprise both di- and tetra-ethers of glycerol and isoprenoid alcohols with most compounds being based on the core lipids archaeol (12) or caldarchaeol (13). Minor core lipids are sn-2- and i -3-hydroxyarchaeol and macrocyclic... 

The important structural lipid in biological membrane is glycero phospholipid which contains glycerol, fatty acids, phosphoric acid and a nitrogenous base. Two fatty acids and a phosphate esterified to glycerol... 

The lipid membrane is made up of a variety of fat-derived chemicals, the most important of which are the phospholipids (or lecithins) and ceramides. Phosphatidylcholine (13.7) is a typical phospholipid. The molecular structure is based on glycerol, propan-1,2,3-triol. Two of the alcohol functions are esterified with fatty acids, stearic acid in this case, and the third (one of the primary alcohol functions) with phosphoric... 

Fig. 1. Structures of lipids covalently attached to proteins. Panel A shows proteins that are lipidated on cytoplasmi-cally exposed amino acids, whereas panel B shows lipidated proteins in the extracellular leaflet. (A) iV-myristoyl glycine, palmitate thioester-linked to cysteine, farnesyl, or geranylgeranyl (prenyl) thioether-linked to cysteine. (B) A/-palmitoyl cysteine, cholesterol ester-linked to glycine, and a minimal GPI anchor linked to the to amino acid in a GPI-anchored protein. The GPI structure is shown with a diacylglycerol moiety containing two ester-linked fatty acids. Other GPI anchors are based on ceramide, while yet others have monoacylglycerol, a fatty acid ether-linked to glycerol, and/or a fatty acid ester-linked to inositol.

Glycerolipids are lipids that contain glycerol in which the three hydroxyl groups are substituted in some way. In terms of quantity, these are by far the most abundant lipids in mammals. Somewhat similar in structure, but occurring at concentrations of less than 1% of the glycerolipids, are lipids that contain diols, i.e., ethylene glycol (ethane diol) and 1,2- and 1,3-propanediol. Because of their rarity, lipids based on diols are not discussed further here. 

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