What is a Glycerophospholipid?

Glycerophospholipid is the most abundantly occuring phospholipids found in the biological membranes. Lipids include a group of organic compounds like fats, hormones, oils, waxes, vitamins etc. They are non-polar molecules and are insoluble in water. Lipids play an important role in biological systems. They are the building blocks of our cell membranes, store energy and are involved in signaling.

Lipids are classified into different classes. One of its classifications is glycerophospholipid. Glycerophospholipids are plentiful in nature and help in metabolism and signaling of various biological pathways. They are important components of lipid bilayers of cells. We can find glycerophospholipids in physiologically active compounds. They make up about a third of the organic phosphorus in plants. Phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol are the most common glycerophospholipids in mammalian cells. Glycerophospholipids mixtures which often contain phosphatidylcholine are commonly referred to as lecithin. It can be used as emulsifiers and surface-active agents. They act as detergents that can lower surface tension of water and will stabilize the dispersion of hydrophobic compounds in aqueous solutions.

Structure

Phosphatidic acids, which are formed by the esterification of two of the hydroxyl groups of glycerol by fatty acids and the esterification of the third hydroxyl group by phosphoric acid, are the source of glycerophospholipids. Fatty acids are joined to the carbons of glycerol, the molecule's backbone, by an ester oxygen. The third carbon is esterified with phosphate and the phosphate group may contain some substituents.

Since glycerophospholipids contain o-acyl group, one o-alkyl group, and o-alk-1-enyl group, it can be considered as a derivative of glycerophosphoric acid. Nonpolar fatty acids are bound to glycerol hence they are hydrophobic in nature. Since phosphate groups attached to glycerol are polar, they are hydrophilic. Thus, glycerophospholipids are amphipathic.  They contain phosphatide in a variety of forms and are used as an intermediate in the manufacture of phosphoglycerates. Examples are:

Plasmalogens

It is an ether phospholipid in which an ether linkage is present in one of the carbon atoms of glycerol in the structure. This linkage is what makes it different from other phospholipids. The third carbon is connected to an ethanolamine or choline via a phosphate ester. The major components of muscles and nerve membranes are plasmalogens. 

Phosphatidylserine

The structure consists of a serine group. It is important in cell cycle signaling, particularly when it comes to apoptosis. Apoptotic mimicry is a primary mechanism for viruses to invade cells. 

Phosphatidylinositol

In this the third carbon of glycerol (backbone) is attached to a phosphate group containing inositol polar head group. Stearic acid and arachidonic acid are the two fatty acids involved in its formation. Its isomer functions as a sensory receptor in the taste function.

Phosphatidylcholine

In addition to fatty acids, it contains a choline head group and glycerophosphoric acid. Here one fatty acid is saturated and the other one is unsaturated. Egg yolk and soybeans are easily available sources of phosphatidylcholine. They are an important component of biological membranes and pulmonary surfactant.

Uses

Membranes

The major function of glycerophospholipid is to serve as a structural component of cell membrane. Since they are amphipathic, the ionic parts are pointing towards the inner outer parts of the cell membrane and the hydrophobic tail forms a hydrophobic center. This kind of arrangement results in a barrier between the cell's interior and its surroundings.

In addition to this they are also involved in signaling and transport and act as a source for the precursors for prostaglandins and other leukotrienes. The ability to function as transporters is aided by their position as storage centers for secondary messengers in the membrane. 

Brain

Several types of glycerophospholipids are found in neural membranes and they function at different rates depending on their composition and location in cell membranes. Stability, permeability and fluidity can be given to the neural membrane by three types of glycerophospholipids. They are 1-alkyl-2-acyl glycerophospholipid, 1,2-diacyl glycerophospholipid and plasmalogen.  Many membrane characteristics, including the development of lateral domains rich polyunsaturated fatty acids are influenced by the length of glycerophospholipid acyl chain and the degree of saturation. Phospholipases degrade glycerophospholipids through receptors, resulting in the production of second messengers such as prostaglandins, eicosanoids platelet activating factor and diacylglycerol. As a result, second messengers are stored in the phospholipids of neural membranes. They also play a role in apoptosis, transporter activity regulation and membrane-bound enzymes. Neurological conditions have been linked to significant changes in the glycerophospholipid composition of the neural membrane. Membrane fluidity and permeability are affected as a result of these changes. The neurodegeneration seen in neurological disorders may be caused by these mechanisms, as well as accumulation of lipid peroxides and a faulty energy metabolism.

Metabolism

Glycerophospholipid metabolism differs between eukaryotes, tumor cells and prokaryotes. Glycerophospholipid phosphatidic acid and polar head groups are all synthesized in prokaryotes. In eukaryotes phosphatidic acid is synthesized in two ways, one leading to phosphatidylcholine and other to phosphatidylethanolamine. They are usually broken down into several stages, each with its own set of intermediates. The formation of the intermediate lysophosphatidic acid is the first step in this metabolism and is done by the addition or transfer of fatty acid chains to the glycerol backbone. The next intermediate is phosphatidic acid which is formed by the acylation of lysophosphatidic acid. DE phosphorylation of phosphatidic acid results in the formation of diacylglycerol, which is needed for the synthesis of phosphatidylcholine. The complete structure can be obtained through the Kennedy pathway in which polar heads are introduced, finalizing polar head regions, two fatty acid chains and phosphate part connected to the glycerol spine. The complete formation of phosphatidylcholine is done by the transfer of polar head groups which is formed from choline. Phosphatidylserine and phosphatidylethanolamine are other glycerophospholipids which can be derived from phosphatidylcholine.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Molecular Biology, Biochemistry and molecular biology.