Glycolipid

The glycolipids, or also called sphingoglycolipids, are composed of a ceramide (sphingosine + fatty acid) and a short-chain carbohydrate; They lack a phosphate group. Glycolipids are part of the lipid bilayer of the cell membrane; the carbohydrate part of the molecule is oriented towards the outside of the plasma membrane and is a fundamental component of the glycocalyx, where it acts in cell recognition and as an antigen receptor.

Depending on the glycolipid, the carbohydrate chain can contain anywhere from one to seven monosaccharide monomers. Like the phosphate head of a phospholipid, the fatty acid head of a glycolipid is hydrophilic, and the hydrocarbon tails are hydrophobic. In aqueous solution, glycolipids behave in a similar way to phospholipids.

Structure

Chemical structure of glucolipids

The essential feature of a glycolipid is the presence of a monosaccharide or oligosaccharide attached to a lipid moiety. The most common lipids in cell membranes are glycerolipids and sphingolipids, which have glycerol or sphingosine backbones, respectively. Fatty acids are connected to this backbone, so that the lipid as a whole has a polar head and a nonpolar tail. The lipid bilayer of the cell membrane consists of two lipid layers, with the inner and outer surfaces of the membrane formed by the polar head groups and the inner part of the membrane formed by the nonpolar fatty acid tails.

Saccharides that are attached to the polar head groups on the outside of the cell are the ligand components of glycolipids and are equally polar, allowing them to be soluble in the aqueous environment surrounding the cell. The lipid and saccharide form a glycoconjugate through a glycosidic bond, which is a covalent bond. The anomeric carbon of the sugar is attached to a free hydroxyl group on the lipid backbone. The structure of these saccharides varies depending on the structure of the molecules to which they are attached.

Function

Cell-Cell Interactions

The main function of glycolipids in the body is to serve as recognition sites for cell-cell interactions. The glycolipid saccharide will bind to a specific complementary carbohydrate or lectin (carbohydrate-binding protein) from a neighboring cell. The interaction of these cell surface markers is the basis of cell recognition and initiates cellular responses that contribute to activities such as regulation, growth, and apoptosis.

Immune response

An example of how glycolipids work within the body is the interaction between leukocytes and endothelial cells during inflammation. Selectins, a class of lectins found on the surface of leukocytes and endothelial cells, bind glycolipid-bound carbohydrates to initiate the immune response. This binding causes leukocytes to leave the circulation and congregate near the site of inflammation. This is the initial binding mechanism, which is followed by the expression of integrins that form stronger bonds and allow leukocytes to migrate to the site of inflammation. Glycolipids are also responsible for other responses, notably the recognition of host cells by viruses.

Blood groups

Blood types are an example of how glycolipids in cell membranes mediate cellular interactions with the surrounding environment. The four main types of human blood (A, B, AB, O) are determined by the oligosaccharide attached to a specific glycolipid on the surface of red blood cells, which acts as an antigen. The unmodified antigen, called the H antigen, is characteristic of type O and is present on red blood cells of all blood types. Blood type A has an added N-acetylgalactosamine as the main determining structure, type B has a galactose, and type AB has all three antigens. Antigens that are not present in an individual's blood will cause the production of antibodies, which will bind to the foreign glycolipids. For this reason, people with blood type AB can receive transfusions of all blood types (the universal acceptor) and people with blood type O can act as donors of all blood types (the universal donor).

Metabolism

Glycosyltransferases

Enzymes called glycosyltransferases bind the saccharide to the lipid molecule and also play a role in assembling the correct oligosaccharide so that the correct receptor can be activated in the cell that responds to the presence of the glycolipid on the cell's surface. The glycolipid is assembled in the Golgi apparatus and embedded in the surface of a vesicle which is then transported to the cell membrane. The vesicle fuses with the cell membrane so that the glycolipid can be presented on the outer surface of the cell.

Glycoside hydrolases

Glycoside hydrolases catalyze the breaking of glycosidic bonds. They are used to modify the oligosaccharide structure of the glycan after it has been added to the lipid. They can also remove glycans from glycolipids to convert them back to unmodified lipids.

Metabolic defects

Sphingolipidoses are a group of diseases associated with the accumulation of sphingolipids that have not been broken down correctly, usually due to a defect in a glycoside hydrolase enzyme. Sphingolipidoses are generally inherited and their effects depend on the enzyme affected and the degree of impairment. A notable example is Niemann-Pick disease, which can cause pain and damage to neural networks and is often fatal in early childhood.

Types

β-D-Galactosilceramide, a galactocerebrósido; R is the alkylic chain of fatty acid.

• Brains. Cerebrósides have a united sugar that by linking β-glucosydic to the hydroxyl group of ceramide; those who have galactosa are called galactocerebrósides (such as brasine) and are found in a characteristic way in the plasma membranes of nerve tissues; those that contain glucose (glucocerebrósides) are found in the non-nervous plasma membranes. The sulphate has a stencil with carbon sulfate 3.

• Globosides. Glybnosides are glucosfingolipids with neutral oligosaccharides attached to the ceramide.

• Ganglyosides. They are the most complex sphingolipids by virtue of containing very large polar heads formed by negatively charged oligosaccharides as they possess one or more N-acetyneuramine or sialic acid units that have a negative load to pH 7. The lymph nodes differ from the previous ones because they possess this acid. They are concentrated in large numbers on the lymph node cells of the central nervous system, especially in nerve endings. The ganglysides constitute 6% of the membrane lipids of the gray matter of the human brain and are found in a lesser amount in the membranes of most non-nervorous animal tissues. They occur in the outer area of the membrane and serve to recognize the cells, so they are considered membrane receptors. Its name is due to the insulation for the first time of the mitochondria membrane of the lymph nodes.