Glycocalyx - Definition,Function

Glycocalyx

• A layer of carbohydrate on the surface of the plasma membrane of most eukaryotic cells. It is made up of the oligosaccharide side-chains of the glycolipid and glycoprotein components of the membrane and may include oligosaccharides secreted by the cell. It plays a role in cell–cell adhesion and in regulating the exchange of materials between a cell and its environment.
• The glycocalyx is a carbohydrate-enriched coating that covers the outside of many eukaryotic cells and prokaryotic cells, particularly bacteria .
• On bacterial cells, the glycocalyx provides a protective coat from host factors.
• The outermost layer of a bacterium, typically consisting of numerous polysaccharides plus various glycoproteins. The glycocalyx varies in thickness and consistency: in some species it forms a flexible slime layer while in others it forms a rigid and relatively impermeable capsule.
• Generally, the glycocalyx is constructed of one or more sugars that are called saccharides.If more than one saccharide is present, the glycocalyx is described as being made of polysaccharide. In some glycocalyces, protein can also be present.

Components and structure

• The glycocalyx is a layer that lines the luminal surface of vascular endothelial cells,The thickness and structure of the glycocalyx vary across different species,Its observed thickness in humans is approximately 0.5 to 5.0 μm
• The glycocalyx consists of proteoglycans (PG), glycoproteins bound with sialic acid, glycosaminoglycans (GAG), and associated plasma proteins  
• Proteoglycans are core proteins anchored to the apical membrane of endothelial cells, to which several GAG chains are covalently attached,There are many different types of proteoglycans, but syndecan-1 (a subtype of syndecan family from 1 to 4) is a primary target of prior endothelial glycocalyx investigations
• GAG chains that bind to proteoglycans include heparan sulfate (a major component of GAGs, comprising over 50% of GAGs in glycocalyx), chondroitin sulfates, dermatan sulfates, and possibly keratin sulfates,These sulfated GAGs are negatively charged, enabling electrostatic interaction with plasma proteins.
• In contrast, hyaluronan (hyaluronic acid) is a large linear molecule that does not bind to proteoglycans, but instead interacts with cell-membrane CD44. Hyaluronan also differs from other GAGs as it is not sulfated and is therefore uncharged.
• However, hyaluronan can form complexes with other sulfated GAGs, enabling it to sequester water and stabilize the gel-like structure of the glycocalyx . Proteins such as albumin, fibrinogen, fibronectin, thrombomodulin, antithrombin III, superoxide dismutase, and cell-adhesion molecules all interact with GAGs.
  

Function

There are two important functions of the glycocalyx.

• The first function is to enable bacteria to become harder for the immune cells called phagocytes so surround and engulf. This is because the presence of a glycocalyx increases the effective diameter of a bacterium and also covers up components of the bacterium that the immune system would detect and be stimulated by. Thus, in a sense, a bacterium with a glycocalyx becomes more invisible to the immune system of a host.  EXAMPLE:Infectious strains of bacteria such as Staphylococcus, Streptococcus, and Pseudomonas tend to elaborate more glycocalyx than their corresponding non-infectious counterparts.
• The second function of a bacterial glycocalyx is to promote the adhesion of the bacteria to living and inert surfaces and the subsequent formation of adherent, glycocalyx-enclosed populations that are called biofilms . Biofilm bacteria can become very hard to kill, party due to the presence of the glycocalyx material. Many persistent infections in the body are caused by bacterial biofilms. One EXEMPLE: The dental plaque formed by glycocalyx-producing Streptococcus mutans, which can become a focus for tooth enamel-digesting acid formed by the bacteria.