Glycoproteins, an overview

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(Last Updated On: October 21, 2017)
Different types of glycosylation

Different Types of N-Linked Glycoproteins Source: Common Wikimedia

Proteins and their classification

Proteins are the polymers of amino acids in which each amino acids are linked together through a special type of the amide bond called as a peptide bond. A peptide bond is formed in between the carbonyl carbon attached to the alpha carbon of first amino acid and amino nitrogen attached to the alpha carbon of the next amino acid. There are many ways of classifying proteins based on their shape, solubility, biological function, density, structure, complexity, etc.

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Depending on their complexity, proteins are of two types simple and conjugated. Simple proteins are those which contain only polypeptide and no other things. For example, albumin, globulin, prolamin, etc. are some examples which contain polypeptide only while conjugated proteins contain polypeptides and some non-protein component called as a cofactor. Transferrin, hemoglobin, etc. are examples of this class.

Among the conjugated proteins, there are many other sub-classes like glycoproteins, nucleoproteins, lipoproteins, heme proteins, etc. This classification is based on the type of non-protein parts (cofactors like NADP, ATP, Iron etc) attached with. As, for example, in glycoproteins, the non-protein part is glycomes (carbohydrate moiety), nucleoproteins are those which are associated with nucleic acids (histone proteins) similarly lipoproteins are those which carry lipids and fatty acids (chylomicrons, LDL, VLDL, and HDL). Heme proteins are those which contain heme as a prosthetic group and examples include hemoglobin, myoglobin, cytochrome etc.

Based on Overall Shape 
Fibrous ProteinsElastin, Collagen, Keratin
Globular ProteinsAlbumin, Globulin, Hemoglobin, Myoglobin
Based on Biological Functions
Transfer ProteinsHemoglobin, Transferrin, Lipoproteins
Defense ProteinsThrombin, Fibrinogen, Immunoglobin
Catalytic ProteinsAlsolase, Kinase, Dehydrogenase
Regulatory ProteinsHormones (insulin, glucagon)
Structural ProteinsCollagen, Elastin, Keratin
Storage ProteinsMyoglobin, Ferritin
Nutrient ProteinsAvalbumin
Based on sedimentation behaviorLDL, HDL, VLDL, Chylomicron
Based on Composition
Simple ProteinsAlbumin, Globulin, Prolamine, Scleroprotein
Conjugated ProteinsGlycoproteins, Hemeproteins, Lipoproteins, Metalloproteins, Nucleoproteins


Glycoproteins and its type

Glycoproteins are also of different types based on the site of attachment of the carbohydrate moiety to the polypeptide chain. As, for example, N-linked glycoproteins in which carbohydrate moiety is attached to the protein through amide nitrogen of the side chain of asparagine residue of the polypeptide. Similarly, the second one is O-linked glycoproteins in which carbohydrate moiety is attached through the O atom of the side chain of serine or training residue of the polypeptide chain.

Glycoproteins have great importance in a biological system and the glycosylation of the protein is necessary for the glycoprotein synthesis. Glycosylation is the process of attaching carbohydrate moiety to the polypeptide formed after translation of the mRNA. Glycosylation is a type of post-translational modification which is necessary for some proteins to become biologically active and functional. Though there are many types of post-translational modifications like a cleavage of some portion of the polypeptide chain, glycosylation is one of the most important post-translational modification.



Glycosylation and its importance

Glycosylation occurs in the Golgi complex. Polypeptide formed after translation of mRNA in a ribosome is transported into the Golgi complex through vesicle transport where it is modified to the glycoproteins. Sequential addition of hexoses to the specific amino acid residue of the polypeptide catalyzed by different enzymes results in the glycoprotein formation. Once glycoprotein of a specific type is formed in the Golgi complex it is transported to the target place through the vesicle transport.

Glycosylation is a necessary process in many ways and glycosylation of the proteins is most important cellular/biochemical process. Carbohydrates attached to the protein act as a tag and thus glycoproteins are targeted to their destination according to the information displayed on them (in the form of carbohydrate moiety). Therefore, glycosylation is necessary for protein targeting. If proteins need to be targeted to membranes they have a special carbohydrate moiety that is only recognized by that membrane similarly, each glycoprotein is recognized by specific receptors displayed on the target.

Proteins that are defective, old are also glycosylated to display tag for degradation, which is recognized by proteolytic enzymes. Some protective proteins such as immunoglobulins are also glycosylated so that they are recognized by a specific antigen. Antigens recognize carbohydrate moiety of the glycoproteins and antigen-antibody interaction initiates.

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