Proteins and their classification
Proteins are the polymers of amino acids in which each amino acid is linked together through a special type of amide bond called a peptide bond. A peptide bond is formed between the carbonyl carbon attached to the alpha carbon of the 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.
Depending on their complexity, proteins are of two types simple and conjugated. Simple proteins are those which contain only polypeptides and no other things. For example, albumin, globulin, prolamin, etc. are some examples that contain polypeptides only while conjugated proteins contain polypeptides and some non-protein components called 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. 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 Proteins | Elastin, Collagen, Keratin |
| Globular Proteins | Albumin, Globulin, Hemoglobin, Myoglobin |
| Based on Biological Functions | |
| Transfer Proteins | Hemoglobin, Transferrin, Lipoproteins |
| Defense Proteins | Thrombin, Fibrinogen, Immunoglobin |
| Catalytic Proteins | Alsolase, Kinase, Dehydrogenase |
| Regulatory Proteins | Hormones (insulin, glucagon) |
| Structural Proteins | Collagen, Elastin, Keratin |
| Storage Proteins | Myoglobin, Ferritin |
| Nutrient Proteins | Avalbumin |
| Based on sedimentation behavior | LDL, HDL, VLDL, Chylomicron |
| Based on Composition | |
| Simple Proteins | Albumin, Globulin, Prolamine, Scleroprotein |
| Conjugated Proteins | Glycoproteins, Hemeproteins, Lipoproteins, Metalloproteins, Nucleoproteins |
Glycoproteins and their 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 glycoprotein synthesis. Glycosylation is the process of attaching carbohydrate moiety to the polypeptide formed after the translation of the mRNA. Glycosylation is a type of post-translational modification that 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 modifications.
Glycosylation and its importance
Glycosylation occurs in the Golgi complex. Polypeptide formed after the translation of mRNA in a ribosome is transported into the Golgi complex through vesicle transport where it is modified to the glycoproteins. The sequential addition of hexoses to the specific amino acid residue of the polypeptide catalyzed by different enzymes results in glycoprotein formation. Once a glycoprotein of a specific type is formed in the Golgi complex it is transported to the target place through vesicle transport.
Glycosylation is a necessary process in many ways and glycosylation of proteins is the 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 and old are also glycosylated to display tags 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.
