Introduction
Post-translational modification is the process by which a protein (polypeptide) is modified chemically after it has gone through the third stage of protein biosynthesis. This third stage is referred to as translation. It is part of the whole process of gene expression. Basically, a protein is made up of chains of amino acids. This means that twenty different amino acids may be used to make a protein during protein synthesis. The post-translational modification of proteins is meant to enable the proteins to perform various functions. This is ensured by the addition of a biochemical functional group by attachment to the protein. This modification occurs after the process of translation (Walsh, 2006).
Examples of the functional groups include a phosphate group, acetate group, carious carbohydrates or lipids. These functional groups may alter the chemical nature of the proteins or change their structure. An example of a chemically altered amino acid is citrullination. This is where the amino acid arginine undergoes posttranslational modification into amino acid citrulline. An example of a structural change is the formation of disulfide bridges.
In a post-translational modification, an enzyme may cause the removal of an amino acid from the amino end of a protein. It may as well cut the chain of the peptide into two. For example, the peptide hormone insulin is cut in the middle. This is after the disulfide bonds are formed. During this process, the propeptide that was present in the middle of the chain is removed. The protein that results would be one with two polypeptide chains joined by a disulfide bond in the middle.
Other modifications cause the proteins to perform certain functions (Bartova et al., 2008). They may be modified to allow for the activation or deactivation of enzymes. For instance, phosphorylation performs this function. This changes the activities and functions of the enzymes. These post-translational modifications are detectable using certain tools. This may be through the use of the Eastern blotting technique or by the use of a mass spectrometer (Brennan and Barford, 2009).
How proteins are modified
During post-translation, the proteins are attached to a biochemical functional group. This may be in form of the addition of a hydrophobic group. An example includes myristoylation. This is whereby a myristate is attached to the protein. Usually, this protein modification is irreversible. It occurs in animals, fungi, viruses, protozoa and plants. The myristate is covalently bonded to the amino acid using an amide group. An enzyme N-myristoyltransferase is necessary as a catalyst in the modification process. Myristoylation performs specific functions in plants. This process is important when the plant requires responding to changes in the environment especially when it is undergoing stress. This process enables responses in the membrane and performs signal transduction.
Another form of modification is palmitoylation. Hemagglutinin is an example of a protein that undergoes this form of modification. This is whereby a palmitoyl group (derived from palmitic acid) is added to the protein. Different from myristoylation, this process is reversible. It is reversible because of the nature of the bond between the protein and palmitate. However, the reverse reaction needs to be catalyzed using the palmitoyl protein thioesterases. This process of palmitoylation may play a role in the regulation of the release of neurotransmitters.
Other post-translational modifications involve the addition of cofactors that are necessary to enhance enzyme activity. An example includes lipoylation, which involves the addition of a lipoate functional group. Other post-translation modifications involve the unique modification of translation factors. Examples include the formation of a diphthamide and hypusine. Other modifications may involve the addition of small chemical groups to the proteins. An example of this is acylation. This involves the addition of a functional group using acyl links. Fatty acylation, for example, involves the attachment of a fatty acid to an amino acid.
Another form of this type of modification is alkylation. It involves the attachment of an alkyl group to the amino acid. These groups include methyl groups or ethyl groups. When a methyl group is added, it is referred to as methylation. They are mainly added to the arginine or lysine residues (Yang and Seto, 2008). This process is reversible and is referred to as demethylation. Glycosylation may also occur. It involves the addition of a glycosyl group to an amino acid. The amino acids involved may include serine, cysteine, tyrosine, arginine, threonine, asparagines, and tryptophan. When this occurs, it causes the formation of a glycoprotein (Drickamer and Taylor, 2006).
Post-translational modifications may also involve additions that are not enzymatic in nature. An example is glycation. This is a process whereby a sugar molecule is added to a protein. This occurs without the involvement of an enzyme. Other examples occurring in vitro include pegylation and biotinylation (Olsen et al., 2006). Modifications may also occur when peptides or other proteins are added to the amino acids (proteins).
Examples include ubiquitination, neddylation and pupylation. All these processes involve the use of covalent linkages to the proteins. Post-translational modifications may lead to the alteration of the chemical nature of proteins. Examples include deamination, deimination, carbamylation and eliminylation. Those involving structural change include the racemization of proline. It may also occur in the form of the formation of disulfide bridges orproteolytic cleavages.
Examples of post-translational modification
Glycosylation
This is a type of posttranslational modification. It is the most frequent and important type. This involves the attachment of glycans (oligosaccharides) to proteins to form glycoproteins. The two are held together using covalent bonds. Different glycoproteins contain different amounts of carbohydrates, ranging from as low as less than 1% to more than 90%. They are made up of a varied number of monosaccharide units. These chains may be straight or branched. All living organisms have glycoproteins in their bodies and they appear in almost all biological processes. Almost all protein activities revolve around aspects of glycoproteins. These include forms such an enzyme, a receptor, hormone, transport protein or structural protein. The carbohydrate chains are also found as part of the secretory proteins and plasma membranes (Drickamer and Taylor, 2006).
Proteins may also be covalently bonded with glycosaminoglycans. This forms proteoglycans, which differ from glycoproteins by virtue of the level of carbohydrate modification. These are also very important for living organisms. Apart from proteins, the oligosaccharides may also be bonded to the lipids. However, this is at a lesser extent as compared to the proteins. The resulting carbohydrate chains in the glycoprotein come in various forms. They may be classified into five groups. The first group consists of the N-linked glycans (oligosaccharides). The others include the O-linked glycans, phosphor-glycans, C-linked oligosaccharides and glypiation.
There are various types of glycosylation. The N-linked glycosylation is most frequent. It is useful in the folding of some proteins of the eukaryotes. It is also useful because it enables the attachment between cells and between the cells and the extracellular matrix. In multi-cellular organisms, the process of glycosylation occurs in the lumen and particularly those of the endoplasmic reticulum. It may also occur in archaea. However, they rarely occur in bacteria. O-linked glycosylation, on the other hand, occur in the Golgi bodies in the multi-cellular organisms. However, they may also be present in bacteria and archaea.
Phospho-serine glycosylation is another form of glycosylation in organisms. With research, several forms of phospho-serine glycosylation have been reported. They include the mannose, fucose, GlcNac and Xylose forms. Two of these were located in Dictyostelium discoideum. These were the GlcNac and Fucose phospho-serine glycans. Research has suggested that mannose was found in mice. It was found on the lamina receptor found on the surface of the cell.
C-mannosylation is another type of glycosylation. Thrombospondins are among the most common type of protein arising from the process. This type of glycosylation is rather unusual. This is because the sugar is not attached to a reactive atom (such as O2 or N2). Instead, it is attached to a carbon atom. Glypiation is a special type of glycosylation. It is also referred to as the formation of GPI anchors. It involves the attachment of a protein to a lipid anchor. This is occurs through a glycan chain.
Roles of glycosylation
The monosaccharide units that attach to the glycoprotein cause them to carry more information per unit weight. The amount of information carried surpasses that carried by the nucleic acids or proteins. Therefore, the process of glycosylation causes the proteins to have various properties depending on the type of glycosylation. The different roles of the glycoproteins are directly related to the nature of their structures. One of the roles of glycosylation is protein folding. Carbohydrates are important for this process. Since they are negatively charged, they can associate with the side chains of the amino acids. Without glycosylation, the protein would not be properly folded and it would not be possible to export it from the endoplasmic reticulum.
Glycosylation is also vital for metabolism (enzyme action). This is because many enzymes are glycoproteins by nature. For example, the enzymes at the small intestines (at the microvilli) are glycoproteins. They have many sugar residues. Glycoproteins may also be important for transportation because they act as carriers. They may bind to hormones or vitamins. An example of such a glycoprotein is ceruloplasmin. It aids in the transportation of iron into the serum. They may also help in the maintenance of structure of the proteins. This is made possible by the aid of the carbohydrate chains that wrap around proteins.
Another role of glycosylation is the formation of glycoproteins, which allow for recognition and adhesion. Glycoproteins appear as cell adhesion molecules on the surfaces of cells. They work to recognize the carbohydrates by virtue of the presence of sugars or the way the sugars present themselves or by their accessibility. This explains why the surface carbohydrates on the cells act as areas of attachment for other cells and various other molecules. Infectious viruses and bacteria, hormones and toxins also find their attachments there. The infectious viruses and bacteria use this opportunity (presence of cell surface oligosaccharides) to intrude the cell.
Phosphorylation
This is another example of post-translational modification of the protein. It occurs when a phosphate group is added to the protein (Ciesla et al., 2011). This process is responsible for turning on or off the protein enzymes. It is through this function that the functions and activities of the enzymes are altered. This process occurs in both unicellular and multi-cellular organisms. In the proteins, this process mainly occurs in the residue of the threonine, histadine, tyrosine and serine. However, histadine phosphorylation occurs more frequently than those that occur on the tyrosine.
The process of phosphorylation occurs on the cells and particularly on certain phosphorylation sites. The many sites available for the process are due to the fact that there are thousands of different types of proteins in any body cell. Research estimates that between 10 percent and 50 percent of all the proteins undergo phosphorylation. In addition, phosphorylation does not only target a single site on a particular protein.
The process may occur at various distinct sites on a protein. Phosphorylation of a particular part of the protein may cause a functional change or an alteration of its localization, it is important to be familiar with the process in order to understand the state of the cells. Understanding the state of the cell will put one in a position to understand the irregularities that comes about during the advent of disease. This would enable disease prevention and treatment (Olive, 2004).
Roles
As earlier explained, the process of phosphorylation turns most of the enzymes and receptors off or on. Dephosphorylation also does the same function. Another significant role that this process plays is in the cellular processes. Reverse phosphorylation of the proteins is also useful for mechanisms of regulation. These regulatory roles come in various forms. The first on is in the regulation of the biological thermodynamics. This mainly occurs in the reactions that require energy. Such reactions that require energy include the phosphorylation of sodium or potassium ions. This is necessary during the transportation of the sodium and potassium ions across the cell membrane. Phosphorylation, in this case, is important in the maintenance of homeostasis of the water content within the organism.
Another regulatory role of the process is in the mediation of the enzyme inhibition process. Phosphorylation is also significant during protein deregulation. It is also through this process that the interactions between proteins are made possible. This is made possible through the concept of recognition domains.
This process results in the final structure of receptors and enzymes. This causes them to become either inactive or active. Phosphorylation may also contribute to the change in structure of a protein. This occurs when a phosphate attaches to a polar R group of amino acids. Through phosphorylation, the hydrophobic end of the protein will become polar and extremely hydrophilic. Due to its hydrophilic nature, it will be able to interact with other residues in the proteins. Both hydrophilic and hydrophobic residues will be involved in this interaction.
Conclusion
Post-translational modification of proteins is a process that increases the diversity of the functions of the protein by adding a functional group. There are different such modifications. They include lipidation, glycosylation, nitrosyoation, methylation, and proteolysis, among others. This process influences almost every aspect of the normal functioning of the cells in the body. Having an understanding of the process of protein post-translational modification is important in various aspects prevention and treatment of diseases.
References
Bartova, E, Krejci, J, Harnicarova, S & Kozubek, S 2008, ‘Histone modifications and nuclear architecture: A review’, J Histochem Cytochem, vol. 56, no. 8, pp. 711-21.
Brennan, DF & Barford, D 2009, ‘Eliminylation: A post-translational modification catalyzed by phosphothreoninelyase’, Trends in Biochemical Sciences, vol. 34, no. 3, pp. 108-114.
Ciesla, F, Fraczyk, T & Rode, W 2011, ‘Phosphorylation in proteins: Important but easily missed’, Acta Biochim Pol, vol. 58, no. 2, pp. 137-147.
Drickamer, K & Taylor, E 2006, Introduction to Glycobiology, Oxford University Press, USA.
Olive, DM 2004, ‘Quantitative methods for the analysis of protein phosphorylation in drug development’, Expert Rev Proteomics, vol. 1, no. 3, pp. 327-341.
Olsen, J, Blagoev, B, Gnad, F, Macek, B, Kumar, C, Mortensen, P & Mann, M 2006, ‘Global, in vivo, and site-specific phosphorylation dynamics in signaling networks’, Cell, vol. 127, no. 3, pp. 635-648.
Walsh, C 2006, Posttranslational modification of proteins: Expanding nature’s inventory, Roberts and Co. Publishers, Englewood.
Yang, XY & Seto, E 2008, ‘Lysine acetylation: Codified crosstalk with other posttranslational modifications’, Mol Cell, vol. 31, no. 4, pp. 449-461.