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Post-translational Modification
Every protein forms a unique three-dimensional structure that allows the protein to be active, but some proteins require further modification after the process of translation so they can form their final activated state.
Post-translational modification involves the enzymatic and covalent modification of proteins and is an important process in cell signalling. Protein modifications occur at the C- or N-terminus of the polypeptide chain, or on the side chains of the amino acids present.
There are hundreds of ways proteins can be modified, but in this article we are only going to discuss the most common mechanisms of post-translational modifications of proteins.
Phosphorylation is the most common mechanism of modifying a protein post-translation, especially enzymes. The process of phosphorylation is reversible and involves adding a phosphate group to specific amino acids such as serine, threonine and tyrosine; the phosphate group is donated from ATP.
Phosphorylation is catalysed by enzymes, these include protein kinases which transfer the terminal phosphate group of ATP to the hydroxyl group present on serine, threonine and tyrosine amino acid residues.
Phosphorylation can be reversed and this involves protein phosphatases, which catalyse the removal of a phosphoryl group from the protein molecule through hydrolysis.
Diagram - The process of phosphorylation and dephosphorylation
Creative commons source by Petaholmes [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Why phosphorylation of proteins is highly effective:
- It adds two negative charges to the modified enzyme or protein. This disrupts the electrostatic interactions allowing new interactions to form, which can affect the activity of the enzyme and the substrate binding.
- The phosphoryl group can form hydrogen bonds.
- The rate of phosphorylation and dephosphorylation can change, either taking less than a second or many hours. This allows the number of phosphorylated proteins to change depending on the needs of the physiological processes taking place.
- As ATP is used as an energy currency and in phosphorylation, this links the regulation of metabolism and the energy status of the cell together.
- Phosphorylation can lead to highly amplified effects within the cell as one single activated kinase can phosphorylate many target proteins in a short space of time. The activated proteins may also be enzymes, which can go on to catalyse further reactions. Therefore, the initial signal can be amplified exponentially throughout the cell through a cascade of kinases in a short time period.
Proteins can be phosphorylated abnormally and this can lead to diseased states. Diseases such as Alzheimer’s disease, Parkinson’s disease and cancer have been linked to abnormal protein phosphorylation.
Another mechanism of post-translational modification is glycosylation, and this process involves the covalent attachment of a carbohydrate to a protein. It is one of the most complicated processes of post-translational modification due to the large number of enzyme-dependent steps involved. The donor molecule is a glycosyl group, which is a mono- or oligosaccharide that forms a glycosidic bond with the acceptor molecule, which in this case is a protein.
There are a number of subtypes of glycosylation which we will discuss further:
N-linked glycosylation is a specific type of glycosylation takes place in the endoplasmic reticulum and involves the attachment of a mono- or oligosaccharide to a nitrogen atom present in the asparagine amino acid residue. N-linked glycosylation requires energy and usually occurs to proteins bound for the membrane or proteins that are going to be secreted.
Diagram - N-linked glycosylation with an asparagine residue in blue
Creative commons source by Lizziechka [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
O-linked glycosylation takes place in the Golgi apparatus and involves glycans being attached to the hydroxyl groups of serine or threonine amino acid residues. This process occurs after N-linked glycosylation and folding of the protein. O-linked glycosylation has been found to be important in the production of proteins released in mucus secretions and proteoglycans, which are a component of the extracellular matrix.
Diagram - O-linked glycosylation with a serine residue in blue
Public Domain source by Tpirojsi
There are other types of glycosylation such as S-linked glycosylation and C-linked glycosylation, but we are not going to discuss these types in this article.
Why glycosylation is an important type of post-translational modification:
- Glycosylation helps in ensuring that proteins fold correctly, it also provides protein stability, helping the protein carry out its function.
- Glycosylation has been shown to be important in how proteins interact with other molecules.
- For example, how a ligand interacts with its complementary receptor or how the receptor interacts with the cell signalling mechanisms inside the cell.
- How these examples interact has been shown to be due to glycosylation, with the process of glycosylation affecting the biological response.
- Glycosylation plays an important role in determining the blood group of a person. The blood group is determined by the presence or absence of glycosyltransferases. These enzymes catalyse the final stage of the transfer of a saccharide molecule to the H antigen present on the outer surface of the red blood cell, and the final result of the carbohydrates added determines the person’s blood group.
It is easy to get confused about the difference between glycosylation and glycation, so we shall go through the differences now:
- Glycation is when a sugar molecule is attached to a protein not under enzymatic control, whereas glycosylation is the attachment of a sugar to a protein under enzyme control using activated sugar donors.
- Glycation often occurs in the blood as simple sugars, such as glucose and galactose, are found here and they can attach to haemoglobin. This results in the formation of glycated haemoglobin which can increase the number of free radicals within the red blood cell leading to damage.
- Glycated haemoglobin or HbA1c can be used as a diagnostic tool for diabetes mellitus as the level of HbA1c can be used to assess how well a patient with diabetes mellitus is controlling their glucose levels. (Check out our article on Diabetes Mellitus).
Acetylation is another example of post-translational modification and it is an important process that occurs in cell biology. In proteins, acetylation usually involves the addition of an acetyl group (CH3CO-) to an amino group, such as the free amino group at the N-terminal or the side chain of a lysine residue, and is catalysed by enzymes.
There are two forms of protein acetylation: N-terminal acetylation and lysine acetylation.
N-terminal acetylation has been shown to be irreversible, and is important for the function and regulation of a variety of proteins. The process involves the addition of an acetyl group to the free amino group present at the N-terminal of the polypeptide chain.
Diagram - The process of N-terminal acetylation
Creative commons source by Ybs.Umich [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Acetylation of lysine residues involves acetyl-CoA which donates its acetyl group to lysine. The acetyl becomes attached to an amino group present on the side chain of a lysine residue.
An example of lysine acetylation is the acetylation of histones and p53. Lysine residues present at the N-terminal of the polypeptide tail extending from the core of the histone protein are acetylated and the acetylated histones leads to the chromatin to be less condensed, therefore higher levels of transcription. The acetylation of p53 is important for its activation and once activated, the protein can suppress the cell cycle, allow for any damaged DNA to be repaired or begin the process of cell apoptosis.
Disulphide bonds are strong bonds formed by the oxidation of the thiol groups (R-SH) of two cysteine residues, with the formation of disulphide bonds occurring in the lumen of the endoplasmic reticulum.
Disulphide bonds are an important in stabilising the tertiary structure of single polypeptide chains and in stabilising multi-subunit proteins such as antibodies and insulin. The majority of proteins that are secreted into the extracellular space contain disulphide bonds, while cytosolic proteins do not contain this type of bond. If a protein contains a high number of disulphide bonds, the molecule will be more resistant to external forces, e.g. heat and detergents, and less likely to denature.
Image - Structure of cysteine
Creative commons source by bigblue0092 [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Image - Polypeptide chain with disulphide bonds
Creative commons source by Jü [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
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