When protein folding into the mature, functional 3D state is complete, it is released from the ribosome but is not necessarily the end of the protein maturation pathway. A folded protein can still undergo further processing through
post-translational modifications (PTMs). As of 2023 there are more than 650 known types of PTM. These modifications can alter protein activity, the ability of the protein to interact with other proteins and where the protein is found within the cell e.g. in the cell nucleus or cytoplasm. Through post-translational modifications, the diversity of proteins encoded by the genome is expanded by 2 to 3
orders of magnitude. There are four key classes of post-translational modification:
Addition of chemical groups Following translation, small chemical groups can be added onto amino acids within the mature protein structure. Examples of processes which add chemical groups to the target protein include methylation,
acetylation and
phosphorylation. Methylation is the reversible addition of a
methyl group onto an amino acid catalyzed by
methyltransferase enzymes. Methylation occurs on at least 9 of the 20 common amino acids, however, it mainly occurs on the amino acids
lysine and
arginine. One example of a protein which is commonly methylated is a
histone. Histones are proteins found in the nucleus of the cell. DNA is tightly wrapped round histones and held in place by other proteins and interactions between negative charges in the DNA and positive charges on the histone. A highly specific pattern of
amino acid methylation on the histone proteins is used to determine which regions of DNA are tightly wound and unable to be transcribed and which regions are loosely wound and able to be transcribed. Histone-based regulation of DNA transcription is also modified by acetylation. Acetylation is the reversible covalent addition of an
acetyl group onto a lysine amino acid by the enzyme
acetyltransferase. The acetyl group is removed from a donor molecule known as
acetyl coenzyme A and transferred onto the target protein.
Histones undergo acetylation on their lysine residues by enzymes known as
histone acetyltransferase. The effect of acetylation is to weaken the charge interactions between the histone and DNA, thereby making more genes in the DNA accessible for transcription. The final, prevalent post-translational chemical group modification is phosphorylation. Phosphorylation is the reversible, covalent addition of a
phosphate group to specific amino acids (
serine,
threonine and
tyrosine) within the protein. The phosphate group is removed from the donor molecule
ATP by a protein
kinase and transferred onto the
hydroxyl group of the target amino acid, this produces
adenosine diphosphate as a byproduct. This process can be reversed and the phosphate group removed by the enzyme protein
phosphatase. Phosphorylation can create a binding site on the phosphorylated protein which enables it to interact with other proteins and generate large, multi-protein complexes. Alternatively, phosphorylation can change the level of protein activity by altering the ability of the protein to bind its substrate.
Addition of complex molecules Post-translational modifications can incorporate more complex, large molecules into the folded protein structure. One common example of this is
glycosylation, the addition of a polysaccharide molecule, which is widely considered to be most common post-translational modification. In glycosylation, a
polysaccharide molecule (known as a
glycan) is covalently added to the target protein by
glycosyltransferases enzymes and modified by
glycosidases in the
endoplasmic reticulum and
Golgi apparatus. Glycosylation can have a critical role in determining the final, folded 3D structure of the target protein. In some cases glycosylation is necessary for correct folding. N-linked glycosylation promotes protein folding by increasing
solubility and mediates the protein binding to
protein chaperones. Chaperones are proteins responsible for folding and maintaining the structure of other proteins. There are broadly two types of glycosylation,
N-linked glycosylation and
O-linked glycosylation. N-linked glycosylation starts in the endoplasmic reticulum with the addition of a precursor glycan. The precursor glycan is modified in the Golgi apparatus to produce complex glycan bound covalently to the nitrogen in an
asparagine amino acid. In contrast, O-linked glycosylation is the sequential covalent addition of
individual sugars onto the oxygen in the amino acids serine and threonine within the mature protein structure.
Formation of covalent bonds Many proteins produced within the cell are secreted outside the cell to function as
extracellular proteins. Extracellular proteins are exposed to a wide variety of conditions. To stabilize the 3D protein structure, covalent bonds are formed either within the protein or between the different polypeptide chains in the quaternary structure. The most prevalent type is a
disulfide bond (also known as a disulfide bridge). A disulfide bond is formed between two
cysteine amino acids using their side chain chemical groups containing a Sulphur atom, these chemical groups are known as
thiol functional groups. Disulfide bonds act to stabilize the
pre-existing structure of the protein. Disulfide bonds are formed in an
oxidation reaction between two thiol groups and therefore, need an oxidizing environment to react. As a result, disulfide bonds are typically formed in the oxidizing environment of the endoplasmic reticulum catalyzed by enzymes called protein disulfide isomerases. Disulfide bonds are rarely formed in the cytoplasm as it is a reducing environment. ==Role of protein synthesis in disease==