Post-translational modification

thumb|250px|Ribbon diagram of a protease ([[TEV protease) complexed with its peptide substrate in black with catalytic residues in red ()]] A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the formation of new protein products. They do this by cleaving the peptide bonds within proteins by hydrolysis, a reaction where water breaks bonds. Proteases are involved in numerous biological pathways, including digestion of ingested proteins, protein catabo

Ubiquitin is a small () regulatory protein found in most tissues of eukaryotic organisms, i.e., it is found ubiquitously. It was discovered in 1975 by Gideon Goldstein and further characterized throughout the late 1970s and 1980s. Four genes in the human genome code for ubiquitin: UBB, UBC, UBA52 and RPS27A.

Methylation, in the chemical sciences, is the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These terms are commonly used in chemistry, biochemistry, soil science, and biology.

thumb|Serine in an amino acid chain, before and after phosphorylation.

Glycosylation is the reaction in which a carbohydrate (or 'glycan'), i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor) in order to form a glycoconjugate. In biology (but not always in chemistry), glycosylation usually refers to an enzyme-catalysed reaction, whereas glycation (also 'non-enzymatic glycation' and 'non-enzymatic glycosylation') may refer to a non-enzymatic reaction.

covalent and generally enzymatic modification of proteins during or after protein biosynthesis

thumb|350px|A tetrapeptide (example: Val-Gly-Ser-Ala) with green highlighted N-terminal α-amino acid (example: L-[[valine) and blue marked C-terminal α-amino acid (example: L-alanine). This tetrapeptide could be encoded by the mRNA sequence 5'-GUU GGU AGU GCU-3'.]] The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amine group is bonded to the carboxylic group of another amino acid, making it a chain. That l

thumb|300px|The hydrolysis of a [[protein (red) by the nucleophilic attack of water (blue). The uncatalysed half-life is several hundred years.]]

process that modulates frequency, rate or extent of gene expression

thumb|500px|A tetrapeptide (example: Valine|Val-Gly-Ser-Ala) with green highlighted N-terminal α-amino acid (example: L-[[valine) and blue marked C-terminal α-amino acid (example: L-alanine).]] The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, carboxy tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is translated from messenger RNA, it is created from N-terminus to C-terminus. The convention for writing peptide sequences is to put the C-terminal

Carboxylation is a chemical reaction in which a carboxylic acid is produced by treating a substrate with carbon dioxide. The opposite reaction is decarboxylation. In chemistry, the term carbonation is sometimes used synonymously with carboxylation, especially when applied to the reaction of carbanionic reagents with CO2. More generally, carbonation usually describes the production of carbonates.

Glycation (non-enzymatic glycosylation) is the covalent attachment of a sugar to a protein, lipid or nucleic acid molecule. Typical sugars that participate in glycation are glucose, fructose, galactose, and their derivatives. Glycation is the non-enzymatic process responsible for many (e.g. micro and macrovascular) complications in diabetes mellitus and is implicated in other diseases and in aging.

In chemistry, hydroxylation refers to the installation of a hydroxyl group () into an organic compound. Hydroxylations generate alcohols and phenols, which are very common functional groups. Hydroxylation confers some degree of water-solubility. Hydroxylation of a hydrocarbon is an oxidation, thus a step in degradation.

process that prevents the expression of a gene

Glycosylphosphatidylinositol () or glycophosphatidylinositol (GPI) is a phosphoglyceride that can be attached to the C-terminus of a protein during posttranslational modification. The resulting GPI-anchored proteins play key roles in a wide variety of biological processes. GPI is composed of a phosphatidylinositol group linked through a carbohydrate-containing linker (glucosamine and mannose glycosidically bound to the inositol residue) and via an ethanolamine phosphate (EtNP) bridge to the C-terminal amino acid of a mature protein. The two fatty acids within the hydrophobic phosphatidyl-inosi

In chemistry, racemization is a conversion, by heat or by chemical reaction, of an optically active compound into a racemic (optically inactive) form. This creates a 1:1 molar ratio of enantiomers and is referred to as a racemic mixture (i.e. contain equal amount of (+) and (−) forms). Plus and minus forms are called dextrorotation and levorotation. The D and L enantiomers are present in equal quantities, the resulting sample is described as a racemic mixture or a racemate. Racemization can proceed through a number of different mechanisms, and it has particular significance in pharmacology in

class of proteins

thumb|Skeletal formula of the prenyl group.Prenylation (also known as isoprenylation or lipidation) is the addition of hydrophobic molecules to a protein or a biomolecule. It is usually assumed that prenyl groups (3-methylbut-2-en-1-yl) facilitate attachment to cell membranes, similar to lipid anchors like the GPI anchor, though direct evidence of this has not been observed. Prenyl groups (also called isoprenyl groups, having one hydrogen atom more than isoprene) have been shown to be important for protein–protein binding through specialized prenyl-binding domains.

proteins or lipids that become glycated as a result of exposure to sugars

Organic reaction: isomerization or rearrangement reaction of the N-glycoside of an aldose or the glycosylamine to the corresponding 1-amino-1-deoxy-ketose

In biochemistry, dephosphorylation is the removal of a phosphate () group from an organic compound by hydrolysis. It is a reversible post-translational modification. Dephosphorylation and its counterpart, phosphorylation, activate and deactivate enzymes by detaching or attaching phosphoric esters and anhydrides. A notable occurrence of dephosphorylation is the conversion of ATP to ADP and inorganic phosphate.

pair of enantiomeric chemical compounds

thumb|250px|Adenosine diphosphate ribose|ADP-ribose ADP-ribosylation is the addition of one or more ADP-ribose moieties to a protein. It is a reversible post-translational modification that is involved in many cellular processes, including cell signaling, DNA repair, gene regulation and apoptosis. Improper ADP-ribosylation has been implicated in some forms of cancer. It is also the basis for the toxicity of bacterial compounds such as cholera toxin, diphtheria toxin, and others.

Sulfation (sometimes spelled sulphation in British English) is the chemical reaction that entails the addition of SO3 group. In principle, many sulfations would involve reactions of sulfur trioxide (SO3). In practice, most sulfations are effected less directly. Regardless of the mechanism, the installation of a sulfate-like group on a substrate leads to substantial changes.

the process of targeting specific proteins to particular regions of the cell, typically membrane-bounded subcellular organelles, usually requiring an organelle-specific protein sequence motif

thumb|upright=1.3|The chemical conversion of arginine to citrulline, known as citrullination or deimination. Citrullination or deimination is the conversion of the amino acid arginine in a protein into the amino acid citrulline. Citrulline is not one of the 20 standard amino acids encoded by DNA in the genetic code. Instead, it is the result of a post-translational modification. Citrullination is distinct from the formation of the free amino acid citrulline as part of the urea cycle or as a byproduct of enzymes of the nitric oxide synthase family.

thumbnail|Formyl functional group is shown in blue.

class of enzymes

thumb|300px|right|In palmitoylation, a palmitoyl group (derived from palmitic acid, pictured above) is added. thumb|Palmitoylation of a cysteine residue thumb|Left Palmitoylation (red) anchors Ankyrin G to the plasma membrane. Right Close up. Palmityl residue in yellow. thumb|Palmitoylation of Gephyrin Controls Receptor Clustering and Plasticity of GABAergic Synapses

Phosphocholine is an intermediate in the synthesis of phosphatidylcholine in tissues. Phosphocholine is made in a reaction, catalyzed by choline kinase, that converts ATP and choline into phosphocholine and ADP. Phosphocholine is a molecule found, for example, in lecithin.

Diphthamide is a post-translationally modified histidine amino acid found in archaeal and eukaryotic elongation factor 2 (eEF-2).

molecular process that occurs within living cells

thumb|250px|right|In myristoylation, a myristoyl group (derived from [[myristic acid, pictured above) is added.]] thumb|300px|right|Co-translational addition of myristic acid by N-myristoyltransferase to N-terminal glycine of a nascent protein. Myristoylation is a lipidation modification where a myristoyl group, derived from myristic acid, is covalently attached by an amide bond to the alpha-amino group of an N-terminal glycine residue. Myristic acid is a 14-carbon saturated fatty acid (14:0) with the systematic name of n-tetradecanoic acid. This modification can be added either co-translation

class of enzymes

proteins located on the surface of the cell membrane that are covalently attached to lipids embedded within the cell membrane

Clostripain (, clostridiopeptidase B, clostridium histolyticum proteinase B, alpha-clostridipain, clostridiopeptidase, Endoproteinase Arg-C) is a cysteine protease that cleaves proteins on the carboxyl peptide bond of arginine. It was isolated from Clostridium histolyticum. The isoelectric point of the enzyme is 4.8-4.9 (at 8 °C), and optimum pH is 7.4~7.8 (against α-benzoyl-arginine ethyl ester). The composition of the enzyme is indicated to be of two chains of relative molecular mass 45,000 and 12,500.

Biological processes used in gene regulation

Glucosepane is a lysine-arginine protein cross-linking product and advanced glycation end product (AGE) derived from D-glucose. It is an irreversible, covalent cross-link product that has been found to make intermolecular and intramolecular cross-links in the collagen of the extracellular matrix (ECM) and crystallin of the eyes. Covalent protein cross-links irreversibly link proteins together in the ECM of tissues. Glucosepane is present in human tissues at levels 10 to 1000 times higher than any other cross-linking AGE, and is currently considered to be the most important cross-linking AGE.

process of protein biosynthesis

protein-coding gene in the species Homo sapiens

The modification of histones by addition of methyl groups.

Polyglycylation is a form of posttranslational modification of glutamate residues of the carboxyl-terminal region tubulin in certain microtubules (e.g., axonemal) originally discovered in Paramecium, and later shown in mammalian neurons as well.

Polyglutamylation is a form of reversible posttranslational modification of glutamate residues seen for example in alpha and beta tubulins, nucleosome assembly proteins NAP1 and NAP2. The γ-carboxy group of glutamate may form peptide-like bond with the amino group of a free glutamate whose α-carboxy group can now be extended into a polyglutamate chain. The glutamylation is done by the enzyme glutamylase and removed by deglutamylase.