post-translational modifications

what do post-translational modifications do?

alter the properties of amino acids

alter structure and function of the protein produced

what are the common post-translational modifications?

• receptor phosphorylation

• acetylation

• N- and O- glycosylation

• ubiquitylation

• proteolysis

what amino acids does receptor phosphorylation target?

serine (S)

threonine (T)

tyrosine (Y)

what is receptor phosphorylation?

receptor is phosphorylated

common in cell signalling e.g. insulin receptors

what are the impacts and effects of receptor phosphorylation

impacts how receptors interact with other molecules and respond to stimuli

what enzymes catalyse phosphorylation and dephosphorylation?

phosphorylation = kinase

dephosphorylation = phosphatase

diagram of receptor phosphorylation

what amino acids does acetylation target?

lysine, serine, threonine

what is an example of acetylation

histone acetylation in DNA

what happens in histone acetylation?

• histones are positively charged and DNA is negatively charged so DNA wraps tightly around histones

• acetylation adds a negative charge to histones, interaction between histone and DNA is weakened making chromatin more accessible

• histone acetyl transferase transfers acetyl group (from acetyl CoA) to lysine residues on histones

diagram of histone acetylation

what amino acids does N- and O- glycosylation impact?

asparagine, serine, threonine

example of N and O glycosylation

erythropoietin

what happens in glycosylation

• Glycosylation is the process of attaching sugar molecules (glycans) to proteins

• polysaccharides are transferred to specific amino acid residues by glycosyltransferases

• key role in regulation of protein function and cell adhesion

• two most common types are N-linked and O-linked glycosylation

what impact does glycosylation have on erythropoietin?

erythropoietin has N and O linked oligosaccharides providing stability and function

diagram of n and o glycosylation

what amino acids does ubiquitylation usually affect?

lysine (K)

what happens in ubiquitylation?

• small, 76-amino-acid protein called ubiquitin is covalently attached to a target substrate

• process dictates protein stability, cellular localization, and signaling pathways

• Ubiquitination is an ATP-dependent cascade facilitated by three primary classes of enzymes: E1, E2, E3

what do E1, E2, and E3 do?

• E1 (Ubiquitin-activating enzyme): Binds and activates ubiquitin before transferring it to the next enzyme.

• E2 (Ubiquitin-conjugating enzyme): Receives the activated ubiquitin from E1.

• E3 (Ubiquitin ligase): Recognizes specific target substrates and facilitates the final transfer of ubiquitin onto the protein

diagram of ubiquitylation

what is proteolysis?

• the process of breaking down proteins into smaller polypeptides or amino acids by hydrolyzing their peptide bonds

• proteases hydrolyse peptide bonds

• irreversible change of structure and function

proteolysis diagram

what is an example of proteolysis?

prothrombin —> thrombin

what happens to prothrombin in proteolysis

• prothrombin is cleaved at two sites to convert into active thrombin (coagulation cascade)

• carried out by the prothrombinase complex

prothrombin proteolysis diagram

what is carboxylation?

• addition of a carboxyl group (-COOH) to specific amino acids in a protein

• used for activating proteins in blood clotting and bone development

what is an example of carboxylation?

• most common type of carboxylation involves changing the amino acid glutamate (Glu) into gamma-carboxyglutamate (Gla)

• prothrombin is rich in ɣ-carboxyglutamate (binds to Ca²⁺)

COLLAGEN AND HYDROXYPROLINE - an example of post-translational modification

• collagen is a protein that forms fibrous structures (abundant in skin, blood vessel walls, teeth)

• each collagen molecule consists of three polypeptide chains coiled together in triple helix

• each polypeptide chain is made up of repeating Gly-X-Y sequences

• X is often proline and Y is often hydroxyproline

how is hydroxyproline generated?

• through post-translational modification of proline

• carried out by the enzyme proline hydroxylase

• requires vitamin C (ascorbic acid) for activity