BS2520 - Protein Structure and Function Lecture 7 Post-Translational Protein Modifications Types of post-translational modifications: AA F Gp Ala NH2 N-acetylation (N-terminus) Arg C(NH2) :NH deimination to citrulline, methylation Modification Asn CONH2 deamidation to Asp or iso(Asp), N-linked glycosylation Asp CH isomerization to isoaspartic acid Cys SH disulfide-bond, oxidation to sulfenic, sulfinic or sulfonic acid, palmitoylation, N-acetylation (N-terminus), S-nitrosylation Gln CONH2 Glu COOH cyclization to Pyroglutamic acid (N-terminus), gamma-carboxylation Gly NH2 N-Myristoylation (N-terminus), N-acetylation (N-terminus) His NH Phosphorylation Lys NH2 CH acetylation, Ubiquitination, SUMOylation, methylation, hydroxylation Met NH2 SH N-acetylation (N-terminus), oxidation to sulfoxide or sulfone Pro CH hydroxylation Ser OH Phosphorylation, O-linked glycosylation, N-acetylation (N-terminus) Thr OH Phosphorylation, O-linked glycosylation, N-acetylation (N-terminus) Trp CH mono- or di-oxidation, formation of Kynurenine Tyr OH sulfation, phosphorylation, glycosylation Val NH2 N-acetylation (N-terminus) cyclization to Pyroglutamic acid (N-terminus), deamidation to Glutamic acid or isopeptide bond formation to a lysine by a transglutaminase - All AA, except from glycine, have a side chain which can be subjected to modification - potential ones listed above. - There is also proteolysis which is the processing of a protein by cleavage. - PTM greatly increases protein diversity - also increases the complexity of the proteome. - There are approx. 400 known different types of PTMs in Biology. Proteolysis Why does this occur? - To introduce diversity - removal on the N-terminal methionine. - Protein translocation - delivery to or across membranes - via signal peptides which are then later removed. - Control of protein function - e.g. insulin and chymotrypsin - require proteolysis for final function to be revealed. - As a mediator in signalling pathways. Why are signal peptides important? - The signal sequence enables translocation to ER for further processing. - Translocation can be co or post-translational.
- The signal peptide (usually 16-30 AA long) is present at the N-terminal of most newly synthesised proteins that are destined towards the secretory pathway. Cytosol Endoplasmic reticulum (ER) P Signal sequence PTMs Protein translation CHS - They are the highly hydrophobic section - this is what inserts itself into the membrane of the ER and promotes entry of the entire protein into the ER. Cleaved protein Gl Function of insulin: - Insulin is biosynthesised as proinsulin which is then translocated to the ER and processed by proteolysis. - From the proinsulin, the free C peptide is removed to give insulin. - Proteases: - e.g. chymotrypsin - chymotrypsinogen as the inactive zymogen - first cleavage at Arginine15 but the 2 sections are still joined by a disulfide bridge - this gives pi chymotrypsin (active enzyme). - This auto-digests to give the final functional form - alpha chymotrypsin. - Changes in active site structure are minor but crucial for activity - so proteolytic modification is common and related to final protein activity and its regulation. Chymotrypsinogen is activated by specific cleavage of a single peptide bond: - Chymotrypsin is a digestive enzyme that hydrolyses proteins - it specifically cleaves peptide bonds on the carboxyl side of AA residues with