PTM

Proteolytic cleavage

  • Carried out by proteases

  • Irreversible

  • Changes the tertiary/quaternary protein structure

  • Can activate/deactivate proteins depending on the protein

 

Example: protease activation - chymotrypsinogen and chymotrypsin

  • Zymogen is inactive when first secreted by the pancreas, this prevents premature digestion, for example prevents digestion of body proteins required for function

  • Inactive precursor is chymotrypsinogen

  • Activated by proteolytic cleavage. Produces 3 polypeptide chains and these are held together by disulphide bridges. The rearrangement and production of a quaternary structure orientates the catalytic triad correctly in the active site enabling nucleophilic attack and protein digestion

 

Lipidation

  • Attachment of hydrophobic groups/fatty acid chains to a protein

  • Allows membrane localisation

  • Changes how the molecule interacts with the membrane and other molecules, changes properties by introducing a hydrophobic group

 

Example: prenylation

  • Attachment of isoprene (conjugated 2 double bonds) containing polymers to cysteine residues of proteins

  • Farnesylation - 15C added

  • Geranylgeranylation - 20C added

  • Can be removed, the protein will no longer be anchored to the membrane, important in the regulation of protein localisation in the membrane due to its reversibility

  • Important in signalling

 

Example: myristylation

  • Irreversible attachment of a 14C fatty acid chain to the amino terminus of protein - generally a glycine residue

  • Irreversible

  • Protein targeting to membranes

 

Example: palmitoylation

  • Reversible attachment of fatty acid chain to cysteine residue

  • Localises proteins to the cytoplasmic leaflet of the plasma membrane

 

Example: GPI anchor

  • Protein attached through C terminus to carbohydrate groups and phosphatidylinositol group with various fatty acid groups

  • Common amongst adhesion molecules and membrane receptors

  • Helps membrane loacalisation

 

Phosphorylation

  • Addition of the terminal phsphate from ATP to a molecule

  • Covalent attachment

  • Very stable PTM

  • Done by kinases, specific, serine, threonine or tyrosine kinases, attach onto the OH of a side chain

  • Addition adds a large negative charge onto the molecule, this can induce large conformational changes on the protein which can be activating or deactivating to a protein depedning on what they change

  • Can produce binding sites for other proteins - such as SH2 domain proteins that can recognise the phosphprylated state of the protein due to the altered charge and electrostatic potential

  • Removed by phosphatatses, hydrolyse the covalent bond and revert the protein back to its original structure

  • Involved in protein kinase cascades

 

Example: Signalling and MAPK pathways

  • Ligand binfing to a RTK causes dimerisation of the intracellular domains, this causes transphosphorylation to occur

  • This produces a binding site for a SH2 domain proteins

  • Phsphprylates these and activats further downstream pathways

  • Example: phosphorylation of a transcirption factor activates it and allows it to bind to DNA asnd cause transcription of mRNA for a specific protein - eg enzyme for metabolism

 

Acetylation/deacetylation

  • Addition of acetly group to a protein, more specifically on a lysine residue

  • Alters the amino acids around it

  • Associated with histones, alters the reading of DNA

  • Done by acetyltransferases and reversed by deacetylases

 

Example: histone acetylation

  • Addition of acetyl group onto lysine residue causes the conversion of heterochromatin to euchromatin

  • Addition removes the +ve charge on lysine, therefore less attracted to the negatively charged DNA backbone, less attraction means increased accessibility by RNA polymerase for transcription

 

Methylation

  • Addtion of a methyl group, commonly onto arginine and lysine, can be signle, double or triple

  • Mehtylation remains the +ve charge on lysine

  • Importsnt for tranxriptional repression or activation

 

Example: mehtylation for repression

  • Produces binding sites for proteins that induce compaction of genes

  • Changes the conformation of the transcription factor binding site, reducing transcription and therefore expression