Question

predict what would happen if the tyrosines in the activation loop of insulin were mutated to phenylalanine

phenylalanines will likely be phosphorylated but insulin recptor will not be activated

phenyalanines cannot be phosphorylated and insulin would not be activated

phenylalanine will likely be phosphorylated and irs 1 will bind and be phosphorylated

phenylalanine cannot be phosphorylated but irs 1 will bind and be phosphorylated

Answer 1

Insulin receptor is a receptor tyrosine kinase family of receptors. It binds to insulin, a hormone that is involved in control of blood sugar levels. Insulin binds to two sites on each of the two receptors. This leads to crosslinking of the tow receptors by insulin, forming a dimer. Receptor dimerization will activate autophosphorylation of three tyrosine residues on the beta subunit of the receptor by tyrosine kinase.

The receptor tyrosine kinase domain is formed by two lobes N and C terminal lobes linked together by a hinge linker. They form the catalytic site of the kinase. It is at this site that ATP, bivalent cation and the substrate tyrosine binds. The tyrosine residues are present in an activation loop that is inactive when insulin is not bound to the receptor. One of the tyrosine in the receptor Tyr 1162 is bound to the active site of the kinase domain is hydrogen bonded to an aspartate residue, Asp 1132. This binding in the catalytic loop will cause autoinhibition of the kinase as this tyrosine competes with the protein substrates. However, when the tyrosine kinase activity is stimulated by receptor dimerization, all three tyrosine residues will be autophosphorylated. Autophosphorylation cause these tyrosine residues to move out of the active site. Further, Tyr 1163 is now hydrogen bonded to Arg1155. This bonding is required to stabilize the activation loop of the receptor, leading to this loop being in open activated configuration.

The autophosphorylation of the tyrosine kinases leads to formation of binding sites for SH2 or PHB containing domain partners in receptor tyrosine kinases. IRS1 (insulin receptor substrate 1) contains PHB domains and will bind to the phosphorylated tyrosine residues. IRS1 is phosphorylated upon activation and will then activate downstream signaling via the P13K pathway or ERK MAPK pathway.

If the tyrosines are replaced by phenylalanine, these residues cannot be phosphorylated by tyrosine kinase domain. These phenylalanine residues are not bound to the active site and hence, will not be phosphorylated. The tyrosine kinase activity of the receptor is specific for tyrosine kinase. There is no activation loop formed and hence, no auto phosphorylation of phenylalanine occurs. Hence, IRS1 recruitment will also be inhibited as phosphotyrosine residues are needed for their recruitment. This will lead to no insulin signaling.

Right choice: Phenylalanines cannot be phosphorylated and insulin would not be activated