Insulin Receptor - Ligand Binding

Ligand Binding

The insulin receptors endogenous ligands include insulin, IGF-I and IGF-II. The binding of ligand to the α-chains of the IR ectodomain induces a structural changes within the receptor leading to autophosphorylation of various tyrosine residues within the intracellular TK domain of the β-chain. These changes facilitate the recruitment of specific adapter proteins such as the insulin receptor substrate proteins (IRS) in addition to Src, Src Homology 2 - B (SH2-B), APS and proteins phosphatases including PTP1B, eventually promoting those downstream processes involving blood glucose homeostasis.

Strictly speaking the relationship between IR and ligand shows complex allosteric properties. This was indicated with the use of a Scatchard plots which identified that the measurement of the ratio of IR bound ligand to unbound ligand does not follow a linear relationship with respect to changes in the concentration of IR bound ligand, suggesting that the IR and its respective ligand share a relationship of cooperative binding. Furthermore, the observation that the rate of IR-ligand dissociation is accelerated upon addition of unbound ligand implies that the nature of this cooperation is negative; said differently, that the initial binding of ligand to the IR inhibits further binding to its second active site - exhibition of allosteric inhibition.

Although as of yet the precise binding mechanism of IR and its lignad has not been elucidated structurally, as identified using a systems biology approach, biologically relevant prediction of the IR-ligand kinetics (insulin/IGF-I) has been identified in the context of the currently available IR ectodomain structure.

These models state that each IR monomer possesses 2 insulin binding sites; site 1, which binds to the 'classical' binding surface of insulin: consisting of L1 plus αCT domains and site 2, consisting of loops at the junction of FnIII-1 and FnIII-2 predicted to bind to the 'novel' hexamer face binding site of insulin. As each monomer contributing to the IR ectodomain exhibits 3D 'mirrored' complementarity, N-terminal site 1 of one monomer ultimately faces C-terminal site 2 of the second monomer, where this is also true for each monomers mirrored complement (the opposite side of the ectodomain structure). Current literature distinguishes the complement binding sites by designating the second monomer's site 1 and site 2 nomenclature as either site 3 and site 4 or as site 1' and site 2' respectively.

As such, these models state that each IR may bind to an insulin molecule (which has two binding surfaces) via 4 locations, being site 1, 2, (3/1') or (4/2'). As each site 1 proximally faces site 2, upon insulin binding to a specific site, 'crosslinking' via ligand between monomers is predicted to occur (i.e. as or as ). In accordance with current mathematical modelling of IR-insulin kinetics, there are two important consequences to the events of insulin crosslinking; 1. that by the aforementioned observation of negative cooperation between IR and its ligand that subsequent binding of ligand to the IR is reduced and 2. that the physical action of crosslinking brings the ectodomain into such a conformation that is required for intracellular tyrosine phosphorylation events to ensue (i.e. these events serve as the requirements for receptor activation and eventual maintenance of blood glucose homeostasis).