Introduction
Glucose-dependent insulinotropic polypeptide receptor (GIP-R) is a G-protein coupled receptor stimulated by gastric inhibitory peptide (GIP). is released from endocrine cells in the small intestine and binds to GIP-R, which is commonly expressed in pancreatic ß-cells, adipose tissue, osteoblasts, and the hypothalamus. GIP-R was biochemically discovered in 1969 and its structure was later determined using cryo-electron microscopy.
The GIP-R plays a key role in mitigating Type II diabetes symptoms by promoting both weight loss and insulin secretion. Type II diabetes, sometimes referred to as insulin resistance, is caused by two variables: a decrease in the response of insulin-sensitive cells and a decrease in insulin secretion from beta cells. Insulin is required for glucose uptake in cells, lowering blood glucose levels. Without adequate insulin secretion, cells do not receive enough glucose, and blood glucose levels remain unhealthily high. Excess blood glucose is stored in the liver as glycogen and can be converted to fatty acids stored in adipose tissue.
GIP-R functions to alleviate Type II diabetes symptoms by using a GPCR to promote insulin secretion from beta cells. Increasing insulin secretion increases cellular glucose absorption. This results in lower blood glucose levels and less glucose being converted to fat. GIP-R can be activated naturally by the GIP ligand or a (dual) agonist such as Tirzepatide
Function & Mechanism
When the , a GPCR pathway is activated and a cascade effect is initiated. When the GIP ligand is bound, the beta-adrenergic receptor induces a conformational change, allowing for the G-alpha subunit to travel down the phospholipid bilayer, dissociating from the G-beta and G-gamma subunits. A palmitic fatty acid keeps the G-alpha subunit tightly bound to the lipid bilayer during its journey to the first enzyme, adenylyl cyclase. The phosphorylation of adenylyl cyclase produces the second messenger, cyclic adenosine monophosphate (cAMP). cAMP then activates the second enzyme, Protein Kinase A (PKA), which inhibits the potassium channel, and increases the calcium channels, allowing calcium to enter the cell. PKA also activates a transcription factor (CREB) that induces transcription allowing the release of insulin. The release of insulin into the bloodstream promotes the uptake of circulatory insulin into target tissues. This pathway reduces blood sugar levels in the body.
Structure
Transmembrane Domain
There are a couple of important interactions that occur with the binding of both the GIP and Tirzepatide ligands.
Specifically, with the GIP ligand, Y1 on the N-term is a crucial residue that helps the ligand bind to the receptor. Y1 hydrogen bonds to Q224 on the receptor allowing for a tight binding deep in the transmembrane. Y1 also has parallel pi-stacking with W296 on the receptor promoting further bonding of the ligand. Analogs of the GIP ligand were made without Y1, and no binding in the receptor occurred, validating the importance of this specific residue. Additionally, another important factor is I7 on the GIP ligand which is mutated in the Tirzepatide ligand. Being hydrophobic, I7 does not hydrogen bond to R190 on the receptor as it does in the Tirzepatide ligand.
Specifically, with the Tirzepatide ligand, Y1 still has the same parallel pi-stacking interaction with W296 of the receptor. Furthermore, as stated above, the seventh residue on the Tirzepatide ligand is mutated to a T7, which induces hydrogen bonding between R190, unlike in the GIP receptor. Due to this hydrogen bonding induction, and with additional water-mediated polarization, the alcohol group on Y1 of the ligand points towards R190 and Q220 causing some hydrophilic interactions. All of these interactions help bind the ligand tightly in the receptor for activation to occur.
Extracellular Domain
In the extracellular domain, there is extensive pi-stacking between both and . The aromatic residues F22 and W25 are conserved on Tirzepatide to maintain those strong hydrophobic interactions with Y36 and W39 on GIP-R. From the original GIP ligand, Tirzeptide has a H18A mutation. While this mutation does result in the loss of a hydrogen bond between the ligand and the receptor, it likely increases Tirzepatide’s affinity for the GLP-R. The I7T mutation on Tirzepatide allowed the ligand to bind deeper in the GIP-R active site. The deeper binding slightly strengthened the pi-stacking interactions present between Tirzepatide and GIP-R by decreasing the distance between the residues.
