G Protein-Coupled Receptors (GPCRs)

G protein-coupled receptors (GPCRs) are integral membrane proteins with seven transmembrane alpha-helices that detect extracellular ligands and transmit the signal across the membrane. Ligand binding stabilizes an active receptor conformation that engages heterotrimeric G proteins (such as Gs, Gi/o and Gq/11), which in turn regulate effectors such as adenylyl cyclase and phospholipase C and generate second messengers like cyclic AMP and calcium. Activated GPCRs are also phosphorylated and bind arrestins, which can end G protein signaling, drive receptor internalization and start separate signaling pathways. GPCRs mediate most physiological responses to hormones and neurotransmitters, and many endogenous peptides, including GLP-1, GIP, glucagon, oxytocin and the melanocortins, act through them.

GPCRs are the most intensively studied class of drug targets. An analysis published in 2017 found that about 34% of all drugs approved by the US FDA act at GPCRs, and nearly 50 approved peptide drugs target them. Examples include GLP-1 receptor agonists such as semaglutide, the dual GIP and GLP-1 receptor agonist tirzepatide, and setmelanotide, which activates the melanocortin-4 receptor.

A large share of approved peptide medicines work by binding a GPCR. Many natural peptide hormones signal through these receptors, so the usual drug discovery strategy has been to modify the natural peptide so that it activates the same receptor for longer or more selectively. Knowing the receptor a peptide targets is therefore the first step in understanding its likely effects, its side effects and how it compares with related drugs acting at the same or neighboring receptors.

GPCR signaling is more complex than a simple on-off switch. One receptor can couple to more than one type of G protein and can also signal through arrestins, and different ligands can favor different pathways, an effect called biased agonism. Newer peptide drugs are designed to activate several related receptors at once, such as GLP-1 and GIP receptors. These design choices can change both how well a drug works and which side effects appear.

Because GPCRs are found throughout the body, a single receptor type can influence several organs. Melanocortin receptors, for example, are involved in skin pigmentation, appetite, energy balance and sexual function, depending on the subtype and tissue. This is one reason claims about unapproved peptides acting on such receptors deserve caution: activating a widely distributed receptor can produce effects far from the intended target, and those effects are only reliably described in controlled human trials.

“A receptor either turns on or turns off”

GPCRs can adopt several active shapes and couple to different G proteins and to arrestins. Agonists can be full or partial, and some favor one signaling pathway over another. Two drugs acting on the same receptor can therefore produce different patterns of response, which is why they are compared in their own clinical trials rather than assumed to be equivalent.

“All peptide drugs act on GPCRs”

Many do, but not all. Insulin acts through a receptor tyrosine kinase, not a GPCR, and some peptide drugs act on enzymes, ion channels or other targets. Analyses of approved drugs show that GPCR-targeting peptides are a large group, mostly for metabolic disease and oncology, rather than the whole field.

“If a peptide binds a receptor in a lab test, it will work in people”

Receptor binding or activation in cell assays is an early step. Whether a peptide reaches the receptor in the body, how long it lasts and what the overall effect is depend on pharmacokinetics, tissue distribution and the wider physiology. Those questions are answered by animal studies and then by human clinical trials.

How do G protein-coupled receptors work?

A GPCR sits in the cell membrane with seven helices crossing it. When a hormone, neurotransmitter or drug binds on the outside, the receptor changes shape and activates a G protein on the inside. The G protein's alpha subunit swaps GDP for GTP and separates from its beta-gamma partner, and these pieces switch enzymes and ion channels on or off, producing messengers such as cyclic AMP or calcium. The signal ends when the G protein converts GTP back to GDP and when the receptor is phosphorylated and bound by arrestins, which can also pull the receptor into the cell.

Why are they called G protein-coupled receptors?

The name comes from the way these receptors signal. When activated, they interact with G proteins, which are named for their ability to bind guanine nucleotides (GTP and GDP). The activated G protein then switches on or off enzymes and ion channels inside the cell, producing messengers such as cyclic AMP or calcium.

What are the main types of GPCRs?

Human GPCRs are usually grouped into five families: rhodopsin (class A, by far the largest), secretin (class B), glutamate (class C), adhesion, and frizzled/taste2, known together as the GRAFS system. The 2003 genome analysis that proposed it identified more than 800 human GPCR sequences, many of them smell (olfactory) receptors. Peptide drugs mostly act at class A receptors, such as the melanocortin and oxytocin receptors, or at class B receptors, such as the GLP-1, GIP and glucagon receptors.

What share of drugs target GPCRs?

A 2017 analysis in Nature Reviews Drug Discovery found that 475 drugs, about 34% of all drugs approved by the US FDA, act at 108 different GPCRs. Many other GPCRs have no approved drug yet, and a separate 2020 review counted nearly 50 approved peptide drugs that target GPCRs.

Is the GLP-1 receptor a GPCR?

Yes. The GLP-1 receptor belongs to the class B (secretin-family) GPCRs, which bind peptide hormones such as GLP-1, GIP and glucagon. Activation of the GLP-1 receptor on pancreatic beta-cells raises cyclic AMP and enhances glucose-dependent insulin secretion, which is the main glucose-lowering action of GLP-1 receptor agonists.