How many transmembrane alpha-helices are present in the G-protein coupled receptors?

(a) Two

(b) Four

(c) Five

(d) Seven

The correct answer is (d) Seven.

G-protein coupled receptors (GPCRs) are a large and diverse family of proteins that act as cell surface receptors. A defining characteristic of their structure is that they are comprised of a single polypeptide chain that threads its way back and forth across the lipid bilayer seven times.

The Serpentine Structure

This conserved structure is often called a serpentine receptor because of the way it snakes through the plasma membrane. These seven transmembrane segments are all alpha-helical in conformation. They are connected by loops of varying lengths, with the N-terminus of the protein on the outside of the cell and the C-terminus on the inside.

This seven-transmembrane architecture creates a specific pocket for signaling molecules (ligands) to bind on the extracellular side, and a surface for interacting with G-proteins on the intracellular side, allowing the receptor to transmit signals from outside to inside the cell.