What are the types of GPCRs?

G-Protein Coupled Receptors (GPCRs) are classified into six types based on sequence and function. The six types are: Class A, Class B, Class C, Class D, Class E, and Class F. 

• Class A GPCRS are rhodopsin-like receptors. They have 719 members and make up for approximately 80% of the overall species. Its endogenous ligand is recognized by a ligand-binding-site at the 7TM region. This class is divided into subgroups including: peptide, lipid, protein, melatonin, steroid, sensory, and alicarboxylic acid. Its transmembrane forms a ligand-binding pocket and 8 helices with a palmitoylated cysteine at its C terminal. 
• Class B GPCRs consist of secretin receptors. Their ligands are recognized by both extracellular and 7TM domains. This class is divided into 2 subfamilies: secretin (B1) and adhesion (B2), which have 15 and 33 members respectively. Secretion members typically have traits of large extracellular domains and attach to parathyroid peptide hormone, growth hormone-releasing hormone, calcitonin gene-related peptide, and vasoactive intestinal peptide as some examples. The adhesion family has 9 subgroups, containing specific N-terminal motifs (e.g. cadherin, epidermal growth factor). 
• In class C GPCRs are metabotropic glutamate receptors. Their ligand-binding pocket exists in the extracellular domain which contains a venus flytrap module. This class contains 22 receptors which are broken down into 5 subfamilies including: 1 calcium-sensing receptor, 8 metabotropic glutamate receptors, 8 orphan GPCRs, 2 gamma-aminobutyric acid type B receptors, and 3 tase 1 receptors. The glutamate subfamily  is characterized by their large ECD and constitutive dimer for activation (of a receptor). 
• Class D GPRCRs consist of the fungal pheromone mating factors receptors (STE2 and STE3), which are involved in the response to mating factors on the cell membrane. The amino acid sequences of these receptors have a high quantity of hydrophobic residue divided into 7 domains.
• Class E GPCRs contain the cAMP receptors from an amoeba (Dictyostelium discoideum) which is involved in various chemotaxis signaling pathways. 
• The frizzled receptors of Drosophila make up Class F GPCRs. They are vital for regulating Wnt binding and hedgehog signaling, which are very common experiments used in biology to study cells.