# Enzyme-linked receptor

An enzyme-linked receptor, also known as a catalytic receptor, is a transmembrane receptor, where the binding of an extracellular ligand causes enzymatic activity on the intracellular side. Hence a catalytic receptor is an integral membrane protein possessing both catalytic, and receptor functions.
They have two important domains, an extra-cellular ligand binding domain and an intracellular domain, which has a catalytic function; and a single transmembrane helix. The signaling molecule binds to the receptor on the outside of the cell and causes a conformational change on the catalytic function located on the receptor inside the cell.
Examples of the enzymatic activity include:
* Receptor tyrosine kinase, as in fibroblast growth factor receptor. Most enzyme-linked receptors are of this type.
* Serine/threonine-specific protein kinase, as in bone morphogenetic protein
* Guanylate cyclase, as in atrial natriuretic factor receptor

## Types

The following is a list of the five major families of catalytic receptors:
| Family | Member | Gene | Catalytic activity | Endogenous ligands | Synthetic ligands |
| Erb | ErbB1 (epidermal growth factor receptor) | EGFR | tyrosine kinase EC 2.7.10.1 | Epidermal growth factor, amphiregulin, betacellulin, epigen, epiregulin, HB-EGF, TGFa | GW583340, gefitinib, erlotinib, tyrphostins AG879 and AG1478 |
| Erb | ErbB2 | ERBB2 | " | | |
| Erb | ErbB3 | ERBB3 | " | NRG-1, NRG-2 | GW583340, gefitinib, erlotinib, tyrphostins AG879 and AG1478 |
| Erb | ErbB4 | ERBB4 | " | Betacellulin, epiregulin, HB-EGF, NRG-1, NRG-2, NRG-3, NRG-4 | GW583340, gefitinib, erlotinib, tyrphostins AG879 and AG1478 |
| GDNF (glial cell-derived neurotrophic factor) | GFRa1 | GFRa1 | " | GDNF > neurturin > artemin | |
| GDNF (glial cell-derived neurotrophic factor) | GFRa2 | GFRa2 | " | Neurturin > GDNF | |
| GDNF (glial cell-derived neurotrophic factor) | GFRa3 | GFRa3 | " | Artemin | |
| GDNF (glial cell-derived neurotrophic factor) | GFRa4 | GFRa4 | " | Persephin | |
| NPR (natriuretic peptide receptor) | NPR1 | NPR1 | guanylyl cyclase EC 4.6.1.2 | Atrial natriuretic peptide | |
| NPR (natriuretic peptide receptor) | NPR2 | NPR2 | " | C-type natriuretic peptide | |
| NPR (natriuretic peptide receptor) | NPR3 | NPR3 | " | Atrial natriuretic peptide | |
| NPR (natriuretic peptide receptor) | NPR4 | NPR4 | " | Uroguanylin | |
| trk neurotrophin receptor | TrkA | NTRK1 | tyrosine kinase EC 2.7.10.1 | Nerve growth factor > NT-3 | GW441756, tyrphostin AG879 |
| trk neurotrophin receptor | TrkB | NTRK2 | " | Brain-derived neurotrophic factor, NT-4/NT-5 > NT-3 | |
| trk neurotrophin receptor | TrkC | NTRK3 | " | NT-3 | |
| trk neurotrophin receptor | p75 | NGFR | " | NGF, BDNF, NT3, NT4/5 | |
| Toll-like | TLR1 | TLR1 | " | | |
| Toll-like | TLR2 | TLR2 | " | Peptidoglycan | |
| Toll-like | TLR3 | TLR3 | " | polyIC, polyinosine-polycytosine | |
| Toll-like | TLR4 | TLR4 | " | LPS, lipopolysaccharide derived from Gram-negative bacteria | |
| Toll-like | TLR5 | TLR5 | " | Flagellin | |
| Toll-like | TLR6 | TLR6 | " | | |
| Toll-like | TLR7 | TLR7 | " | | resiquimod, imiquimod |
| Toll-like | TLR8 | TLR8 | " | | |
| Toll-like | TLR9 | TLR9 | " | CpG, DNA enriched in cytosine:guanosine pairs | |
| Toll-like | TLR10 | TLR10 | " | | |