# Enzyme-Linked Receptors --- **Enzyme-linked receptors**, or **tyrosine kinase receptors (RTK)**[^1] are a type of [[receptor proteins]] that bind with lipid-insoluble [[signaling molecule|signaling molecules]] and activate signal transduction in order to activate the [[signal response]]. The signal they activate involves kicking off a chain reaction called a *phosphorylation cascade*. [^1]:The most well-known examples of enzyme linked receptors are tyrosine kinase receptors (or, "receptor tyrosine kinase", so sometimes they are used interchangeably in these notes. But please note there are other types out there. Some notable enzyme-linked receptors are [[growth factors]], the receptors for [[insulin]] or [[fibroblasts]], and [[toll-like receptor|TLRs]] found on certain cells in our immune system. ## Structure These receptors have their receptors on the outside of the cell, and are attached to an [[enzyme]] on the inside of the cell. The receptor parts are incredibly varied, and the inside enzymes are incredibly conserved. ## The Phosphorylation Cascade The **phosphorylation cascade** is a process that amplifies and diversifies the initial signal, much like [[second messengers]] in [[g protein-coupled receptors]]. Step 1: The [[signaling molecule]] binds to two different membrane-bound receptors, which dimerizes them. Step 2: Step 1's conformational change allows the RTK to [[phosphorylation|phosphorylate]] itself using [[adenosine triphosphate]] inside the cell. Step 3: Some "bridging proteins" then link the RTK with a membrane-bound protein called "Ras". Step 4: The activated Ras triggers the [[phosphorylation]] and activation of a [[enzyme|protein kinase]]. Step 5: A "phosphorylation cascade" occurs, which amplifies and diversifies the signal. ![[enzyme linked transduction pathway.png]] %%this line intentionally left blank%%