Recognition of Hormones by Receptors

The receptors of the target cell can be divided into two classes the membrane-bound receptors and the soluble cytoplasmic or nuclear localized receptors (Fig. 3.8). 

Membrane-bound receptors are actually transmembrane proteins they display an extracellular domain linked to an intracellular domain by a transmembrane domain. Binding of a hormone to the extracellular side induces a specific reaction on the cytosolic side, which then triggers further reactions in the target cell. The mechanism of signal transmission over the membrane will be discussed in more detail in Chapters 5, 

8 and 11. Characteristic of signal transduction via membrane bound receptors is that the signaling molecule does not need to penetrate the target cell to activate the intracellular signal chain. 

In the case of intraceilularly localized receptors the hormone must enter the cell in order to be able to interact with the receptor. The hormone usually penetrates the target cell by passive diffusion. The nuclear receptors can be classified as ligand-con-trolled transcription activators. The hormone acts as the activating ligand the activated receptor stimulates the transcriptional activity of genes which carry DNA elements specific for the receptor. 

Hormones are usually produced by specialized cells and initiate a reaction in only a certain cell type. Only those cells that possess a cognate protein, the receptor of the hormone, can act as target cells. Receptors specifically recognize and bind the cognate hormone based on their chemical nature. The binding of the hormone to the receptor in the target cell induces an intracellular cascade of reactions at whose end lies a defined biochemical response. The pathway from receptor bound signaling molecule to final biochemical response is complex and occurs under the participation of many proteins. 

---