Categories of Receptors

• Ion channels receptors

• Ligand gated ion channels

• Voltage gated ion channels

• Receptors that are linked to cytoplasmic enzymes

• Cytokine receptors

• Receptors that have intrinsic enzymatic activity (they are enzymes)

• Receptor tyrosine kinases

• Receptors that activate intracellular second messengers

• G protein coupled receptors

• Cell adhesion receptors

Cell Surface Receptors

Three kinds:

1. Enzyme-linked receptors (RTK)

2. G-protein coupled receptors (GPCR)

3. Ligand-gated ion channels

Enzyme-linked Receptors

• Found in all living species

• Extracellular domain binds signal

• Intracellular domain becomes functional catalyst

• Most are protein kinases

• Many receptors have intracellular domains with enzyme function

• Most are receptor tyrosine-kinase

• They phosphorylate tyrosine residues in selected intracellular proteins

• These receptors are activated by growth factors, thus being important in cell proliferation

Receptor Tyrosine Kinases

• Receptor tyrosine kinases: category of enzyme-linked receptors found in animals

• Recognize various types of signaling molecules

  • Growth factor – hormone that acts to stimulate cell growth or division

• example: Epidermal Growth Factor (EGF)

  • One function is to stimulate epidermal cells to divide

  • Functions in many different contexts

G-Protein Couples Receptors

• Found in all eukaryotes, common in animals

• 7 transmembrane segments

• Activated receptor activates G protein

• Releases GDP and binds GTP instead

• GTP causes G protein to dissociate

• α subunit and β/γ dimer interact with other proteins in a signaling pathway

• Signals binding to cell surface are “first messenger”

Intracellular Receptors

• Intracellular receptors: found in the cytosol or nucleus of target cells

• Small or hydrophobic chemical messengers can readily cross the membrane and activate receptors

• Examples of hydrophobic messengers are the steroid and thyroid hormones of animals

  • An activated hormone-receptor complex can act as a transcription factor, turning on specific genes