Membrane receptors

Question 1

4 main receptor types

Answer 1

1. Ligand-gated ion channels (ionotropic) eg. nAChR
2. G-protein coupled receptors (metabotropic) eg. mAChR
3. Kinase-linked receptors eg. RTKs, cytokine receptors
4. Nuclear receptors eg. oestrogen receptor

Question 2

G-protein coupled receptors (GPCRs)

Answer 2

• largest family of membrane receptors (~800)
• mediate effects of many hormones and neurotransmitters eg. mACh
• 7 transmembrane domain
• targeted by 40% of drugs on the market

Question 3

Enzymes of phosphorylation

Answer 3

Kinases add phosphate, phosphatases remove phosphate.

Question 4

Receptor tyrosine kinases (RTKs)

Answer 4

Upon binding of a first messenger (ie. growth factor), these receptors dimerise and are cross-phosphorylated. They are catalytically active, triggering phosphorylation of cytosolic RAS proteins. These act as second messengers, inducing the transcription of genes.

Question 5

Ga receptors

Answer 5

Upon binding of a first messenger, the alpha subunit of the G-protein is phosphorylated. Gas receptors stimulate the conversion of adenylyl cyclase (AC) to cyclic AMP, while Gai receptors are inhibitory. The cAMP acts as a second messenger to activate protein kinase A (PKA).

Question 6

Gq receptors

Answer 6

Upon binding of a first messenger, the alpha subunit of the G-protein is phosphorylated. This activates phospholipase C (PLC) to convert PIP2 into IP3 and DAG, which both act as second messengers. IP3 stimulates the release of Ca2+ from the ER, while DAG activates protein kinase C (PKC).