The absorption of UV light by nucleic acids arises from n ® p* and p ® p* transitions of the purine and pyrimidine bases that occur between 260nm and 275 nm.

TLS Online TPP Program

#Id: 9072

#Unit 4. Cell Communication and Cell Signaling

In resting state 

basal level of PDE activity keeps cAMP levels below those necessary for PKA activation.

TLS Online TPP Program

#Id: 9073

#Unit 4. Cell Communication and Cell Signaling

TLS Online TPP Program

#Id: 9074

#Unit 4. Cell Communication and Cell Signaling

Activation of β-adrenergic receptors causes activation of C subunits of PKA which phosphorylate PDE, stimulating its catalytic activity.

TLS Online TPP Program

#Id: 9075

#Unit 4. Cell Communication and Cell Signaling

Active PDE hydrolyzes cAMP, driving cAMP levels back to basal levels.

TLS Online TPP Program

#Id: 9076

#Unit 4. Cell Communication and Cell Signaling

A G protein– coupled receptor kinase phosphorylates serine and threonine residues in the

cytoplasmic segments of activated GPCRs

TLS Online TPP Program

#Id: 9077

#Unit 4. Cell Communication and Cell Signaling

β-Arrestin (Arr) binds to these phosphorylated receptors. Clathrin and AP2, two other proteins bound by β-arrestin, promote endocytosis of the receptor.