made from ATP by adenylate cyclase (AC); [cAMP] = 10-7 M, increased 2-100X upon AC activation
6 TM, cytoplasmic catalytic domain, two quasisymmetric regions req. for catalysis
alpha of Gs stimulates
alpha of Gi inhibits (cancel out Gs effect or lower from normal baseline levels)
beta-gamma can either stimulate or inhibit (different isoforms)
forskolin directly activates AC, cholera toxin indirectly through Gs
cytosolic Ca++ concentrations also control activity
destroyed by phosphodiesterases (themselves inhibited by xanthine derivatives like theophylline, caffeine)
turns on PKA, a tetramer of two catalytic (kinase, C) and two regulatory (inhibitory, R) subunits
tetramer is inactive until the R-subunit binds cAMP and releases the C-subunits
targets of PKA phosphorylation themselves are often kinases, so this forms a cascade
Regulation by cAMP
metabolic: lipid/carbohydrate metabolism, steroidogenic response to pituitary hormones
electrophysiological: phoshorylate ion channels, turning them on or off (esp Ca++)
gene expression: (CRE-cAMP sens reg element in 5’ flanking, PKA phoshorylates CREB protein)
cGMP as a Second Messenger
3 cGMP-Guanylate Cyclases(GC)
(1) intrinsic membrane-receptor for ANF (atrial natriuretic factor) (smooth muscle/renal, large extracellular domain,probably dimerizes)
(2) soluble, heme-binding GC regulated by NO (each dimer binds heme, NO increases activity)
(3) membrane protein regulated by Ca++ levels
phosphodiesterases—some passive, others regulated
rhodopsin activation of transducin (Gt) activates cGMP PDE for visual processing
many protein kinases and ion channels are mediated by cGMP
PKG: cGMP associated protein kinase (especially smooth muscle, regions of brain)
PKG dependent protein phosphetase activates Ca++-dependent K+ channels which cause hyperpolarization of plasma membrane, inactivation of voltage-dependent Ca++ channels, and reduction of cytoplasmic Ca++, leading to relaxation and vasodilation
since NO from endothelial cells (paracrine) regulates cGMP levels, nitroglycerine is a vasodilator
An Example: Visual Signal Transduction
retinal cells express a Na+/Ca++ channel gated by intracellular cGMP (open when high cGMP)
in the dark
:
no photoisomerization of rhodopsin or activation of transducin
less PDE so lots of cGMP; Na+/Ca++ channels open, causing depolarization
in light
:
activate rhodopsin/transducin
PDE on so cGMP down, channels close, cell is hyperpolarized
less inhibitory neurotransmitters released, suppressing electrical activity in retinal cell neuronal targets