Revision 7: Signal Transduction in Biological Membranes Flashcards
Explain how G-prot.s work
Guanine nucleotide binding proteins
GPCRs are bound to G-proteins
- agonist binds to G-protein in heterotrimeric form (alpha-betatgamma)
- >GDP released from alpha subunit and GTP binds instead (the agonist acts as a guanine nucleotide exchange factor -GEF)
- >affinity of receptor for G protein decreases and teh subunits are released -> alpha-GTP and beta-gamma
- > action eventually terminated by hydrolysis of GTP->GDP, so the heterotetramer reforms
light G-protein
receptor is rhodopsin, G protein is Gt, stimulates cyclic GMP phosphodiesterase, response is visual excitation
Toxins used in manipulation of G-protein cycle
Cholera (CTx) and Pertussus (PTx) toxins are ADP-ribosylate specific G-proteins
CTx: eliminate GTPase activity of G(alpha)s, leads to it becoming irreversibly activated
PTx: interferes w/ GDP/GTP exchange on G(alpha)i, leads to it becoming irreversibly inactivated
mutations affecting G proteins
Retinitis Pigmentosa: loss of function mutation to rhodopsin, causes severe visual impairment/blindness
Nephrogenic Diabetes Insipidus: loss of function mutation to V2 vasopressin receptor, ADH cannot bind to receptors, leads to less aquaporins and too much water being lost from the body
Familial Male Precocious Puberty: gain of function mutation (ie. receptor active w/o ligand) to Luteinising Hormone (LH) receptor, leads to very early onset of puberty in boys (sometimes as early as 1 year)
retinitis pigmentosa
loss of function mutation of rhodopsin
causes severe visual impairment/blindness
nephrogenic diabetes insipidus
loss of function mutation of V2 vasopressin receptor, ADH cannot act on these receptors in the kidney, less aquaporins, too much water loss
familial male precocious puberty
gain of function of LH receptor, leads to early (sometimes as early as 1 year) progression of puberty in boys
2nd messenger effects of adenyl cyclase, phospholipase C, cGMP phosphodiesterase
Ad. Cyc.: Gi inhibits, Gs stimulates, ATP->cGMP->cAMP-dependent Protein kinase -> phosphorylation -> glycogenolysis, GNG in liver, lipolysis in adipose, relaxation of SMCs, +ve ino and chronotopic effects of heart
Phospholipase C: activated by Gq, PIP2->IP3 ->ER -> release of Calcium
cGMP phosphodiesterase: found in photoreceptor cells of retina, activated and broken down by Gt (transducin)
Deactivation of GPCR pathways
1 when active, GPCRs are susceptible to variety of prot. kinases -> phosphorylation -> prevent more activation of G-proteins
2 active lifetime of alpha-GTP limited by cellular functions -> stim. GTPase activity
3 Cells have high energy enz.s -> metabolise 2nd messengers -> levels returned to basal state
4 enz. cascades -> activated downstream of 2nd messenger/prot. kinase activation -> oppose effect of GPCRs (-ve feedback)
regulation of chronotropy of the heart
ACh -> M2 receptors in SAN -> Gi -> inhibit Adenyl cyclase/(N)Adr -> Beta-1 -> Gs -> stimulate adenyl cyclase
Inhibited Ad. Cyc. -> more open K+ channels -> inc. PM permeability to K+ -> hyperpolarisation -> slowing intrinsic firing rate by decreasing gradient of funny current -> -ve chronotropic effect
regulation of inotropy of the heart
(N)Adr -> Beta (predominately 1) -> Gs -> inc. open probablity of VOCa2+Cs -> influx of calcium -> inc. inotrophic effect
Gs inc. probablity of VOCCs opening both directly and indirectly (+ ad. cyc. -> cAMP -> PKA -> phosphorylation and activation of VOCCs)
arteriolar vasoconstriction mechanism
(N)Adr -> alpha 1 -> Gq -> stimulate phospholipase C -> PIP2 -> IP3 (and DAG) -> release of calcium -> inc. contraction of SMCs in blood vessels
modulation of NT release
endogenous opioids/analgesics such as morphine -> presynaptic mu-opioid receptor stimulated -> Gi -> liberation G protein heterotrimer -> interact w/ VOCCs -> reduce calcium entry -> therefore less NT release
