G-protein mechanisms and PKA phosphorylation
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| What are G-protein coupled receptors? | Integral membrane proteins
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| How are G-protein coupled receptors characterised? | By 7 transmembrane-spanning helices within their structure
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| The activity of ion channels (include K+, Cl-, Ca2+ channels) is mediated by what? | PKA-mediated phosphorylation
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| What allows modulation of transcription factors? | Translocation of PKA into the nucleus
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| How is a beta-adrenergic receptor coupled to adenylyl cyclase activation? | By the action of the alpha-subunit of the stimulatory G-protein called Gs
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| How is the original hormone signal amplified in trans-membrane signalling? | Each molecule of bound hormone can stimulate many Gs-alpha-subunits
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| What effect does epinephrine have in muscle tissue? | Converts glycogen to glucose
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| What do phosphodiesterases do? | They terminate the cAMP signal
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| How do phosphodiesterases terminate the cAMP signal? | By converting cAMP to its 5'-AMP metabolite
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| What is cAMP derived from? | ATP
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| Which enzyme catalyses this conversion? | Adenylyl cyclase
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| How is the activity of cAMP terminated? | By the hydrolysis of cAMP to 5'-AMP by specific cAMP phosphodiesterases
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| How is adenylyl cyclase activation switched off? | By the hydrolysis of GTP
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| As well as the adenylyl cyclase activation, what must also be switched off to return the cell to its resting, unstimulated state? | The hormone-receptor complex
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| What are the three types of G-protein subunits? | alpha, beta, gamma
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| Which subunit is specifically usually conferred by? | alpha subunit, because it contains the GTP-binding site and an intrinsic GTPase activity
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| What are the 4 alpha subfamilies, and how have they been identified? | Gs, Gi, Gq, and G12. Identified on the basis of their cDNA homology and function
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| How do G-protein coupled receptors bring about an intracellular response? | They recruit guanine nucleotide-binding proteins (G-proteins) via their third cytoplasmic loop, to couple their signal transduction elements
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| What two groups can G-proteins be split into? | Monomeric RAS-like G-proteins and heterotrimetric G-proteins
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| What second messenger are beta-adrenergic receptors coupled to? | cAMP
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| Give an example of a beta-adrenergic hormone | epinephrine
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| What does protein kinase A phosphorylate? | Target proteins on serine and threonine residues
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| What subunits does PKA comprise of? | Two regulatory subunits, and two catalytic subunits
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| How do ligands typically bind to G-protein coupled receptors? | By sitting in a pocket formed by the 7 intracellular transmembrane-spanning alpha helices
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| What does ligation of a receptor initiate? | A reaction with the GDP-bound heterotrimeric G-proteins
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| What does ligation of a receptor initiate? | The exchange of GDP for GTP (including a change in G-alpha)
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| What do G-protein coupled receptors generally comprise of? | -An extracellular N-terminus
-Seven transmembrane-spanning alpha helices (20-28 hydrophobic amino acids each)
-Three extracellular and intracellular loops
-An intracellular C-terminal tail
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| What two families of transcription factors can be modulated by the translocation of PKA into the nucleus? | cAMP-response-element-binding protein (CREB) and activation transcription factor (ATF)
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| What does the modulation of the transcription factors lead to? | Either induction or repression of expression of specific genes
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| How do G-proteins exist at the inactive site? | As heterotrimers with GDP bound to the alpha-subunit
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| How is downstream signalling activated by G-proteins? | Activated G-alpha or G-beta subunits can interact with one or more effectors to induce intracellular secondary messengers, which activate downstream signalling
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| How is the G-protein anchored to the plasma membrane? | By lipid modification of the gamma-subunits, and some of the alpha-subunits
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