1. In studies trying to understand the regulation of hormone-sensitive adenylate
ID: 59304 • Letter: 1
Question
1. In studies trying to understand the regulation of hormone-sensitive adenylate cyclase, it was discovered that non-hydrolyzable GTP analogs and cholera toxin (that inhibits the GTPase activity of Gs) were able to activate the enzyme in a sustained (irreversible) profile, therefore indicating that GTP hydrolysis represents a deactivation (turn-off) regulatory step.
a) Will these analogs or toxin have any effect on adenylate cyclase activity in the absence of any added hormone?
b) If you add an factor that increases the rate of GTPase hydrolysis of the Gs protein, will this activate or inhibit the observed downstream effect upon addition of the hormone (this is in the absence of GTP analogs or toxin). Explain your answer.
Explanation / Answer
a. Analogs like cholera toxin act independent of the hormone. Hormones transmit signal across the membrane so as to activate the inactive g protein bound to GDP. Cholera toxin transfers ADP-ribose to P site of G-alpha-s subunit. This leads to inactivation of teh Galpha-s subunit which loses its catalytic activity, and so cannot hydrolyze GTP to GDP and pi. Thus, the activity is independent of hormones.
b. It enhances the downstream effect. If GTP hydrolysis increases, the inactive GDP is regained quick. As a result, less cAMP is formed. The GDP form again reaches the membrane, binds to the receptor, and is activated through the receptor binding to its ligand. Thus, a factor that increases rate of GTP hydrolysis decrease the downstream action, whereas any factor that decrease the rate of GTP hydrolysis increase
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