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Old 04-02-2013
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Default BioChem question

Q. The binding of a ligand to a membrane bound receptor can result in conformational change receptor leading to activation of an enzyme such as adenylyl cyclase. This conformational change is initiated by serise of events that requires the binding of whic of the following nucleotides?
a) ADP

Why or why is it not ATP? Does adenyl cyclase increase ATP and CAMP through Gs g protien coupled receptor ?????
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Old 04-02-2013
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i think its GTP. the alpha subunit breaks away and does the needful on adenyl cyclase ?
or is it GDP?
the later effects are ATP - cAMP - PKA.
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Old 04-02-2013
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looks like there is a bit of the question stem missing, or not exactly complete. because it looks like its describing a G-protein. which of course, binds GTP on the alpha subunit, which dissociates and travels to an effector enzyme.

the alpha subunit also has intrinsic GTPase activity, which inactivates the G protein, and returns the alpha subunit back to its home with beta/gamma subunits.

(btw, like the post above, i sometimes get confused whether GDP or GTP binds when active. just remember that the alpha protein has intrinsic GTPase activity to switch it off.

Now you know that:

1) its GTP that binds to make it active.
2) the intrinsinc GTPase inactivates it (cleaves a phosphate off)
3) GDP is now left on it, which must get replaced by another GTP to become active again.
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