Debbie Hay-2

Question 1

What are the general structural features of G-proteins?

Answer 1

Has three subunits (and alpha and a beta/gamma dimer)
The inactive form has a GDP bound to the alpha subunit
The active state has GTP bound and triggers dissociation of the alpha unit from the beta/gamma dimer

Question 2

What is the G-Protein cycle?

Answer 2

Begins with inactive receptor and unactivated G protein which is anchored in the membrane
The receptor is activated by a hormone causing a conformational change in the receptor allowing interaction between the receptor and the G-Protein with the receptor binding to the alpha subunit
this opens up the nucleotide binding site on the G protein allowing for exchange of GDP for GTP and for the alpha subunit to dissociate from the beta/gamma dimer
The alpha subunit then diffuse to a downstream effector
The effector then propagates the signal
The signal is eventually terminated by the slow GTPase activity of the alpha subunit allowing the alpha unit to reassociate with the beta/gamma dimer

Question 3

What is the role of adenylate cyclase in cell signalling?

Answer 3

Family of enzymes that form cAMP
Is a downstream effector protein for Galpha(i) and Galpha(s) as it can bind these proteins by interacting with the same component of the G proteins as the beta/gamma dimer
Changes in cAMP can have many downstream effects including activation of protein kinase A

Question 4

How does adenylate cyclase form cAMP?

Answer 4

ATP is an adenosine molecule which has three terminal Phosphates in the 5’ position
Adenylate cyclase removes two of these phosphates and forms a diester bond which connects the phosphate group from the 5’ to 3’ position
The phosphodiester bond is broken by cAMP phosphodiesterase forming 5’AMP allowing inactivation of this second messengers

Question 5

What is protein kinase A?

Answer 5

A enzyme composed of 2 regulatory and 2 catalytic chains
The binding of cAMP to both of these regulatory chains causes release of the catalytic units which phosphorylate serine and threonine residues on target proteins including glycogen phosphorylase kinase, glycogen synthase and cREBP (cAMP response element binding protein) transcriptional activation protein

Question 6

What is phospholipase C?

Answer 6

This is a pathway stimulated by Galpha(q)
This binds to the beta form pf phospholipase C and activates it
This enzyme catalyses the Cleavage of PIP2 (Phosphatidylinositol-4,5-bisphosphate) found in cell membranes resulting in the formation of DAG (Diacylglycerol) and IP3 (inositol-1,4,5-triphosphate)
Soluble IP3 diffuse through the cell and binds to receptors in the ER membrane which act as calcium channels triggering the release of Ca2+ into the cytosol allowing regulation of many cellular processes
DAG remains in the membrane where it can activate protein kinase C which phosphorylates serine and threonine residues in target proteins also aiding in increasing intracellular proteins
IP3 and DAG are readily converted into other species via phosphorylation and are therefore present only transiently