Question

3. Think about what G-proteins look like before they bind to a G-protein-coupled receptor. These G-proteins would have...circle your answer GTP attached to them OR GDP attached to them (a) Think about what G-proteins look like after they bind to a G-protein-coupled receptor. These G-proteins would have...circle your answer GTP attached to them OR GDP attached to them (b) A molecule of GTP contains how many phosphate groups? (c) Think about a G-protein that has a GTP attached to it. During the process of signal transduction, something happens to one of the phosphate groups on the GTP molecule. Explain what this is. (d) Generally speaking, attaching a phosphate group to a protein would cause that protein to... Remain stuck in its current shape OR Change shape (e) In the signal transduction example that we discussed in class, an activated G-protein causes activation of an enzyme. Once activated, what does this particular enzyme do? (f) in the signal transduction example that we discussed in class, CAMP causes activation of an enzyme. Once activated, what does this particular enzyme do?

Answer 1

Ans 3: Before binding to the G-protein-coupled receptor the GDP is attached with the G-proteins. G-protein is inactive when bounded with the GDP.

The explanation for incorrect option: When G-protein coupled receptor is bounded with G-protein upon activation the GDP is replaced with GTP.

Ans (a): Once the G-protein binds with the G-protein coupled receptor the GDP is replaced with GTP and now G-protein has GTP attached to them.

The explanation for incorrect option: GDP is attached with G-protein before it's binding with G-protein coupled receptor. It is inactive in this state.

Ans (b): The full form of GTP is guanosine triphosphate. A molecule of GTP has three phosphate groups( tri means three).

Ans (c): During the process of signal transduction one phosphate group is released from the GTP and forms GDP (guanosine diphosphate) or we can say GTP is hydrolyzed to GDP.

Ans (d): Phosphorylation (attachment of phosphate group) causes a change in the shape of the protein. phosphorylation regulates the function of the proteins.

Ans (e): Activate G-protein causes the activation of the enzyme adenylyl cyclase that converts ATP (adenosine triphosphate) into cyclic AMP (adenosine monophosphate). Two phosphate groups are removed in this process as pyrophosphate. The cAMP is called the second messenger.

Ans (f): Binding of cAMP causes the activation of the Protein kinase A enzyme. This enzyme has regulatory and catalytic subunits. Binding of CAMP causes the dissociation of the regulatory subunits and catalytic subunits and thus makes the enzyme active.

Now catalytic subunits of protein kinase A will enter the nucleus and phosphorylates the transcription regulatory proteins thereby activating them. Then will lead to the expression of a particular gene.

Here is the sequence of steps for the signaling mediated through G-protein coupled receptor.

1. Initially, G-protein has not bounded with the G-protein coupled receptor and at this time it has bound GDP and it is inactive.
2. The first messenger that may be hormone or growth factor or neurotransmitter binds with the receptor and activates the receptor by causing a conformational change.
3. Now receptor will bind to the G-protein and GDP is replaced with GTP and G-protein becomes active.
4. This causes dissociation of the alpha subunit of the G-protein from the beta and gamma subunits.
5. This activated alpha subunit causes the activation of adenylyl cyclase that converts ATP into cyclic AMP.
6. This cyclic-AMP activates protein kinase A.
7. This activated protein kinase A enters in the nucleus and causes expression of genes by activating transcription factors.