Question

1. Which of the following statements is correct about cAMP and signal transduction pathways?

cAMP promotes the degradation of adenylate cyclase.

cAMP modulates the activity of specific protein kinases.

cAMP acts to dissociate the hormone from the hormone-receptor complex.

The hormone stimulus leads to increased amounts of adenylate cyclase.

The hormone-receptor complex enters the cell and activates the adenylate cyclase.

2. Which of the following is characteristic of diacylglycerol (DAG)?

has inducible tyrosine kinase activity

converts cAMP to AMP

is cleaved by phospholipase C

activates protein kinase C

is derived from sphingolipids

Answer 1

The cAMP is a second messenger. When stimulus encounters its receptor, and receptor-ligand binding occur the G protein converted into active GTP form from its inactive eGDP form. G alpha subunit GTP will diffuse and activate adenylyl cycles. adenylyl cyclase will convert ATP into cAMP. Four cAMP required for the activation of PKA. because PKA has four cAMP binding site on its regulatory subunit. When the cAMP level in the cell increased by the action of adenylyl cyclase PKA activated. Catalytic subunit of PKA will released from regulatory and phosphorylate its targets including some proteins even inside the nuclease. There are different desensitization mechanisms are employed for the inactivation of GPCR like PDE will act to hydrolyze cAMP to AMP , Arrestin mediated and some intrinsic GAP activities, etc.   

cAMP, not mediate degradation of adenylyl cyclase. Adenylyl cyclase is the enzyme responsible for the conversion of ATP to cyclic AMP. There is a feedback mechanism that occurs by PDE but its not degradation of the enzyme by the product.  

There is no such activity like the dissociation of the receptor from cell membrane because all the receptors in the GPCR pathway are exposed on the cell membrane because its ligand is impermeable (proteins). Only the activated G protein subunit G alpha GTP is dissociated and interacts with adenylyl cyclase to activate it.

Hormone receptor interaction only leads to the activation of Adenylyl cyclase already present in the cell membrane towards the cytosolic side, not any synthetic pathways or more production.

Already mentioned that the ligand cant enters into the cell because the membrane is impermeable. So G protein alpha subunit will activate adenylyl cyclase.   

2) DAG activate protein kinase C (PKC). PIP2 on the membrane will cleaved by PLC and give IP3 and DAG, both will act as a second messenger. It doesn't have any tyrosine kinase activity. DAG only activate PLC not any effect on the conversion of cAMP to AMP. DAG will not cleaved. DAG itself is a cleaved product by PLC from PIP2 (mentioned in the figure down below). DAG is glycerol derived not from sphingolipid.  

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