Neurology MCQ Answer 500

Figure 3. cAMP as a second messanger. cAMP is made from ATP by the enzyme adenylate yclase. CAMP is broken down by phosphodiesterase (PDE). The activity of adenylate cyclase can be modulated in by G-proteins. When stimulatory agonixts (As)bind with their receptors the stimulate G-proteins of the Gs class. This increases adenylate cyclase activity and cAMP levels. When inhibitory agonixts (Ai)bind with their receptors the stimulate G-proteins of the Gi class. This decreases adenylate cyclase activity and cAMP levels. cAMP exerts its effect by activating protein kinase A (PKA) which phosphorylate proteins, e.g. enzymes and pumps, and in turn increases or decreases their activity.

1. The relevant G protein (termed G_q), activated by a first-messenger-bound receptor, activates a plasma-membrane effector enzyme called phospholipase C.

1. This enzyme catalyzes the breakdown of a plasma-membrane phospholipid known as phosphatidylinositol bisphosphate, abbreviated PIP₂, to diacylglycerol (DAG) and inositol trisphosphate (IP₃).

2. Both DAG and IP₃ then function as second messengers but in very different ways.

2. DAG activates a particular protein kinase known as protein kinase C, which then phosphorylates a large number of other proteins, leading to the cell's response.

3. IP₃, does not exert its second messenger role by directly activating a protein kinase.

1. IP₃, after entering the cytosol, binds to calcium channels on the outer membranes of the endoplasmic reticulum and opens them.

2. Because the concentration of calcium is much higher in the endoplasmic reticulum than in the cytosol, calcium diffuses out of this organelle into the cytosol

3. Increases cytosolic calcium concentration.

4. This increased calcium concentration then continues the sequence of events leading to the cell's response to the first messenger

Category: Neurology MCQs