Question

In the presence of active nitric oxide synthase but absence of substrate, A. electrons cannot be transferred to heme. B. calmodulin cannot bind \(\mathrm{Ca}^{2+}\). C. the oxygenase and reductase domains dissociate. D. \(c\) GMP production is increased. E. superoxide and/or hydrogen peroxide will be formed.

Short answer

A) Electrons will not be transferred to the heme group. B) Calcium ions will not bind to calmodulin. C) Oxygenase and reductase domains will dissociate. D) There will be an increase in cGMP production. E) Superoxide and/or hydrogen peroxide will be formed. Answer: E) Superoxide and/or hydrogen peroxide will be formed.

Step-by-step solution

Background information on Nitric oxide synthase (NOS)

Nitric oxide synthase (NOS) is an enzyme that catalyzes the production of nitric oxide (NO) from L-arginine. NOS contains an oxygenase domain, which binds to the substrate and cofactors, and a reductase domain, transferring electrons from NADPH to the heme group. Calcium ions play a vital role in NOS activity by binding to calmodulin, allowing it to activate NOS.

Evaluate statement A

Since NOS is active, it means electrons can be transferred to the heme group, even if the substrate is not present. Thus, statement A is incorrect.

Evaluate statement B

The absence of substrate has no direct relation to calmodulin's ability to bind \(\mathrm{Ca}^{2+}\). Calcium ions can still bind to calmodulin and activate NOS. Therefore, statement B is incorrect.

Evaluate statement C

The presence of active NOS indicates that oxygenase and reductase domains remain intact. They don't dissociate in the absence of substrate. Thus, statement C is incorrect.

Evaluate statement D

NOS catalyzes the production of nitric oxide (NO), which activates guanylate cyclase, increasing \(c\)GMP production. However, since the substrate is absent, NO will not be produced, and therefore there won't be an increase in \(c\)GMP production. Statement D is incorrect.

Evaluate statement E

In the absence of substrate but the presence of an active NOS enzyme, the enzyme will still receive electrons from NADPH and transfer them to the heme group, leading to the reduction of molecular oxygen to form ROS such as superoxide or hydrogen peroxide. Thus, statement E is correct. The correct answer to this exercise is E. In the presence of active nitric oxide synthase but absence of substrate, superoxide and/or hydrogen peroxide will be formed.