Question

Some patients show profound hypotension after abdominal surgery complicated by bacterial infections that produce endotoxins. Such hypotension is often refractory to treatment with conventional vasoconstrictor drugs. This hypotension may be caused by an overproduction of nitric oxide by the induced form of nitric oxide synthase (NOS). Administration of NOS inhibitors specific to this form might be an appropriate treatment for such patients. Nitric oxide synthase A. catalyzes a dioxygenase reaction. B. is similar to cytochromes P450 in binding zinc and tetrahydrobiopterin. C. accepts electrons from NADH. D. uses a flow of electrons from NADPH to FAD to FMN to heme-iron. E. is inhibited by \(\mathrm{Ca}^{+2}\)

Short answer

A. Nitric oxide synthase catalyzes a dioxygenase reaction B. Similar to cytochromes P450 in binding zinc and tetrahydrobiopterin C. Accepts electrons from NADH D. Uses a flow of electrons from NADPH to FAD to FMN to heme-iron E. Is inhibited by \(\mathrm{Ca}^{+2}\) Answer: A, D

Step-by-step solution

1. Understand the role of Nitric oxide synthase (NOS)

Nitric oxide synthase (NOS) is an enzyme responsible for the production of nitric oxide (NO), which is an important signaling molecule that plays various roles in the body, such as vasodilation, immune response, and neurotransmission. Knowing the proper functions and characteristics of NOS will help us choose the correct option.

2. Analyze each option

Let's analyze each option: A. Catalyzes a dioxygenase reaction: True. NOS does catalyze a dioxygenase reaction, as it uses oxygen molecules to convert L-arginine into NO (nitric oxide) and L-citrulline. B. Similar to cytochromes P450 in binding zinc and tetrahydrobiopterin: False. NOS binds to a heme group, not zinc. It does utilize tetrahydrobiopterin (BH4) as a cofactor, but cytochromes P450 mainly bind to P450 iron-sulfur proteins or cytochrome b5, not tetrahydrobiopterin. C. Accepts electrons from NADH: False. NOS accepts electrons from NADPH (Nicotinamide Adenine Dinucleotide Phosphate), not NADH (Nicotinamide Adenine Dinucleotide). D. Uses a flow of electrons from NADPH to FAD to FMN to heme-iron: True. NOS electron flow starts with NADPH, which transfers electrons to FAD (Flavin Adenine Dinucleotide), which then transfers to FMN (Flavin Mononucleotide), and finally to the heme-iron, where they engage in a reaction with molecular oxygen to produce NO. E. Is inhibited by \(\mathrm{Ca}^{+2}\) : False. NOS is activated by the binding of \(\mathrm{Ca}^{+2}\) -calmodulin complex, not inhibited.

3. Choose the correct option

Based on the analysis of each option, we can conclude that both option A and D are correct statements about nitric oxide synthase (NOS): A. Nitric oxide synthase catalyzes a dioxygenase reaction D. Nitric oxide synthase uses a flow of electrons from NADPH to FAD to FMN to heme-iron