Background
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Nitric oxide is a physiological signaling molecule involved in a number of cellular processes. Some examples include:
- Relaxation of smooth muscle
- Neurotransmission and neuromodulation
- Inhibition of platelet aggregation and adhesion
- Killing of pathogens (Brown, 1995)
Mammalian cells synthesize nitric oxide from the amino acid L-arginine by nitric oxide synthases through the L-arginine-nitric oxide pathway (Figure 3) (Moncada et. al, 1993).
Nitric oxide synthases (NOS) are a class of enzymes that synthesize nitric oxide from the guanidino nitrogen atom of L-arginine in the presence of O2 and using the cofactors NADPH, FAD, FMN, and tetrahydrobiopterin (BH4).
Nitric
oxide synthase is functional as a homodimer (Figure 2). Each subunit of
NOS has a carboxyl-terminal reductase domain and an amino-terminal oxygenase
domain that contains a heme group. These domains are linked with a central
calmodulin-binding domain.
Figure 2: Nitric Oxide Synthase in its functional homodimeric form. Reproductive and Cardiovascular Disease Research Group
Each domain of NOS has a different function for enzyme activity, as is summarized in Table 1.
Table 1: Function of Nitric Oxide Synthase Domains
|
Domain
|
Function
|
|
Reductase
Domain
|
This
domain contains the FAD and FMN cofactors and is responsible for
transferring electrons from NADPH to the nearby oxygenase domain
of the other subunit of the dimer.
|
|
Calmodulin-Binding
Domain
|
Binding of calmodulin is required for NOS activity. This domain detects changes in intracellular calcium levels. |
|
Oxygenase
Domain
|
This domain contains the binding sites for BH4, heme, and arginine. This domain catalyzes the reaction that converts arginine into citrulline and NO. |
Visit Nitric Oxide Synthases for more information.
There are three different forms of nitric oxide synthase. For more information pertaining to the NOS isoforms visit www.answers.com.
-
Neuronal NOS (nNOS or NOS1) produces NO in the central and peripheral neuronal tissue.
- Inducible NOS (iNOS or NOS2) is found in the immune and cardiovascular systems. Macrophages use the oxidative stress of NO in the response to pathogens.
- Endothelial NOS (eNOS or NOS3) produces NO in blood vessels and regulates vascular function.
In the nitric oxide synthase reaction (Figure 3), the guanidino nitrogen of L-arginine undergoes a five-electron oxidation to yield L-citrulline and NO. Binding of calmodulin to the enzyme is thought to enable electron flow from flavin groups in the reductase domain to the heme. This electron flow is accompanied by conversion of O2 and L-arginine to NO and L-citrulline.
Figure 3: The reaction catalyzed by nitric oxide synthase. www.answers.com
The
steps of the NOS reaction pathway in relation to the different domains
and cofactors of the enzyme are shown in Figure 4. Electrons are donated
from NADPH to the reductase domain and then proceed via FAD and FMN
to the oxygenase domain. There the electrons interact with the heme
iron and BH4 at the active site in order to catalyze the reaction of
oxygen with L-arginine, generating L-citrulline and NO as products.
Electron flow through the reductase domain requires the presence of
bound calmodulin (Table 1) (Alderton et. al,
2001).
Figure 4: The NOS reaction and electron transfer between cofactors. (Alderton et. al, 2001)