Biological Chemistry - Berezov, T. T., & Korovkin, B. F. 1998

Hormones
Molecular mechanisms of hormonal signal transduction
Guanylyl cyclase messenger system

For quite a long time, cyclic guanosine monophosphate (cGMP) was regarded as the antipode of cAMP, and was credited with Functions opposite to those of cAMP. To date, however, a substantial body of evidence indicates that cGMP plays an independent role in regulating cellular functions. Specifically, in the Kidneys and intestines, it controls Ion transport and Water balance, whereas in The Heart Muscle, it serves as a relaxation signal, and so forth.

The Biosynthesis of cGMP from GTP is catalyzed by a specific guanylyl cyclase, analogous to the synthesis of cAMP:

Class="center">

* Liver Glycogen phosphorylase is also regulated by Hormones (epinephrine and Glucagon) as well as by Blood glucose concentration. Glucose, much like glucose-6-phosphate, acts as an allosteric inhibitor of phosphorylase a; binding of glucose (when blood glucose levels rise) to the allosteric site of active phosphorylase a induces a series of conformational changes, facilitating the activation of phosphorylase phosphatase a (see Fig. 10.1), which in turn leads to the dephosphorylation of phosphorylase a and its conversion into inactive phosphorylase b.

Fig. 8.8. Schematic representation of The Central Role of cAMP and protein kinase in the hormonal Regulation of Glycogen Synthesis and Breakdown.

Epinephrine-receptor complex: AC - adenylate cyclase, G - G-protein; C and R - catalytic and Regulatory Subunits of protein kinase, respectively; PhK - phosphorylase b kinase; Ph - phosphorylase; Glc-1-P - glucose-1-phosphate; Glc-6-P - glucose-6-phosphate; UDP-Glc - uridine diphosphate glucose; GS - glycogen synthase.

Four distinct forms of guanylyl cyclase are currently known: three are membrane-bound, and one is soluble, located in the Cytosol. Studies have shown that the membrane-bound forms (with a Molecular Weight of ~ 180,000) consist of three domains: a receptor domain localized on the outer surface of Cell/30.html">The Plasma Membrane, an intramembrane domain, and a catalytic component that is identical across different forms of the enzyme. Guanylyl cyclase has been identified in numerous Organs (heart, Lungs, kidneys, Adrenal Glands, intestinal endothelium, retina, etc.), indicating its widespread involvement in The regulation of intracellular METABOLISM mediated through cGMP. The membrane-bound enzyme is activated via specific receptors by short extracellular Peptides (18–20 amino acid residues)—most notably the atrial natriuretic peptide (ANP), as well as heat-stable toxins from Gram-negative Bacteria, and others. ANP, as is well known, is synthesized in the atrium in response to an increase in blood volume, is transported via the blood to the kidneys, where it activates guanylyl cyclase (thereby elevating cGMP levels), and promotes The excretion of Na and water. Vascular smooth muscle Cells also contain a similar receptor-guanylyl cyclase system, through which receptor-bound ANP exerts a vasodilatory effect, helping to lower blood pressure. In intestinal epithelial cells, the receptor-guanylyl cyclase system can be activated by bacterial endotoxins, which slow down intestinal water absorption and lead to diarrhea.

The soluble form of guanylyl cyclase (molecular weight 152,000) is a heme-containing enzyme composed of two subunits. Nitrovasodilators and free radicals—products of Lipid Peroxidation—participate in the regulation of this form of guanylyl cyclase. One of the best-known activators is the endothelium-derived relaxing factor (EDRF), which induces vascular relaxation. The active component and natural Ligand of this factor is nitric oxide (NO). This form of the enzyme is also activated by certain nitrovasodilators (such as nitroglycerin and nitroprusside) used in the Treatment of heart diseases; The breakdown of these drugs also releases NO.

Nitric oxide is generated from The amino acid Arginine through the action of a complex Ca²⁺-dependent mixed-function enzyme system known as nitric oxide synthase (NOS):

Upon interacting with the heme of guanylyl cyclase, nitric oxide promotes the rapid synthesis of cGMP, which decreases the force of cardiac contractions by stimulating ion pumps that operate at low Ca²⁺ concentrations. However, the action of NO is short-lived (lasting a few seconds) and localized strictly to the site of its synthesis. Nitroglycerin produces a similar yet more sustained effect, as it releases NO at a slower rate.

Evidence has been obtained indicating that most of the effects of cGMP are mediated by a cGMP-dependent protein kinase, designated as protein kinase G. This enzyme, widely distributed in Eukaryotic cells, has been isolated in pure form (molecular weight 80,000). It consists of two subunits: a catalytic domain with a sequence analogous to that of the C-subunit of protein kinase A (cAMP-dependent), and a regulatory domain similar to the R-subunit of protein kinase A (see above). Nonetheless, protein Kinases A and G recognize distinct protein sequences, thereby regulating the phosphorylation of Serine and Threonine OH groups on different intracellular Proteins and eliciting distinct biological effects.

The intracellular levels of the Cyclic NUCLEOTIDES cAMP and cGMP are controlled by respective phosphodiesterases, which catalyze their Hydrolysis into 5'-nucleotide monophosphates and differ in their affinities for cAMP and cGMP. A soluble calmodulin-dependent phosphodiesterase and a membrane-bound isoform not regulated by Ca²⁺ and calmodulin have been isolated and characterized.