Fundamentals of Biochemistry - A. A. Anisimov 1986

Integration and Regulation of Metabolism
Cyclic Nucleotides

Extracellular signals in the form of primary messengers—Hormones and Neurotransmitters, as well as certain other BIOLOGICALLY ACTIVE SUBSTANCES—upon reaching target Cells trigger The formation of intracellular secondary messengers, namely cyclic NUCLEOTIDES (cAMP and cGMP). An increase or decrease in the intracellular concentration of these substances leads to various metabolic shifts, depending on The Cell type and its current state. cAMP was first isolated and identified in 1958 by E. Sutherland, who was subsequently awarded the Nobel Prize for his research into the Properties and Functions of this compound. cAMP is formed from ATP with the participation of the enzyme adenylate cyclase:

Class="center">

Adenylate cyclase is a component of The cell membrane and consists of three subunits. The regulatory subunit is located on the outer surface of the cytoplasmic membrane and constitutes a part of the hormone receptor or is closely associated with it. The catalytic subunit, responsible for converting ATP into 3', 5'-AMP, is situated on the inner side of the membrane. The coupling subunit lies within the membrane thickness, transmitting signals from the receptor to the catalytic subunit (see Fig. 12.1). Phospholipids are essential Components of the coupling subunit; Treatment of the enzyme with phospholipases results in a loss of hormone sensitivity.

The catalytic subunit is active only when hormones interact with the receptor subunit. The activation of adenylate cyclase and the elevation of cAMP levels are induced by

catecholamines (adrenaline, noradrenaline), which function simultaneously as both hormones and neurotransmitters, many Other Hormones (ACTH, Glucagon, parathyroid hormone, vasopressin, etc.), and A number of biologically active substances such as Prostaglandins E₁ and E₂, histamine, cholera toxin, etc. Mg²⁺ and GTP serve as Cofactors for adenylate cyclase.

Unlike the preceding enzyme, guanylate cyclase is loosely bound to the cytoplasmic membrane. The reaction producing cGMP is activated by Mg²⁺, with Ca²⁺ enhancing this activation:

Furthermore, guanylate cyclase differs from adenylate cyclase in its insensitivity to most hormones. Certain Steroid Hormones, such as estrogens, are an exception. Other stimulators of adenylate cyclase likewise have no effect on guanylate cyclase. This enzyme is activated by acetylcholine (in the presence of Ca²⁺) and prostaglandins F_2α. The concentration of cGMP also increases under METABOLISM/18.html">The Influence of phytohemagglutinin (see Section 2.5.3). The Effect of Insulin on Tissues is similarly accompanied by an elevation in intracellular cGMP, through which insulin apparently mediates its action to some extent. However, the underlying mechanisms of this phenomenon and the reasons for these correlations remain insufficiently clear.

Phosphodiesterase 3', 5'-AMP and phosphodiesterase 3', 5'-GMP catalyze the hydrolytic Cleavage of cyclic nucleotides to form non-cyclic 3'-nucleoside monophosphates:

The regulatory function of cyclic nucleotides is executed through the activation of corresponding protein Kinases. These Enzymes consist of two subunits: a catalytic subunit and a receptor (or regulatory) subunit containing an allosteric cAMP-binding site. In the absence of cAMP, both subunits form a lowly active complex; upon cAMP binding, dissociation occurs, and the catalytic subunit achieves maximal activity.

Proteins phosphorylated by cAMP-dependent protein kinases are highly diverse and found in all cellular components: Chromatin proteins (Histones and non-histones), ribosomal and Membrane Proteins, numerous enzymes, etc. Nevertheless, mechanisms exist that determine the Specificity of phosphorylation for strictly defined proteins in any given situation. Such targeting is ensured by the Specific features of Cell Differentiation, The Nature of the external signal, and the functional state of the cell.

The accessibility of a given protein to a protein kinase depends on its conformation, which is influenced by ion composition and concentration, the presence of various regulators, etc. cGMP-dependent protein kinases also consist of catalytic and Regulatory Subunits. The presence of these protein kinases in the cell suggests that cGMP acts as an independent intracellular messenger, separate from cAMP. Consequently, for physiological and biological processes, The ratio of their concentrations is far more significant than the concentration of either cyclic nucleotide alone. Protein phosphorylation by protein kinases typically activates these proteins (e.g., enzymes) or triggers other biochemical effects, such as Gene activation or Changes in membrane permeability. Phosphorylated Proteins can be rapidly dephosphorylated via Hydrolysis involving phosphoprotein Phosphatases, which thereby counteract the activating effect of protein kinases.

Although cAMP and cGMP are regarded as independently functioning regulators of cellular metabolism, they frequently exhibit antagonistic and reciprocal relationships: 1) cGMP (like adrenaline) stimulates Heart contractions, whereas cGMP (like acetylcholine) inhibits them; 2) cAMP (like adrenaline) suppresses smooth Muscle contraction (in the intestine, Uterus, and Blood Vessels), whereas cGMP (or acetylcholine) activates it; 3) cAMP inhibits cell proliferation, while cGMP stimulates it; 4) cGMP, similar to acetylcholine, promotes the depolarization of postganglionic Neurons, whereas cAMP induces their hyperpolarization.

Alongside this, instances of unidirectional action of cAMP and cGMP are also known: 1) pancreatic amylase secretion is stimulated to approximately the same extent by both cAMP and cGMP; 2) within specific concentration ranges, both nucleotides stimulate Glycogenolysis.

Among the biochemical and physiological effects induced by cAMP, aside from those already listed, the following also merit attention: 1) activation of lipolysis and Glycogen phosphorolysis; 2) stimulation of the secretion of insulin, prostaglandins, Calcitonin, and gastric Hydrochloric acid; 3) effects on Protein Synthesis (see Section 5.4); 4) inhibition of cell growth; 5) inhibition of erythrocyte aggregation.

cAMP easily diffuses from cells into various Body Fluids—blood, CEREBROSPINAL FLUID, gastric juice—and participates in a variety of vital processes: it increases membrane permeability, takes part in corticosteroidogenesis and The Development of adaptive responses, influences all classes of metabolic pathways, mediates neuromuscular excitation transmission, and contributes to Central Nervous system functions. It is believed that such diverse effects of the adenylate cyclase system are realized through the formation of third-order messengers (second messengers)—various biologically active substances.

In recent years, accumulating evidence highlights the crucial role of Ca²⁺ in regulating cellular metabolism in connection with the action of cyclic nucleotides, hormones, and neurotransmitters.

It has been suggested (M. A. Fedorov, 1979) that Ca²⁺ is the primary and more evolutionarily ancient intracellular mediator of various factors acting on the cell, with The Role of cAMP often consisting mainly in stimulating the influx of Ca²⁺ from the extracellular medium and Mitochondria. Under the Influence of External stimuli, the intracellular concentration of Ca²⁺ can change rapidly and substantially (from 10^-5 to 10^-8 M). It is quite plausible that the intracellular regulatory system (at least in certain cases) can be represented as a chain of three messengers: hormones → cyclic nucleotides → Ca²⁺.

Cyclic nucleotides have been discovered in many animal species, Bacteria, and a number of unicellular organisms. In prokaryotes, cAMP plays a vital role in regulating Protein Biosynthesis (see Section 5.4). In certain myxomycetes (slime Molds), cAMP acts as a unique hormone and intercellular chemical signaling molecule. It can be synthesized during the initial phase of myxomycete development when food reserves are depleted. Individual myxomycetes move toward the original source of cAMP, aggregate, form fruiting bodies, and produce spores.

The presence of cyclic nucleotides in higher plants has not been conclusively proven. However, the detection by some authors of adenylate cyclase activity and cAMP-dependent protein kinases in higher plants suggests caution against taking a categorical stance on this issue.