Hydrolysis 2 ,3 -cAMP

A critical component of the G-protein effector cascade is the hydrolysis of GTP by the activated a-subunit (GTPase). This provides not only a component of the amplification process of the G-protein cascade (63) but also serves to provide further measures of dmg efficacy. Additionally, the scheme of Figure 10 indicates that the coupling process also depends on the stoichiometry of receptors and G-proteins. A reduction in receptor number should diminish the efficacy of coupling and thus reduce dmg efficacy. This is seen in Figure 11, which indicates that the abiUty of the muscarinic dmg carbachol [51 -83-2] to inhibit cAMP formation and to stimulate inositol triphosphate, IP, formation yields different dose—response curves, and that after receptor removal by irreversible alkylation, carbachol becomes a partial agonist (68). 

Phosphodiesterase Inhibitors. Because of the complexity of the biochemical processes involved in cardiac muscle contraction, investigators have looked at these pathways for other means of dmg intervention for CHF. One of the areas of investigation involves increased cycHc adenosine monophosphate [60-92-4] (cAMP) through inhibition of phosphodiesterase [9025-82-5] (PDE). This class of compounds includes amrinone, considered beneficial for CHF because of positive inotropic and vasodilator activity. The mechanism of inotropic action involves the inhibition of PDE, which in turn inhibits the intracellular hydrolysis of cAMP (130). In cascade fashion, cAMP-catalyzed phosphorylation of sarcolemmal calcium-channels follows, activating the calcium pump (131). A series of synthetic moieties including the bipyridines, amrinone and milrinone, piroximone and enoximone, [77671-31-9], C22H22N2O2S, all of which have been shown to improve cardiac contractiUty in short-term studies, were developed (132,133). These dmgs... 

The tethering of PKA through AKAPs by itself is not sufficient to compartmentalize and control a cAMP/ PKA-dependent pathway. Cyclic AMP readily diffuses throughout the cell. Therefore, discrete cAMP/PKA signalling compartments are only conceivable if this diffusion is limited. Phosphodiesterases (PDE) establish gradients of cAMP by local hydrolysis of the... 

Increased lipid synthesis/inhibi-tion of lipolysis Activation of lipoprotein lipase (LPL)/induc-tion of fatty acid synthase (FAS)/inactivation of hormone sensitive lipase (HSL) Facilitated uptake of fatty acids by LPL-dependent hydrolysis of triacylglycerol from circulating lipoproteins. Increased lipid synthesis through Akt-mediated FAS-expression. Inhibition of lipolysis by preventing cAMP-dependent activation of HSL (insulin-dependent activation of phosphodiesterases )... 

Cyclic nucleotide phosphodiesterases (PDEs) are a class of enzymes that catalyze the hydrolysis of 3, 5 -cyclic guanosine monophosphate (cGMP) or 3, 5 -cyclic adenosine monophosphate (cAMP) to 5 -guanosine monophosphate (GMP) or 5 -adenosine monophosphate (AMP), respectively. 

Effector Gq/11 Preferentially increases Pi hydrolysis and elevates [Ca2+]j Gq/n Preferentially increases Pi hydrolysis and elevates [Ca2+]j (in recombinant systems) Gq/nPreferentially increases Pi hydrolysis and elevates [Ca2+], Intrinsic ligandgated ion channel GsPreferentially increases cAMP formation. 

A significant functional and structural feature of the plasma membrane Ca pumps is the presence of the calmodulin-binding subdomains A and B near the C-terminus (Fig. 3), that imparts calmodulin sensitivity on the Ca transport and ATP hydrolysis [3]. Adjacent to the calmodulin-binding region are two acidic segments (AC) and the P(S) region containing a serine residue that is susceptible to phosphorylation by cAMP-dependent protein kinase [34]. A unique feature of the plasma membrane Ca pump is its activation by acidic phospholipids that are presumed to... 

The effect of receptor stimulation is thus to catalyze a reaction cycle. This leads to considerable amplification of the initial signal. For example, in the process of visual excitation, the photoisomerization of one rhodopsin molecule leads to the activation of approximately 500 to 1000 transdudn (Gt) molecules, each of which in turn catalyzes the hydrolysis of many hundreds of cyclic guanosine monophosphate (cGMP) molecules by phosphodiesterase. Amplification in the adenylate cyclase cascade is less but still substantial each ligand-bound P-adrenoceptor activates approximately 10 to 20 Gs molecules, each of which in turn catalyzes the production of hundreds of cyclic adenosine monophosphate (cAMP) molecules by adenylate cyclase. 

FIGURE 50-5 A model for the transduction of odors in OSNs. The individual steps are detailed in the text. Note that several feedback loops modulate the odor response, including inhibition of the CNG channel by Ca2+ ions (purple balls) that permeate the channel, and a Ca2+/calmodulin (CaM) -mediated desensitization of the channel that underlies rapid odor adaptation. Several other mechanisms, including phosphodiesterase-mediated hydrolysis of the second messenger, cAMP, and phosphorylation of the OR by various kinases, have also been described. 

Even more efficient bimetallic cooperativity was achieved by the dinuclear complex 36 [53]. It was demonstrated to cleave 2, 3 -cAMP (298 K) and ApA (323 K) with high efficiency at pH 6, which results in 300-500-fold rate increase compared to the mononuclear complex Cu(II)-[9]aneN at pH 7.3. The pH-metric study showed two overlapped deprotonations of the metal-bound water molecules near pH 6. The observed bell-shaped pH-rate profiles indicate that the monohydroxy form is the active species. The proposed mechanism for both 2, 3 -cAMP and ApA hydrolysis consists of a double Lewis-acid activation of the substrates, while the metal-bound hydroxide acts as general base for activating the nucleophilic 2 -OH group in the case of ApA (36a). Based on the 1000-fold higher activity of the dinuclear complex toward 2, 3 -cAMP, the authors suggest nucleophilic catalysis of the Cu(II)-OH unit in 36b. The latter mechanism is comparable to those of protein phosphatase 1 and fructose 1,6-diphosphatase. 

In adipose tissue, insulin stimulation suppresses triglyceride hydrolysis (to free fatty acids and glycerol) by activating cAMP phosphodiesterase (cAMP PDE). Cyclic AMP, (3, 5 cAMP), is required to stimulate hormone sensitive lipase (HSL), the enzyme which hydrolyses triglyceride within adipocytes PDE converts active 3, 5 cAMP to inactive 5 AMP thus preventing the stimulation of HSL. The net effect of insulin on lipid metabolism is to promote storage. 

J. Fassberg, V. J. Stella, A Kinetic and Mechanistic Study of the Hydrolysis of Camp-tothecin and Some Analogues , J. Pharm. Sci. 1992, 81, 676-684. 

The excitatoiy amino acids (EAA), glutamate and aspartate, are the principal excitatory neurotransmitters in the brain. They are released by neurons in several distinct anatomical pathways, such as corticofugal projections, but their distribution is practically ubiquitous in the central nervous system. There are both metabotropic and ionotropic EAA receptors. The metabotropic receptors bind glutamate and are labeled mGluRl to mGluRB. They are coupled via G-proteins to phosphoinositide hydrolysis, phospholipase D, and cAMP production. Ionotropic EAA receptors have been divided into three subtypes /V-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionic acid (AMPA), and kainate receptors (Nakanishi 1992). 

The nucleotide cyclic AMP (3, 5 -cyclic adenosine monophosphate, cAMP) is a cyclic phosphate ester of particular biochemical significance. It is formed from the triester ATP by the action of the enzyme adenylate cyclase, via nucleophilic attack of the ribose 3 -hydroxyl onto the nearest P=0 group, displacing diphosphate as leaving group. It is subsequently inactivated by hydrolysis to 5 -AMP through the action of a phosphodiesterase enzyme. 

This enzyme [EC 3.1.4.17] catalyzes the hydrolysis of 3, 5 -cAMP or other 3, 5 cyclic nucleotides (cNMP) to... 

This protein kinase (known as protein kinase A or PK-A) has an R2C2 quaternary structure that binds 3, 5 -cAMP at its dimeric regulatory (R) subunit with resultant release of two catalytic (C) subunits. The free energy of hydrolysis of the cychc nucleotide activator is large (AG 13 kcal/mol) and allows the 3, 5 -cAMP to be virtually irreversibly converted to AMP by the action of a specific phosphodiesterase. This protein kinase, originally discovered by the Nobelists Edwin Krebs and Edward Fischer, is now considered to be the prototype for over two thousand members of the protein kinase superfamily. 

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