Cyclic 3 , 5*-AMP

It now appears that many hormones (e.g. glucagon and adrenaline) in both animals and plants exert their effects by, as a first step, decreasing or increasing cyclic AMP within the cell. This may possibly occur by modification of the activity of the enzyme AMP cyclase which generates cyclic AMP from ATP. 

Taylor, S. S., Radzio-Andzelm, E. Cyclic AMP-dependent protein kinase. In Protein Kinases, Woodgett, J. R., editor, IRL Press, Oxford, 1994. 

Steinberg, R. A. A kinase-negative mutant of s49 mouse lymphoma cells is defective in posttranslational maturation of catalytic subunitof cyclic amp-dependent protein kinase. Mol. Cell Biol. 11 (1991) 705-712. 

Fig. 11. Dose—response curves for (A,A) inhibition of cyclic AMP formation and stimulation of IP formation by carbachol (A,D) before and (A,H) after reduction of receptor number by irreversible alkylation (carbachol) is in M. Error bars ( ) are shown for some studies.

Protamine kinase (from rainbow trout testes) [37278-10-7] [EC 2.7.1.70]. Partial purification by hydoxylapatite chromatography followed by biospecific chromatography on nucleotide coupled Sepharose 4B (the nucleotide was 8-(6-aminohexyl)amine coupled cyclic-AMP). [Jergil et al. Biochem J139 441 1974.]... 

CAP controls a number of operons, all of which are involved in the breakdown of sugar molecules and one of which is the lac operon. When the level of the breakdown products of lactose is low, the concentration of cyclic AMP in the cell increases and CAP is switched on, binds to its specific operators, and increases the rate of transcription of adjacent operons. 

Many biochemical and biophysical studies of CAP-DNA complexes in solution have demonstrated that CAP induces a sharp bend in DNA upon binding. This was confirmed when the group of Thomas Steitz at Yale University determined the crystal structure of cyclic AMP-DNA complex to 3 A resolution. The CAP molecule comprises two identical polypeptide chains of 209 amino acid residues (Figure 8.24). Each chain is folded into two domains that have separate functions (Figure 8.24b). The larger N-terminal domain binds the allosteric effector molecule, cyclic AMP, and provides all the subunit interactions that form the dimer. The C-terminal domain contains the helix-tum-helix motif that binds DNA. 

Weber, L, Steitz, T.A. The structure of a complex of catabo-lite gene activator protein and cyclic AMP refined at 2.S A resolution. /. Mol. Biol. 198 311-326, 1987. 

A number of kinase structures have been determined in various catalytic states. For example, structures of the cyclin-dependent kinase, CDK2, in its inactive state and in a partially active state after cyclin binding have been discussed in Chapter 6. The most thoroughly studied kinase is the cyclic AMP-dependent protein kinase the structure of both the inactive and the active... 

Depletion of ATP in the cells prevents maintenance of the membrane potential, inhibits the functioning of ion pumps, and attenuates cellular signal transduction (e.g., formation of second messengers such as inositol phos phates or cyclic AMP). A marked ATP depletion ultimately impairs the activ-itv of the cell and leads to ceil death. 

Cyclic AMP-dependent protein kinase is shown complexed with a pseudosubstrate peptide (red). This complex also includes ATP (yellow) and two Mn ions (violet) bound at the active site. 

FIGURE 15.7 Cyclic AMP-dependent protein kinase (also known as PKA) is a 150- to l70-kD R9C9 tetramer in mammalian cells. The two R (regulatory) subunits bind cAMP ( = 3 X 10 M) cAMP binding releases the R subunits from the C (catalytic) subunits. C subunits are enzymatically active as monomers. 

FIGURE 15.20 The adenylyl cyclase reaction yields 3, 5 -cyclic AMP and pyrophosphate. The reaction is driven forward by subsequent hydrolysis of pyrophosphate by the enzyme inorganic pyrophosphatase. 

Smooth muscle contractions are subject to the actions of hormones and related agents. As shown in Figure 17.32, binding of the hormone epinephrine to smooth muscle receptors activates an intracellular adenylyl cyclase reaction that produces cyclic AMP (cAMP). The cAMP serves to activate a protein kinase that phosphorylates the myosin light chain kinase. The phosphorylated MLCK has a lower affinity for the Ca -calmodulin complex and thus is physiologically inactive. Reversal of this inactivation occurs via myosin light chain kinase phosphatase. 

Earner, J., 1990. Insulin and the stimulation of glycogen synthesis The road from glycogen structure to glycogen synthase to cyclic AMP-dependent protein kinase to insulin mediators. Advances in Enzymology 63 173-231. 

There is some evidence to suggest that these drugs may owe their activity to inhibition of the enzyme that is responsible for hydrolysis of 3, 5 -cyclic AMP (itself a guanine derivative) and thus prolong the action of cyclic AMP. 

Draw the structure of cyclic adenosine monophosphate (cAMP), a messenger involved in the regulation of glucose production in the body. Cyclic AMP has a phosphate ring connecting the 3 and 5 hydroxyl groups on adenosine. 

Cyclic adenosine monophosphate (cyclic AMP), a modulator of hormone action, is related to AMP (Problem 29.24) but has its phosphate group linked to two hydroxyl groups at C3 and C5 of the sugar. Draw the structure of cyclic AMP. 

FIGURE 2.4 Occupancy-response curves for P-adrenoceptor agonists in transfected CHO cells. Occupancy (abscissae) calculated from binding affinity measured by displacement of [125I]iodocyanopindolol. Response measured as increases in cyclic AMP. Drawn from [3],... 

There are numerous second messenger systems such as those utilizing cyclic AMP and cyclic GMP, calcium and calmodulin, phosphoinosiddes, and diacylglerol with accompanying modulatory mechanisms. Each receptor is coupled to these in a variety of ways in different cell types. Therefore, it can be seen that it is impractical to attempt to quantitatively define each stimulus-response mechanism for each receptor system. Fortunately, this is not an... 

FIGURE 2.6 Production of cyclic AMP from ATP by the enzyme adenylate cyclase. Cyclic AMP is a ubiquitous second messenger in cells activating numerous cellular pathways. The adenylate cyclase is activated by the a subunit of Gs-protein and inhibited by the a-subunit of Gj-protein. Cyclic AMP is degraded by phosphodiesterases in the cell. 

FIGURE 2.17 Differential efficiency of receptor coupling for cardiac function, (a) Guinea pig left atrial force of contraction (inotropy, open circles) and rate of relaxation (lusitropy, filled circles) as a function (ordinates) of elevated intracellular cyclic AMP concentration (abscissae). Redrawn from [6]. 

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