3 ,5’-Cyclic AMP-dependent protein

The yeast enzyme is a homodimer of Mr2 X 102 500 and has 49% sequence identity to the muscle enzyme. The yeast enzyme is more simply regulated feedback inhibition by the allosteric inhibitor glucose-6-phosphate and activation by a 3 -5 -cyclic AMP-dependent protein kinase or a yeast phosphorylase that phosphorylates Thr-10.55... 

Cox ME, Deeble PE, Bissonette EA, Parsons SJ. Activated 3, 5 -cyclic AMP-dependent protein kinase is sufficient to induce neuroendocrine-like differentiation of the LNCaP prostate tumor cell line. J Biol Chem 2000 275 13812-8. 

Byus, C. V., Costa, M., Sipes, I. G., Brodie, B. B., and Russell, D. H., Activation of 3 5 -cyclic AMP-dependent protein kinase and induction of ornithine decarboxylase as early events in induction of mixed-function oxygenases, Proc. Natl. Acad. Sci. U.S.A., 73, 1241, 1976. 

Cyclic AMP (adenosine 3, 5 -cyclic monophosphate) is anotter secondary messenger that acts as an intracellular mediator for many different hormones, communicating the signal through the cyclic AMP-dependent protein kinase. This, in turn, phosphorylates other proteins at ine and threonine residues. Certain cell-surfece receptors act by increasing the concentration of intracellular cyclic AMP. A long-duration sudden increase of intracellular cyclic AMP takes place with cholera toxins in intestinal epithelial cells. Other cell-surfece receptors play the opposite role of decreasing the concentration of cyclic AMP. 

Some of the best known protein kinases (designated PKA or cAPK) are those that depend upon 3 , 5 -cyclic AMP (cAMP) as an allosteric effector. They are oligomeric proteins of composition R2C2, where R is a regulatory subunit and C is a catalytic subunit. Unless... 

A relative of the kinases is adenylate cyclase, whose role in forming the allosteric effector 3, 5 -cyclic AMP (cAMP) was considered in Chapter 11. This enzyme catalyzes a displacement on Pa of ATP by the 3 -hydroxyl group of its ribose ring (see Eq. 11-8, step a). The structure of the active site is known.905 Studies with ATPaS suggest an in-line mechanism resembling that of ribonuclease (step a, Eq. 12-25). However, it is Mg2+ dependent, does not utilize the two-histidine mechanism of ribonuclease A, and involves an aspartate carboxylate as catalytic base.906 All isoforms of adenylate cyclase are activated by the a subunits of some G proteins (Chapter 11). The structures907 of Gsa and of its complex with adenylate kinase905 have been determined. The Gsa activator appears to serve as an allosteric effector. 

Hormonal control of the activity of phosphorylase kinase. Just as the activity of phosphorylase is increased by phosphorylation, so is the activity of its phosphorylase kinase (which may be phosphorylated on two serine residues, one in an a subunit and one in a /3 subunit). Hormonal stimulation (/3-adrenergic) leads to the production of 3, 5 -cyclic AMP ( second messenger ), which stimulates the activity of the cyclic-AMP-dependent protein kinase that catalyzes the phosphorylation of phosphorylase kinase. 

Catabolite activator protein, CAP (also called cAMP receptor protein, CRP) is an activator required for high level transcription of the lac operon. The active molecule is a CRP dimer that binds 3 5 cyclic AMP to form a CRP-cAMP complex. CRP-cAMP binds to the lac promoter and increases the binding of RNA polymerase, stimulating transcription of the lac operon. CRP dimer without cAMP cannot bind to this DNA. The action of CRP depends upon the carbon source available to the bacterium. When glucose is present, the intracellular level of cAMP falls, CRP cannot bind to the lac promoter and the lac operon is only weakly transcribed. When glucose is absent, the level of intracellular cAMP rises, the CRP-cAMP complex stimulates transcription of the lac operon and allows lactose to be used as an alternative carbon source. 

Phosphorylase kinase is one of the best characterized enzyme systems to illustrate the role of calcium ions in regulation of intermediary metabolism. Phosphorylase kinase is composed of four different subunits termed a (Mr 145000), /3 (MT 128000), y (A/r 45000) and 5 (Mr 17000) and has the structure (a/3y8)A [106]. Only one of its four subunits actually catalyses the phosphorylation reaction the other three subunits are regulatory and enable the enzyme complex to be activated both by calcium and cyclic AMP. The y subunit carries the catalytic activity the 8 subunit is the calcium binding protein calmodulin and is responsible for the calcium dependence of the enzyme. The a and /3 subunits are the targets for cyclic-AMP mediated regulation, both being phosphorylated by the cyclic-AMP dependent protein kinase. Calmodulin appears to interact with phosphorylase kinase in a different manner from other enzymes, since it is an integral component of the enzyme. Phosphorylase kinase has an absolute requirement for calcium, and is inactive in its absence. 

Fig. 8. Known and potential interactions of the ACTH/adenylate cyclase/cyclic AMP-dependent protein kinase system in the adrenocortical cell with other hormones and intracellular messengers. Epinephrine activates adenylate cyclase in the adrenocortical cell [33]. Adrenocortical cells have receptors for several hormones which may activate G, including angiotensin II [34], acetylcholine [35], and endogenous opioid peptides [36]. Angiotensin II, acetylcholine and vasopressin [37-39] have all been demonstrated to activate the breakdown of PIP2 in adrenocortical cells and to stimulate steroidogenesis 5-hydroxytrypt-amine is also a known steroidogenic agent [40]. Probable receptor subtypes involved are indicated (/3 M (muscarinic) 5-HT, and V,). This is not a comprehensive diagramming of all stimuli or all possible interactions. Modified from Ref. 7.

Figure 5.3 shows how the second messenger, cAMP, formed fi om ATP by adenylyl cyclase, transmits the signal by activating a second-messenger-dependent serine/ threonine protein kinase, the cyclic AMP-dependent protein kinase A (PKA). 

An example of specific transcriptional control is cyclic AMP-dependent regulation of genes that have a cyclic AMP response element (CRE) through the action of the transcription factor CREB (cyclic AMP response element binding protein. Figure 12-5). 

Fig. 7. Adrenergic receptor subtypes and their coupled G proteins and effectors. AC, adenylyl cyclase c-AMP, cyclic adenosine-3, 5 -monophosphate DAG, diacylgly-cerol PKA, c-AMP-dependent protein kinase A PKC, protein kinase C PLC-p, phospholipase CP +, activation -, inhibition t, increase i, decrease.

Recently, studies in mammalian systems on the 3, 5 -cAMP-acti-vated protein kinases (Rubin and Rosen, 1975 Swillens and Dumont, 1976) have disclosed a new variant of the subunit model. The cAMP-dependent protein kinase that converts phosphorylase b to a, and glycogen synthase from I to D is composed of two subunits, one catalytic cally active and the other inhibiting the first. Cyclic AMP binds to the inhibitory subunit, causing its dissociation from the catalytic subunit, thereby permitting expression of the enzyme activity. 

Rosen, O. M., and Erlichman, J., 1975, Reversible autophosphorylation of a cyclic 3 5 -AMP-dependent protein kinase from bovine cardiac muscle, /. Biol. Chem. 250 7788. 

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