CAMP-dependent Protein Kinase A

Cheng Y, Zhang Y, McCammon JA (2006) How does activation loop phosphorylation modulate catalytic activity in the camp-dependent protein kinase a theoretical study. Protein Sci 15 672-683... 

Cyclic nucleotides are made in response to receptor activation. The receptor activates a G-protein that, in turn, activates adenylyl cyclase to make the cyclic nucleotide. To complete the signaling, the increase in cAMP concentration activates a specific protein kinase (serine/threo-nine), cAMP-dependent protein kinase (A kinase) (Fig. 9-7). To turn off the signaling pathway, the cyclic nucleotides are destroyed by enzymes called phosphodiesterases. These cleave cAMP to AMP. 

Insulin activates PFK-2 (via the tyrosine kinase receptor and activation of protein phosphatases), which converts a tiny amovmt of fructose 6-phosphate to fructose 2,6-bisphosphate (F2,6-BP). F2,6-BP activates PFK-1. Glucagon inhibits PFK-2 (via cAMP-dependent protein kinase A), lowering F2,6 BP and thereby inhibiting PPK-1. 

Second-messenger-dependent kinases, such as cAMP-dependent protein kinase A (PKA) and protein kinase C (PKC), are most often implicated in heterologous desensitization (123,129) however, the systems involved may vary between cell types (130). These protein kinases are associated with GPCR desensitization that... 

Affinity Labeling of Catalytic ATP Sites. Residues involved in ATP binding are potentially revealed by the use of affinity labels that are based on ATP s structure. Perhaps the most systematically studied of these compounds is 5 -fluorosulfonylbenzoyladenosine (5 -FSBA) (Figure 3a), which has been reported to label at least six kinases (32-A1). In the case of rabbit muscle pyruvate kinase such work has Indicated the presence of a tyrosine residue within the metal nucleotide binding site and an essential cysteine residue located at or near the free metal binding site (32). A similar reagent, 5 -FSBGuanosine, revealed the presence of two cysteine residues at the catalytic site of this same enzyme, both distinct residues from those modified by 5 -FSBA (33,34). With yeast pyruvate kinase both tyrosine and cysteine residues were modified by 5 -FSBA at the catalytic site ( ), and with porcine cAMP-dependent protein kinase a lysine residue was labeled at the active site (36). 

Fig. 7.2. Structure and substrate binding sites of Ser/Thr-spedfic protein kinases, a) Peptide binding site structure of the catalytic subunit of the cAMP-dependent protein kinase A from mouse, with bound inhibitor peptide PKI (5-22), shown in dark in the figure. PKI (5-22) is a fragment (amino adds 5-22) of the naturally occurring heat-stable protein kinase inhibitor PKI. The inhibitor peptide binds in the region of the substrate binding site between the two lobes of protein kinase A (Knighton et al., 1991). The P-loop is involved in binding the phosphate residue of ATP. b) ATP binding site structure of casein kinase I with bound Mg-ATP. The Mg is shown as a sphere. MOLSKRIPT representation according to Kraulis, (1991).

Variability in hormonal response patterns does not stop at the level of second-messenger synthesis. Thus, cyclic AMP can activate the well-known cAMP-dependent protein kinase A, but the possibility of other cAMP-respon-sive enzymes or cAMP-activated regulatory proteins should not be ruled out. The protein kinase activated by cAMP can activate a number of other enzymes. For example, in the liver, phosphorylase kinase (see fig. 24.15) is activated and catalyzes the breakdown of glycogen. In adipocytes, triacyl-glycerol lipase is activated and catalyzes the breakdown of triacylglycerols. 

Presynaptic Long-Term Plasticity Mediated by cAMP-Dependent Protein Kinase A (PKA)... 

Apart from exocytosis, presynaptic H3 autoreceptors also inhibit the synthesis of histamine at the level of nerve endings, at least in part through pathways distinct from those leading to the inhibition of release. One pathway is inhibition of adenylyl cyclase (Gomez-Ramirez et al. 2002 Moreno-Delgado et al. 2006) activation of cAMP-dependent protein kinase A by cAMP stimulates histamine synthesis through phosphorylation of histidine decarboxylase, and this stimulation is diminished when adenylyl cyclase activity decreases following activation of H3 autoreceptors and Gi/o proteins. 

Another second-messenger-dependent kinase which interacts with targeting proteins, is the cAMP-dependent protein kinase A. PKA interacts with AKAPs (A-kinase-associated proteins). AKAPs bind to the dimeric form of the regulatory subunit of PKA. They are multivalent linkers, which bind not only to PKA, but also to other kinases, such as PKC, and the Ca +Z calmodulin-dependent kinase II and the phosphatase PP2B (Fig. 7.6). 

Some ligand-activated membrane receptors transmit their signal by stimulating adenylate cyclase activity in the cell to produce cAMP. This activation pathway is mediated by a receptor-associated G protein called GS (Chapter 16). In mammals, the most common mechanism by which cAMP serves as a second messenger involves cAMP binding to the regulatory subunit of cAMP-dependent protein kinase A (PKA). Dissociation of the regulatory subunit allows the catalytic sub-... 

Pharmacomechanical mechanisms for relaxation include (1) G kinase-dependent increases in the activity of sarcoplasmic reticulum Ca pumps (SERCA). Ca pumps on the plasma membrane may also be stimulated (not shown). This increase in Ca sequestration and extrusion decreases [Ca +Jj and induces relaxation as shown in the foregoing. (2) Some agents appear to decrease 1,4,5-1P3 formation and may relax smooth muscle by decrease Ca + release (not shown) (3) Finally, increases in [cAMP] activate cAMP-dependent protein kinase (A kinase), which could phosphorylate myosin light chain kinase and decrease its Ca sensitivity (this mechanism has not been demonstrated in intact smooth muscle). 

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