Ca2+ / calmodulin

AMPK can also be activated by a Ca2+-mediated pathway involving phosphorylation at Thr-172 by the Ca2+/calmodulin-dependent protein kinase, CaMKK 3. CaMKKa and CaMKK 3 were discovered as the upstream kinase for the calmodulin-dependent protein kinases-1 and -IV they both activate AMPK in a Ca2+/ calmodulin-dependent manner in cell-free assays, although CaMKK 3 appears to much more active against AMPK in intact cells. Expression of CaMKKa and CaMKK(3 primarily occurs in neural tissues, but CaMKKp is also expressed in some other cell types. Thus, the Ca2+-mediated pathway for AMPK activation has now been shown to occur in response to depolarization in rat neuronal tissue, in response to thrombin (acting via a Gq-coupled receptor) in endothelial cells, and in response to activation of the T cell receptor in T cells. 

Benitez-King, G., Rios, A., Martinez, A. Anton-Tay, F. (1996). In vitro inhibition of Ca2+/calmodulin-dependent kinase n activity by melatonin. Biochim. Biophys. Acta 1290, 191-6. 

Bagni, C., Mannucci, L., Dotti, C. G., and Amaldi, F. (2000). Chemical stimulation of synaptosomes modulates alpha —Ca2+/calmodulin-dependent protein kinase II mRNA association to polysomes. J. Neurosci. 20, RC76. 

Takao, K., Okamoto, K., Nakagawa, T., Neve, R. L., Nagai, T., Miyawaki, A., Hashikawa, T., Kobayashi, S. and Hayashi, Y. (2005). Visualization of synaptic Ca2+/calmodulin-dependent protein kinase II activity in living neurons. J. Neurosci. 25, 3107-12. 

The binding of calcium ion to calmodulin, a major biochemical regulator of ion pumps and receptors, occurs on a time scale about a thousand times shorter than that observed for RNA conformational change. This Ca2+-calmodulin binding, which can be followed successfully by nuclear magnetic resonance (NMR), occurs in about ten milliseconds. 

Secondary signals Glucose 6-phosphate activates synthesis. Ca2+-Calmodulin activates degradation by activating phosphorylase kinase. 

Ca2+/calmodulin. The effect of calmodulin binding is to increase the affinity of the substrate Ca2+ site by 20- to 30-fold. This highly cooperative activation mechanism makes the PMCAs very sensitive to small changes in [Ca2+]j. A group of at least five PMCAs forms a multigene family. Three isoforms, PMCA1-3, occur in brain and each has a distinct distribution [20]. 

Ca2+,calmodulin-dependent May transiently associate with synaptic vesicles to phosphorylate synapsins and rabphilin-3A. May regulate various protein kinases I and II steps in neurotransmitter release. 

The Ca2+-calmodulin complex may also activate nitric oxide synthase (NOS), which binds to a PDZ domain of PSD-95. Activated NOS produces NO from arginine NO, in turn, activates guanylate cyclase, the enzyme that catalyzes the conversion of GTP to the intracellular messenger cGMP, which activates protein kinase G (PKG). 

In general, the 10 different forms of adenylyl cyclase can be divided into three major and two minor groups based on their functional attributes as well as on their sequence homology. The three major families are (1) the Ca2+/calmodulin-stimulated isoforms, which include AC1, AC3, and AC8 certain of these enzymes are inhibited by GPt (2) the G[ -stimulated isoforms, which include AC2, AC4, and AC7 and (3) the Ca2+ and Gai inhibited isoforms, which are AC5 and AC6. The other two categories each have a single member (4) AC9, which is the most divergent of the membrane-bound isoforms, is stimulated by G(IS, inhibited by calcineurin (a protein phosphatase), and is insensitive to forskolin (5) sAC, the only soluble isoform, is the most divergent and is most similar to the adenylyl cyclase found in Cyanobacteria. 

The concentration of Ca2+ required for half-maximal activation of AC1 is approximately 150mmol/l, while AC8 is five times less sensitive. The Clb domain of adenylyl cyclase appears to mediate the activation of the enzyme by Ca2+/calmodulin (see Fig. 21-3). Activation of AC1 by Ca2+/calmodulin can be blocked by a peptide fragment of the Clb portion of this enzyme. This peptide is also capable of binding Ca27calmodulin itself. 

Although AC5 and AC6 are not influenced by Ca2+/ calmodulin, these two forms of the enzymes are inhibited by free Ca2+. The concentrations of Ca2+ required for this inhibition are in the physiological range (0.1-1.0 mmol/1) that would be observed upon activation of voltage-sensitive Ca2+ channels in neurons [ 1 ]. The localization of AC5 and AC6 in the brain indicates that this form of Ca2+ influx is the primary means of Ca2+ regulation of these enzymes. 

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