ATPases phosphorylation

Vanadium. Vanadium is essential in rats and chicks (85,156). Estimated human intake is less than 4 mg/d. In animals, deficiency results in impaired growth, reproduction, and Hpid metaboHsm (157), and altered thyroid peroxidase activities (112). The levels of coen2yme A and coen2yme Q q in rats are reduced and monoamine oxidase activity is increased when rats are given excess vanadium (157). Vanadium may play a role in the regulation of (NaK)—ATPase, phosphoryl transferases, adenylate cyclase, and protein kinases (112). 

Hunter. C. W., Bigellow, D. J., and Squier, T, C- (1990). Lypophosphatidyl choline modulates catalylically motion of the Ca -ATPase phosphorylation domains. Biochemhtry 38(14), 4604-4612. 

Gensburger, C. Freyermuth, S. Klein, C. Malviya, A. N. In vivo nuclear Ca -ATPase phosphorylation triggers intermediate size molecular transport to the nucleus. Biochem. Biophys. Res. Commun. 2003, 303, 1225-1228. 

Aminophenol is a selective nephrotoxic agent and intermpts proximal tubular function (121,122). Disagreement exists concerning the nephrotoxity of the other isomers although they are not as potent as 4-aminophenol (123,124). Respiration, oxidative phosphorylation, and ATPase activity are inhibited in rat kidney mitochondria (125). The aminophenols and their derivatives are inhibitors of 5-Hpoxygenase (126) and prostaglandin synthetase... 

A minimal mechanism for Na, K -ATPase postulates that the enzyme cycles between two principal conformations, denoted Ej and Eg (Figure 10.11). El has a high affinity for Na and ATP and is rapidly phosphorylated in the presence of Mg to form Ei-P, a state which contains three oeeluded Na ions (occluded in the sense that they are tightly bound and not easily dissociated from the enzyme in this conformation). A conformation change yields Eg-P, a form of the enzyme with relatively low affinity for Na, but a high affinity for K. This state presumably releases 3 Na ions and binds 2 ions on the outside of the cell. Dephosphorylation leaves EgKg, a form of the enzyme with two... 

FIGURE 10.10 The reaction of tridated sodium borohydride with the aspartyl phosphate at the active site of Na, K -ATPase. Acid hydrolysis of the enzyme following phosphorylation and sodium borohydride treatment yields a tripeptide containing serine, homoserine (derived from the aspartyl-phosphate), and lysine as shown. The site of phosphorylation is Asp" in the large cytoplasmic domain of the ATPase. 

FIGURE 10.11 A mechanism for Na, K -ATPase. The model assumes two principal conformations, Ei and E9. Binding of Na ions to Ei is followed by phosphorylation and release of ADP. Na ions are transported and released and ions are bound before dephosphorylation of the enzyme. Transport and release of ions complete the cycle. 

FIGURE 10.13 Some of the sequence homologies in the nucleotide binding and phosphorylation domains of Na, K -ATPase, Ca -ATPase, and gastric H, K -ATPase. (Adapted from j0rgensm, P. L., and Andersen, J. R, 1988. Structnral basis for Ei - E2 confoyinational transitions in Na, K -pnmp and Cc -pnmp proteins. Journal of Membrane Biology 103 95-120)... 

Protein kinase A (PKA) is a cyclic AMP-dependent protein kinase, a member of a family of protein kinases that are activated by binding of cAMP to their two regulatory subunits, which results in the release of two active catalytic subunits. Targets of PKA include L-type calcium channels (the relevant subunit and site of phosphorylation is still uncertain), phospholam-ban (the regulator of the sarcoplasmic calcium ATPase, SERCA) and key enzymes of glucose and lipid metabolism. 

Sarcoplasmic calcium ATPase this enzyme utilizes the energy gained from hydrolysis of ATP to pump calcium from the cytosol into the stores of the sarcoplasmic reticulum. Its activity is negatively regulated by the closely associated protein phospholamban, and this inhibition is relieved upon phosphorylation of phospholamban by protein kinase A (PKA). 

Myosin-I molecules have several IQ sequences on or near the head and have light chains associated with them (Cheney and Mooseker, 1992 Cheney et al., 1993). Frequently, the light chains appear to be calmodulin molecules and some myosin-I molecules can bind three to four molecules of calmodulin at one time. Brush-border and adrenal myosin-I also bind calmodulin. Acanthamoeba myosin-I has a light chain that can be removed, in vitro, without adversely affecting the ATPase activity or the heavy chain phosphorylation (Korn and Hammer, 1988). The role of these calmodulin molecules in regulating myosin-I is complex and poorly understood. One possibility is that the calmodulin molecules dissociate from the heavy chains when calcium binds to the calmodulin, thereby imparting greater flexibility to the head of the myosin-I molecules. 

Vance, J.E., eds.), pp. 116-142, Benjamin/Cummings Publishing Co., Menlo Park, California. Senior, A.E. (1988). ATP synthesis by oxidative phosphorylation. Physiological Rev. 68, 177-230. Senior, A.E. (1990). The proton-translocating ATPase of Esherichia colt. Ann. Rev. Biophys. Chem. 19,7- 1. 

Apart from gastropods, harmful effects of TBT have also been demonstrated in oysters (Environmental Health Criteria 116, Thain and Waldock 1986). Early work established that adult Pacific oysters (Crassostrea gigas) showed shell thickening caused by the development of gel centers when exposed to 0.2 pg/L of TBT fluoride (Alzieu et al. 1982). Subsequent work established the no observable effect level (NOEL) for shell thickening in this, the most sensitive of the tested species, at about 20 ng/L. It has been suggested that shell thickening is a consequence of the effect of TBT on mitochondrial oxidative phosphorylation (Alzieu et al. 1982). Reduced ATP production may retard the function of Ca++ ATPase, which is responsible for the Ca++ transport that leads to CaCOj deposition during the course of shell formation. Abnormal calcification causes distortion of the shell layers. 

When smooth muscle myosin is bound to F-actin in the absence of other muscle proteins such as tropomyosin, there is no detectable ATPase activity. This absence of activity is quite unlike the situation described for striated muscle myosin and F-actin, which has abundant ATPase activity. Smooth muscle myosin contains fight chains that prevent the binding of the myosin head to F-actin they must be phosphorylated before they allow F-actin to activate myosin ATPase. The ATPase activity then attained hydrolyzes ATP about tenfold more slowly than the corresponding activity in skeletal muscle. The phosphate on the myosin fight chains may form a chelate with the Ca bound to the tropomyosin-TpC-actin complex, leading to an increased rate of formation of cross-bridges between the myosin heads and actin. The phosphorylation of fight chains initiates the attachment-detachment contraction cycle of smooth muscle. 

Effect of protein-bound Ca TpC 4Ca antagonizes Tpl inhibition of F-actin-myosin interaction (allows F-actin activation of ATPase) Calmodulin 4Ca activates myosin light chain kinase that phosphorylates myosin p-light chain. The phosphorylated p-light chain no longer inhibits F-actin-myosin interaction (allows F-actin activation of ATPase). 

ATP certainly fulfils the criteria for a NT. It is mostly synthesised by mitochondrial oxidative phosphorylation using glucose taken up by the nerve terminal. Much of that ATP is, of course, required to help maintain Na+/K+ ATPase activity and the resting membrane potential as well as a Ca +ATPase, protein kinases and the vesicular binding and release of various NTs. But that leaves some for release as a NT. This has been shown in many peripheral tissues and organs with sympathetic and parasympathetic innervation as well as in brain slices, synaptosomes and from in vivo studies with microdialysis and the cortical cup. There is also evidence that in sympathetically innervated tissue some extracellular ATP originates from the activated postsynaptic cell. While most of the released ATP comes from vesicles containing other NTs, some... 

Figure 13.2 Schematic representation of a possible ATP, purinergic, synapse. The effects of ATP, synthesised intraneuronally by mitochondrial oxidative phosphorylation from glucose, on various neuronal ATPases, are shown together with its actions as a conventional neurotransmitter acting at postsynaptic P2 and presynaptic Pj receptors...

A sequence of ten amino acids (ICS-D-KTGTLT) around the phosphorylation site of Na,K-ATPase (Asp ) is highly conserved among the Na,K-, H,K-, Ca-, and Id-pumps [6]. There is also homology with the subunit of FpATP synthetase of mitochondria and chloroplasts (see [6]) except that Asp is replaced by Thr. Accordingly a covalent phosphorylated intermediate is not formed in Fi-ATPase. Mutagenesis of the phosphorylated aspartate residue in Na,K-ATPase [82], Ca-ATPase [87], or H-ATPase [88] completely blocks activity. 

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