Y-phosphate

Figure 13.5 A Mg + atom links GTP to the Ras protein. Mg Is coordinated to one oxygen atom each from the p and y phosphates of GTP as well as to the side chains of Set 17 and Thr 35 of Ras. Two water molecules complete the octahedral coordination of Mg.

In both structures the ion is coordinated to six ligands with octahedral geometry. Four water molecules as well as the side chain oxygen atom of a serine residue from the P-loop and one oxygen atom from the (3-phosphate bind to Mg + in the GDP structure. Two of the water molecules are replaced in the GTP structure by a threonine residue from switch I and an oxygen atom from the y phosphate (similar to the arrangement shown in... 

Figure 13.10 Rearrangements of the hydrogen bond network between strands 1, 2, and 3 in the p sheet of Go. as a consequence of the switch from the GDP (blue) to the GTP (green) conformation. Strand P3 pulls away from pi and disrupts two hydrogen bonds in order to bring Gly 199 into contact with the y-phosphate of GTP. As a consequence new hydrogen bonds are formed between P2 and P3. (Adapted from D. Lambright et al.. Nature 369 621-628,...

Figure 13.12 A concerted mechanism for GTP hydrolysis by Ga in which transfer of a proton to the y phosphate is coupled to deprotonation of the attacking water by Gin 200. (Adapted from J. Sondek et al., Nature 372 276-279, 1994.)...

The transport of each COg requires the expenditure of two high-energy phosphate bonds. The energy of these bonds is expended in the phosphorylation of pyruvate to PEP (phosphoenolpyruvate) by the plant enzyme pyruvate-Pj dikinase the products are PEP, AMP, and pyrophosphate (PPi). This represents a unique phosphotransferase reaction in that both the /3- and y-phosphates of a single ATP are used to phosphorylate the two substrates, pyruvate and Pj. The reaction mechanism involves an enzyme phosphohistidine intermediate. The y-phosphate of ATP is transferred to Pj, whereas formation of E-His-P occurs by addition of the /3-phosphate from ATP ... 

Tlie Na+/K+-ATPase belongs to the P-type ATPases, a family of more than 50 enzymes that also includes the Ca2+-ATPase of the sarcoplasmic reticulum or the gastric H+/K+-ATPase. P-Type ATPases have in common that during ion transport an aspartyl phos-phointermediate is formed by transfer of the y-phosphate group of ATP to the highly conserved sequence DKTGS/T [1]. 

Tyrosine kinase inhibitors interfere with the function of tyrosine kinases that catalyze the transfer of the y-phosphate group of ATP to tyrosine residues of protein substrates. Tyrosine kinases can be subdivided into two large families, receptor tyrosine kinases (RTKs) and non-receptor tyrosine kinases (NRTKs see corresponding chapter). The human genome... 

Possible modes of regulation of filament assembly may be anticipated from the basic properties of actin. We have shown that the tightly bound divalent metal ion (Ca or Mg ) interacts with the P- and y-phosphates of ATP bound to actin, and that the Me-ATP bidentate chelate is bound to G-actin in the A configuration. The nature of the bound metal ion affects the conformation of actin, the binding kinetics of ATP and ADP, and the rate of ATP hydrolysis. 

Pj release occurs at a relatively apparent slow rate (kobs = 0.005 s" ), so that the transient intermediate F-ADP-Pj in which P is non-covalently bound, has a life time of 2-3 minutes (Carlier and Pantaloni, 1986 Carlier, 1987). While the y-phosphate cleavage step is irreversible as assessed by 0 exchange studies (Carlier et al., 1987), the release of Pi is reversible. Binding of H2PO4 (Kp 10 M) causes the stabilization of actin filaments and the rate of filament growth varies linearly with the concentration of actin monomer in the presence of Pi (Carlier and Pantaloni, 1988). Therefore, Pi release appears as the elementary step responsible for the destabilization of actin-actin interactions in the filament. 

Bigay, J., Deterre, P., Pfister, C., Chabre, M. (1987). Fluoride complexes of aluminium or beryllium act on G-proteins as reversibly bound analogues of the y-phosphate of GTP. EMBO J. 6, 2907-2912. 

Combeau, C. Carlier, M.-F. (1989). Characterization of the aluminium and beryllium fluoride species bound to F-actin and microtubules at the site of the y-phosphate of the nucleotide. J. Biol. Chem. 264,19017-19022. 

The R2C2 complex has no enTymatic activity, but the binding of cAMP by R dissociates R from C, thereby activating the latter (Figure 43-5). The active C subunit catalyzes the transfet of the y phosphate of ATP to a serine or threonine residue in a variety of proteins. The consensus phosphotylation sites are -Arg-Arg/Lys-X-Ser/Thr- and -Arg-Lys-X-X-Ser-, where X can be any amino acid. 

Brinkerhoff CR, Sharma RP, Boucier DR. 1981. The effects of Tn-ortho-to y Phosphate (TOTP) on the immune system of mice. Ecotoxicology and Environmental Safety 5 368-376. 

Mortensen A, Ladefoged O. 1992. Delayed neurotoxicity of trixylenyl phosphate and a trialkyl/aryl phosphate mixture, and the modulating effect of atropine on tn-ortho-to y phosphate-induced neurotoxicity. NeuroToxicology 13 347-354. 

The terminal (y) phosphate of GTP is hydrolyzed by the GTPase activity of the G-protein a subunit, leaving GDP bound instead. This reverses the conformational change in step 5 and allows the a subunit to dissociate from the effector and reassociate with the py subunit. The reassociation will also reverse Py-effector interaction because Ga-GDP effectively competes with the effector for Py-binding. Though of fairly high affinity (e.g.,... 

FIGURE 24-2 Tyrosine phosphorylation and dephosphorylation. Protein tyrosine kinases (PTK) catalyze the transfer of the y-phosphate group from ATP to the hydroxyl group of tyrosine residues, whereas protein tyrosine phosphatases (PTP) remove the phosphate group from phosphotyro-sine. R, protein. 

Protein kinases have a general architecture catalytic domain, a binding domain that orients the substrate to the catalytic site, and phosphate donor binding site which donates the y-phosphate to the acceptor hydroxyl residues. 

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