GTP, hydrolysis

A vital biophysico-chemical problem is to understand how chemical energy (released by ATP or GTP hydrolysis... 

A failure to turn off GTP-activated Ga has dire consequences. For example, in the disease cholera, cholera toxin produced by the bacterium Vibrio cholerae binds to Ga and prevents GTP hydrolysis, resulting in the continued excretion of sodium and water into the gut. 

All the residues involved in important functions in the catalytic mechanism are strictly conserved in all homologous GTPases with one notable exception. Ras does not have the arginine in the switch 1 region that stabilizes the transition state. The assumption that the lack of this catalytically important residue was one reason for the slow rate of GTP hydrolysis by Ras was confirmed when the group of Alfred Wittinghofer, Max-Planck Institute,... 

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.)...

In summary, structural studies of Ras and Gq with GTP-yS and a transition state analog have illuminated the catalytic mechanism of their GTPase activity, as well as the mechanism by which GTP hydrolysis is stimulated by GAP and RGS. In addition, these structural studies have shown how tumor-causing mutations affect the function of Ras and Gq. 

Berghuis, A.M., et al. Structure of the GDP-Pi complex of Gly203-Ala Giai a mimic of the ternary product complex of Ga-catalyzed GTP hydrolysis. Structure 4 1277-1290, 1996. 

Schlichting, L, et al. Time-resolved x-ray crystallographic study of the conformational change in Ha-ras p21 protein on GTP hydrolysis. Nature 345 309-315,... 

Tesmer, J.J.G., et al. Structure of RGS4 bound to AIF4—activated Giai Stabilization of the transition state for GTP hydrolysis. Cell 89 251-261, 1997. 

FIGURE 17.3 A model of the GTP-depeu-deut treadmiUing process. Both a- aud /J-tubuliu possess two different binding sites for GTP. The polymerization of tubuliu to form microtubules is driven by GTP hydrolysis in a process that is only beginning to be understood in detail. 

GTP-binding protein at an arginine residue which is involved in GTP hydrolysis. ADP-ribosylation thus leads to constitutive activation of Gs. 

GTPase activating proteins (GAPs) stimulate the intrinsic GTP hydrolysis of GTPases. 

It should be noted that fast inactivation of receptor signaling not only involves the desensitization of the receptor but also the components of the downstream signaling cascade. The deactivation of active Ga subunits is controlled by the intramolecular hydrolysis of bound GTP, allowing it to reform the inactive heterotrimer. Termination of G protein-mediated signaling in vivo is 10- to 100-fold faster than the in vitro rate of GTP hydrolysis by Ga subunits, suggesting... 

Shutes A, Der CJ (2005) Real-time in vitro measurements of GTP hydrolysis. Methods 37 183-189... 

Several key concepts are worth remembering. GTP is used as an energy source for translation, but ATP is used to form the aminoacyl-tRNA. The ribosome effectively has two kinds of tRNA binding sites. Only tRNAMet can bind to the P (for peptide) site, and this only occurs during the initial formation of the functional ribosome (initiation). All other aminoacyl-tRNAs enter at the A (for amino acid) binding site. After formation of the peptide bond (this doesn t require GTP hydrolysis), the tRNA with the growing peptide attached is moved (translocated) to the other site (this does require GTP hydrolysis). 

All GTP binding proteins in signal transduction share a common structural element - the Ras-like domain which is responsible for the specific complexation of guanosine diphosphate and -triphosphate and which contains catalytic residues that promote GTP-hydrolysis. 

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