PMID: 26272610

Authors:

Rudack T, Jenrich S, Brucker S, Vetter IR, Gerwert K, Kotting C

Title:

Catalysis of GTP hydrolysis by small GTPases at atomic detail by integration of X-ray crystallography, experimental and theoretical IR spectroscopy.

Journal:

J Biol Chem. 2015 Aug 13. pii: jbc.M115.648071.

Abstract:

Small GTPases regulate key processes in cells. Malfunction of their GTPase reaction by mutations is involved in severe diseases. Here, we compare the GTPase reaction of the slower hydrolyzing GTPase Ran with Ras. By combination of time-resolved FTIR-difference spectroscopy and QM/MM simulations we elucidate that the Mg2+ coordination by the phosphate groups, which varies largely among the X-ray structures, is the same for Ran and Ras. A new X-ray structure of a Ran.RanBD1 complex with improved resolution confirmed this finding and revealed a general problem with the refinement of Mg2+ in GTPases. The Mg2+ coordination is not responsible for the much slower GTPase reaction of Ran. Instead, the location of the Tyr39 side chain of Ran between the gamma-phosphate and Gln69 prevents the optimal positioning of the attacking water molecule by the Gln69 relative to the gamma-phosphate. This is confirmed in the RanY39A.RanBD1 crystal structure. The QM/MM simulations provide IR spectra of the catalytic center which agree very nicely with the experimental ones. The combination of both methods can correlate spectra with structure at atomic detail. For example the FTIR difference spectra of RasA18T and RanT25A mutants show that spectral differences are mainly due to the hydrogen bond of Thr25 to the alpha-phosphate in Ran. By integration of X-ray structure analysis, experimental and theoretical IR spectroscopy the catalytic center of the X-ray structural models are further refined to sub-Angstrom resolution, allowing an improved understanding of catalysis.