Simulated 18O kinetic isotope effects in enzymatic hydrolysis of guanosine triphosphate

Invited review: Activation of G proteins by GTP and the mechanism of Gα-catalyzed GTP hydrolysis

This review addresses the regulatory consequences of the binding of GTP to the alpha subunits (Gα) of heterotrimeric G proteins, the reaction mechanism of GTP hydrolysis catalyzed by Gα and the means by which GTPase activating proteins (GAPs) stimulate the GTPase activity of Gα. The high energy of GTP binding is used to restrain and stabilize the conformation of the Gα switch segments, particularly switch II, to afford stable complementary to the surfaces of Gα effectors, while excluding interaction with Gβγ, the regulatory binding partner of GDP-bound Gα. Upon GTP hydrolysis, the energy of these conformational restraints is dissipated and the two switch segments, particularly switch II, become flexible and are able to adopt a conformation suitable for tight binding to Gβγ. Catalytic site pre-organization presents a significant activation energy barrier to Gα GTPase activity. The glutamine residue near the N-terminus of switch II (Glncat ) must adopt a conformation in which it orients and stabilizes the γ phosphate and the water nucleophile for an in-line attack. The transition state is probably loose with dissociative character; phosphoryl transfer may be concerted. The catalytic arginine in switch I (Argcat ), together with amide hydrogen bonds from the phosphate binding loop, stabilize charge at the β-γ bridge oxygen of the leaving group. GAPs that harbor "regulator of protein signaling" (RGS) domains, or structurally unrelated domains within G protein effectors that function as GAPs, accelerate catalysis by stabilizing the pre-transition state for Gα-catalyzed GTP hydrolysis, primarily by restraining Argcat and Glncat to their catalytic conformations. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 449-462, 2016.

Computational characterization of the chemical step in the GTP hydrolysis by Ras-GAP for the wild-type and G13V mutated Ras

The free energy profiles for the chemical reaction of the guanosine triphosphate hydrolysis GTP + H2O → GDP + Pi by Ras-GAP for the wild-type and G13V mutated Ras were computed by using molecular dynamics protocols with the QM(ab initio)/MM potentials. The results are consistent with the recent measurements of reaction kinetics in Ras-GAP showing about two-order reduction of the rate constant upon G13V mutation in Ras: the computed activation barrier on the free energy profile is increased by 3 kcal/mol upon the G13V replacement. The major reason for a higher energy barrier is a shift of the "arginine finger" (R789 from GAP) from the favorable position in the active site. The results of simulations provide support for the mechanism of the reference reaction according to which the Q61 side chain directly participates in chemical transformations at the proton transfer stage.

Modeling the role of G12V and G13V Ras mutations in the Ras-GAP-catalyzed hydrolysis reaction of guanosine triphosphate

Cancer-associated point mutations in Ras, in particular, at glycine 12 and glycine 13, affect the normal cycle between inactive GDP-bound and active GTP-bound states. In this work, the role of G12V and G13V replacements in the GAP-stimulated intrinsic GTP hydrolysis reaction in Ras is studied using molecular dynamics (MD) simulations with quantum mechanics/molecular mechanics (QM/MM) potentials. A model molecular system was constructed by motifs of the relevant crystal structure (Protein Data Bank entry 1WQ1 ). QM/MM optimization of geometry parameters in the Ras-GAP-GTP complex and QM/MM-MD simulations were performed with a quantum subsystem comprising a large fraction of the enzyme active site. For the system with wild-type Ras, the conformations fluctuated near the structure ready to be involved in the efficient chemical reaction leading to the cleavage of the phosphorus-oxygen bond in GTP upon approach of the properly aligned catalytic water molecule. Dynamics of the system with the G13V mutant is characterized by an enhanced flexibility in the area occupied by the γ-phosphate group of GTP, catalytic water, and the side chains of Arg789 and Gln61, which should somewhat hinder fast chemical steps. Conformational dynamics of the system with the G12V mutant shows considerable displacement of the Gln61 side chain and catalytic water from their favorable arrangement in the active site that may lead to a marked reduction in the reaction rate. The obtained computational results correlate well with the recent kinetic measurements of the Ras-GAP-catalyzed hydrolysis of GTP.

Human leptin triggers proliferation of A549 cells via blocking endoplasmic reticulum stress-related apoptosis

Lung cancer is a disease characterized by uncontrolled cell growth in tissues of the lung. Leptin is a pleiotropic hormone with antiapoptotic and proliferative roles involved in several systems. However, there is no known antiapoptotic mechanism of leptin in non-small cell lung cancer (NSCLC). So, we investigated the antiapoptotic mechanism of leptin in NSCLC. Proliferation, apoptosis, and the specific mechanism of leptin-transfected cells were analyzed in this study. Leptin, p-Perk, IRE1, cleaved ATF6, spliced XBP1, eIF2-α, TRAF2, CHOP, and caspase 12 proteins were detected by Western blot, and endoplasmic reticulum (ER) stress-related mRNA was detected by semi-quantitative reverse transcription PCR (RT-PCR). Leptin in A549 and transfected cells inhibited cisplatin-activated ER stress-associated mRNA transcription and activation of proteins. ER stress unfolded protein response (UPR) proteins, PERK and ATF6, were involved in leptin-triggered apoptosis. XBP1 and TRAF2 were increased significantly when treated with cisplatin in A549-siLPT and non-transfected cells. CHOP expression was blocked in A549 and transfected cells (LPT-PeP and LPT-EX cells). In conclusion, leptin can promote the proliferation of A549 cells through blocking ER stress-mediated apoptosis. This blocking is mediated by the p-Perk and ATF6 pathway through blocking activation of CHOP.