Airline environmental efficiency measures considering materials balance principles: an application of a network range-adjusted measure with weak-G disposability

<p>In this paper, we focus on assessing airline environmental efficiency with network structure and build a three-stage efficiency production process. Then, we propose a new model, network range-adjusted measure with weak-G disposability, to measure the environmental efficiency of 29 global airlines based on the data from 2008 to 2015. Finally, a second-stage regression analysis is done to explore the important influencing factors. The main findings are as follows: (1) Eva Air is the airline with the highest efficiency among these 29 airlines; (2) Delta is at the bottom of the efficiency ranking among the 29 airlines and this result is in correlation with its old aircraft fleet; (3) the average efficiency change index in 2014 is the highest in the period 2009–2015; (4) only average fleet age has a slightly significant impact on the overall efficiency and Services efficiency.</p

Charging Free Energy Calculations Using the Generalized Solvent Boundary Potential (GSBP) and Periodic Boundary Condition: A Comparative Analysis Using Ion Solvation and Oxidation Free Energy in Proteins

Free energy simulations using a finite sphere boundary condition rather than a periodic boundary condition (PBC) are attractive in the study of very large biomolecular systems. To understand the quantitative impact of various approximations in such simulations, we compare charging free energies in both solution and protein systems calculated in a linear response framework with the Generalized Solvent Boundary Potential (GSBP) and PBC simulations. For simple ions in solution, we find good agreements between GSBP and PBC charging free energies, once the relevant correction terms are taken into consideration. For PBC simulations with the particle-mesh-Ewald for long-range electrostatics, the contribution (Δ<i>G</i>_P–M) due to the use of a particle rather than molecule based summation scheme in real space is found to be significant, as pointed out by Hünenberger and co-workers. For GSBP, when the inner region is close to be charge neutral, the key correction is the overpolarization of water molecules at the inner/outer dielectric boundary; the magnitude of the correction (Δ<i>G</i>_s–pol), however, is relatively small. For charging (oxidation) free energy in proteins, the situation is more complex, although good agreement between GSBP and PBC can still be obtained when care is exercised. The smooth dielectric boundary approximation inherent to GSBP tends to make significant errors when the inner region is featured with a high net charge. However, the error can be corrected with Poisson–Boltzmann calculations using snapshots from GSBP simulations in a straightforward and robust manner. Because of the more complex charge and solvent distributions, the magnitudes of Δ<i>G</i>_P–M and Δ<i>G</i>_s–pol in protein simulations appear to be different from those derived for solution simulations, leading to uncertainty in directly comparing absolute charging free energies from PBC and GSBP simulations for protein systems. The relative charging/oxidation free energies, however, are robust. With the linear response approximation, for the specific protein system (CueR) studied, the effect of freezing the protein structure in the outer region is found to be small, unless a very small (8 Å) inner region is used; even in the latter case, the result is substantially improved when the nearby metal binding loop is allowed to respond to metal oxidation. The implications of these results to the applicability of GSBP to complex biomolecules and in ab initio QM/MM simulations are discussed

Substrate and Transition State Binding in Alkaline Phosphatase Analyzed by Computation of Oxygen Isotope Effects

Enzymes are powerful catalysts, and a thorough understanding of the sources of their catalytic power will facilitate many medical and industrial applications. Here we have studied the catalytic mechanism of alkaline phosphatase (AP), which is one of the most catalytically proficient enzymes known. We have used quantum mechanics calculations and hybrid quantum mechanics/molecular mechanics (QM/MM) simulations to model a variety of isotope effects relevant to the reaction of AP. We have calculated equilibrium isotope effects (EIEs), binding isotope effects (BIEs), and kinetic isotope effects (KIEs) for a range of phosphate mono- and diester substrates. The results agree well with experimental values, but the model for the reaction’s transition state (TS) differs from the original interpretation of those experiments. Our model indicates that isotope effects on binding make important contributions to measured KIEs on <i>V</i>/<i>K</i>, which complicated interpretation of the measured values. Our results provide a detailed interpretation of the measured isotope effects and make predictions that can test the proposed model. The model indicates that the substrate is deformed in the ground state (GS) of the reaction and partially resembles the TS. The highly preorganized active site preferentially binds conformations that resemble the TS and not the GS, which induces the substrate to adapt to the enzyme, rather than the other way aroundas with classic “induced fit” models. The preferential stabilization of the TS over the GS is what lowers the barrier to the chemical step

Multiple Pathways and Time Scales for Conformational Transitions in apo-Adenylate Kinase

The open/close transition in adenylate kinase (AK) is regarded as a representative example for large-scale conformational transition in proteins, yet its mechanism remains unclear despite numerous experimental and computational studies. Using extensive (∼50 μs) explicit solvent atomistic simulations and Markov state analysis, we shed new lights on the mechanism of this transition in the apo form of AK. The closed basin of apo AK features an open NMP domain while the LID domain closes and rotates toward it. Therefore, although the computed structural properties of the closed ensemble are consistent with previously reported FRET and PRE measurements, our simulations suggest that NMP closure is likely to follow AMP binding, in contrast to the previous interpretation of FRET and PRE data that the apo state was able to sample the fully closed conformation for “ligand selection”. The closed state ensemble is found to be kinetically heterogeneous; multiple pathways and time scales are associated with the open/close transition, providing new clues to the disparate time scales observed in different experiments. Besides interdomain interactions, a novel mutual information analysis identifies specific intradomain interactions that correlate strongly to transition kinetics, supporting observations from previous chimera experiments. While our results underscore the role of internal domain properties in determining the kinetics of open/close transition in apo AK, no evidence is observed for any significant degree of local unfolding during the transition. These observations about AK have general implications to our view of conformational states, transition pathways, and time scales of conformational changes in proteins. The key features and time scales of observed transition pathways are robust and similar from simulations using two popular fixed charge force fields

Stabilization of Different Types of Transition States in a Single Enzyme Active Site: QM/MM Analysis of Enzymes in the Alkaline Phosphatase Superfamily

The first step for the hydrolysis of a phosphate monoester (pNPP^2–) in enzymes of the alkaline phosphatase (AP) superfamily, R166S AP and wild-type NPP, is studied using QM/MM simulations based on an approximate density functional theory (SCC-DFTBPR) and a recently introduced QM/MM interaction Hamiltonian. The calculations suggest that similar loose transition states are involved in both enzymes, despite the fact that phosphate monoesters are the cognate substrates for AP but promiscuous substrates for NPP. The computed loose transition states are clearly different from the more synchronous ones previously calculated for diester reactions in the same AP enzymes. Therefore, our results explicitly support the proposal that AP enzymes are able to recognize and stabilize different types of transition states in a single active site. Analysis of the structural features of computed transition states indicates that the plastic nature of the bimetallic site plays a minor role in accommodating multiple types of transition states and that the high degree of solvent accessibility of the AP active site also contributes to its ability to stabilize diverse transition-state structures without the need of causing large structural distortions of the bimetallic motif. The binding mode of the leaving group in the transition state highlights that vanadate may not always be an ideal transition state analog for loose phosphoryl transfer transition states

Implementation of the Solvent Macromolecule Boundary Potential and Application to Model and Realistic Enzyme Systems

The implementation of the solvent macromolecule boundary potential (SMBP) by Benighaus and Thiel (<i>J. Chem. Theory Comput.</i> <b>2009</b>, <i>5</i>, 3114) into the program package CHARMM is presented. The SMBP allows for the efficient calculation of solvent effects for large macromolecules using irregularly shaped dielectric boundaries. In contrast to the generalized solvent boundary potential (GSBP) by Roux et al. (<i>J. Chem. Phys.</i> <b>2001</b>, <i>114</i>, 2924) from which it is derived, the SMBP is targeted for quantum mechanical/molecular mechanical (QM/MM) setups using ab initio methods for the QM part. After presenting benchmark results for simple model systems, applications of the SMBP for the calculation of geometries, reaction energy barriers, and vibrational frequencies for an alkaline phosphatase (AP) enzyme are discussed. Although the effect of the boundary potential on optimized structures (including the transition state) and vibrational frequencies is relatively small, the energetics of the phosphoryl transfer catalyzed by AP depend significantly on the boundary potential. Finally, to emphasize a unique feature of our implementation, we apply both SMBP and GSBP to the calculation of the energy barrier for a proton transfer reaction in a simple model channel, where the effect of an external transmembrane potential is studied. Due to the dipolar response of the polar environment, the effective charge displacement estimated based on the effect of the membrane potential on the proton transfer energetics deviates from the net charge that passes the membrane

Free Energy Calculations for the Peripheral Binding of Proteins/Peptides to an Anionic Membrane. 1. Implicit Membrane Models

The binding of peptides and proteins to the surface of complex lipid membranes is important in many biological processes such as cell signaling and membrane remodeling. Computational studies can aid experiments by identifying physical interactions and structural motifs that determine the binding affinity and specificity. However, previous studies focused on either qualitative behaviors of protein/membrane interactions or the binding affinity of small peptides. Motivated by this observation, we set out to develop computational protocols for bimolecular binding to charged membranes that are applicable to both peptides and large proteins. In this work, we explore a method based on an implicit membrane/solvent model (generalized Born with a simple switching in combination with the Gouy–Chapman–Stern model for a charged interface), which we expect to lead to useful results when the binding does not implicate significant membrane deformation and local demixing of lipids. We show that the binding free energy can be efficiently computed following a thermodynamic cycle similar to protein–ligand binding calculations, especially when a Bennett acceptance ratio based protocol is used to consider both the membrane bound and solution conformational ensembles. Test calculations on a series of peptides show that our computational approach leads to binding affinities in encouraging agreement with experimental data, including for the challenging example of the bringing of flexible MARCKS-ED peptides to membranes. The calculations highlight that for a membrane with a significant fraction of anionic lipids, it is essential to include the effect of ion adsorption using the Stern model, which significantly modifies the effective surface charge. This implicit membrane model based computational protocol helps lay the groundwork for more systematic analysis of protein/peptide binding to membranes of complex shape and composition

Why Do Arginine and Lysine Organize Lipids Differently? Insights from Coarse-Grained and Atomistic Simulations

An important puzzle in membrane biophysics is the difference in the behaviors of lysine (Lys) and arginine (Arg) based peptides at the membrane. For example, the translocation of poly-Arg is orders of magnitude faster than that of poly-Lys. Recent experimental work suggests that much of the difference can be inferred from the phase behavior of peptide/lipid mixtures. At similar concentrations, mixtures of phosphatidylethanolamine (PE) and phosphatidylserine (PS) lipids display different phases in the presence of these polypeptides, with a bicontinuous phase observed with poly-Arg peptides and an inverted hexagonal phase observed with poly-Lys peptides. Here we show that simulations with the coarse-grained (CG) BMW-MARTINI model reproduce the experimental results. An analysis using atomistic and CG models reveals that electrostatic and glycerol–peptide interactions play a crucial role in determining the phase behavior of peptide–lipid mixtures, with the difference between Arg and Lys arising from the stronger interactions of the former with lipid glycerols. In other words, the multivalent nature of the guanidinium group allows Arg to simultaneously interact with both phosphate and glycerol groups, while Lys engages solely with phosphate; this feature of amino acid/lipid interactions has not been emphasized in previous studies. The Arg peptides colocalize with PS in regions of high negative Gaussian curvature and stabilize the bicontinuous phase. Decreasing the strength of either the electrostatic interactions or the peptide–glycerol interactions causes the inverted hexagonal phase to become more stable. The results highlight the utility of CG models for the investigation of phase behavior but also emphasize the subtlety of the phenomena, with small changes in specific interactions leading to qualitatively different phases

Leaving Group Ability Observably Affects Transition State Structure in a Single Enzyme Active Site

A reaction’s transition state (TS) structure plays a critical role in determining reactivity and has important implications for the design of catalysts, drugs, and other applications. Here, we explore TS structure in the enzyme alkaline phosphatase using hybrid Quantum Mechanics/Molecular Mechanics simulations. We find that minor perturbations to the substrate have major effects on TS structure and the way the enzyme stabilizes the TS. Substrates with good leaving groups (LGs) have little cleavage of the phosphorus–LG bond at the TS, while substrates with poor LGs have substantial cleavage of that bond. The results predict nonlinear free energy relationships for a single rate-determining step, and substantial differences in kinetic isotope effects for different substrates; both trends were observed in previous experimental studies, although the original interpretations differed from the present model. Moreover, due to different degrees of phosphorus–LG bond cleavage at the TS for different substrates, the LG is stabilized by different interactions at the TS: while a poor LG is directly stabilized by an active site zinc ion, a good LG is mainly stabilized by active site water molecules. Our results demonstrate the considerable plasticity of TS structure and stabilization in enzymes. Furthermore, perturbations to reactivity that probe TS structure experimentally (i.e., substituent effects) may substantially perturb the TS they aim to probe, and thus classical experimental approaches such as free energy relations should be interpreted with care

Conformational Disorder Enhances Electron Transfer Through Alkyl Monolayers: Ferrocene on Conductive Diamond

We have investigated the electron-transfer kinetics of ferrocene groups covalently tethered to surfaces of conductive diamond electrodes. Electrochemical measurements show that the rates are only weakly dependent on chain length but are strongly dependent on the surface coverage; these observations are contrary to what is commonly observed for self-assembled monolayers on gold, pointing to important mechanistic differences in electron transfer processes on covalently bonded materials. Molecular dynamics simulations show that dependence on chain length and molecular density can be readily explained in terms of dynamic crowding effects. At low coverage, conformational flexibility of surface-tethered alkyl chains allows the redox-active ferrocene group to dynamically approach the diamond surface, leading to facile electron transfer for all surface molecules. As the coverage is increased, steric crowding causes the average ferrocene-to-surface distance to increase, decreasing the electron transfer rate. Even at the most dense packings, the mismatch between the spacing of surface lattice sites and the optimum alkyl chain density leads to voids and inherent disorder that facilitates electron transfer. These results are significant in the design and optimization of electrocatalytically active surfaces on covalently bonded materials relevant for electro- and photocatalysis