Publications by authors named "Filip Persson"

8 Publications

  • Page 1 of 1

The geometry of protein hydration.

J Chem Phys 2018 Jun;148(21):215101

Division of Biophysical Chemistry, Department of Chemistry, Lund University, P.O. Box 124, SE-22100 Lund, Sweden.

Based on molecular dynamics simulations of four globular proteins in dilute aqueous solution, with three different water models, we examine several, essentially geometrical, aspects of the protein-water interface that remain controversial or incompletely understood. First, we compare different hydration shell definitions, based on spatial or topological proximity criteria. We find that the best method for constructing monolayer shells with nearly complete coverage is to use a 5 Å water-carbon cutoff and a 4 Å water-water cutoff. Using this method, we determine a mean interfacial water area of 11.1 Å which appears to be a universal property of the protein-water interface. We then analyze the local coordination and packing density of water molecules in the hydration shells and in subsets of the first shell. The mean polar water coordination number in the first shell remains within 1% of the bulk-water value, and it is 5% lower in the nonpolar part of the first shell. The local packing density is obtained from additively weighted Voronoi tessellation, arguably the most physically realistic method for allocating space between protein and water. We find that water in all parts of the first hydration shell, including the nonpolar part, is more densely packed than in the bulk, with a shell-averaged density excess of 6% for all four proteins. We suggest reasons why this value differs from previous experimental and computational results, emphasizing the importance of a realistic placement of the protein-water dividing surface and the distinction between spatial correlation and packing density. The protein-induced perturbation of water coordination and packing density is found to be short-ranged, with an exponential decay "length" of 0.6 shells. We also compute the protein partial volume, analyze its decomposition, and argue against the relevance of electrostriction.
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http://dx.doi.org/10.1063/1.5026744DOI Listing
June 2018

Compressibility of the protein-water interface.

J Chem Phys 2018 Jun;148(21):215102

Division of Biophysical Chemistry, Department of Chemistry, Lund University, P.O. Box 124, SE-22100 Lund, Sweden.

The compressibility of a protein relates to its stability, flexibility, and hydrophobic interactions, but the measurement, interpretation, and computation of this important thermodynamic parameter present technical and conceptual challenges. Here, we present a theoretical analysis of protein compressibility and apply it to molecular dynamics simulations of four globular proteins. Using additively weighted Voronoi tessellation, we decompose the solution compressibility into contributions from the protein and its hydration shells. We find that positively cross-correlated protein-water volume fluctuations account for more than half of the protein compressibility that governs the protein's pressure response, while the self correlations correspond to small (∼0.7%) fluctuations of the protein volume. The self compressibility is nearly the same as for ice, whereas the total protein compressibility, including cross correlations, is ∼45% of the bulk-water value. Taking the inhomogeneous solvent density into account, we decompose the experimentally accessible protein partial compressibility into intrinsic, hydration, and molecular exchange contributions and show how they can be computed with good statistical accuracy despite the dominant bulk-water contribution. The exchange contribution describes how the protein solution responds to an applied pressure by redistributing water molecules from lower to higher density; it is negligibly small for native proteins, but potentially important for non-native states. Because the hydration shell is an open system, the conventional closed-system compressibility definitions yield a pseudo-compressibility. We define an intrinsic shell compressibility, unaffected by occupation number fluctuations, and show that it approaches the bulk-water value exponentially with a decay "length" of one shell, less than the bulk-water compressibility correlation length. In the first hydration shell, the intrinsic compressibility is 25%-30% lower than in bulk water, whereas its self part is 15%-20% lower. These large reductions are caused mainly by the proximity to the more rigid protein and are not a consequence of the perturbed water structure.
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http://dx.doi.org/10.1063/1.5026774DOI Listing
June 2018

The spatial range of protein hydration.

J Chem Phys 2018 Jun;148(21):215104

Division of Biophysical Chemistry, Department of Chemistry, Lund University, P.O. Box 124, SE-22100 Lund, Sweden.

Proteins interact with their aqueous surroundings, thereby modifying the physical properties of the solvent. The extent of this perturbation has been investigated by numerous methods in the past half-century, but a consensus has still not emerged regarding the spatial range of the perturbation. To a large extent, the disparate views found in the current literature can be traced to the lack of a rigorous definition of the perturbation range. Stating that a particular solvent property differs from its bulk value at a certain distance from the protein is not particularly helpful since such findings depend on the sensitivity and precision of the technique used to probe the system. What is needed is a well-defined decay length, an intrinsic property of the protein in a dilute aqueous solution, that specifies the length scale on which a given physical property approaches its bulk-water value. Based on molecular dynamics simulations of four small globular proteins, we present such an analysis of the structural and dynamic properties of the hydrogen-bonded solvent network. The results demonstrate unequivocally that the solvent perturbation is short-ranged, with all investigated properties having exponential decay lengths of less than one hydration shell. The short range of the perturbation is a consequence of the high energy density of bulk water, rendering this solvent highly resistant to structural perturbations. The electric field from the protein, which under certain conditions can be long-ranged, induces a weak alignment of water dipoles, which, however, is merely the linear dielectric response of bulk water and, therefore, should not be thought of as a structural perturbation. By decomposing the first hydration shell into polarity-based subsets, we find that the hydration structure of the nonpolar parts of the protein surface is similar to that of small nonpolar solutes. For all four examined proteins, the mean number of water-water hydrogen bonds in the nonpolar subset is within 1% of the value in bulk water, suggesting that the fragmentation and topography of the nonpolar protein-water interface has evolved to minimize the propensity for protein aggregation by reducing the unfavorable free energy of hydrophobic hydration.
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http://dx.doi.org/10.1063/1.5031005DOI Listing
June 2018

How proteins modify water dynamics.

J Chem Phys 2018 Jun;148(21):215103

Division of Biophysical Chemistry, Department of Chemistry, Lund University, P.O. Box 124, SE-22100 Lund, Sweden.

Much of biology happens at the protein-water interface, so all dynamical processes in this region are of fundamental importance. Local structural fluctuations in the hydration layer can be probed by O magnetic relaxation dispersion (MRD), which, at high frequencies, measures the integral of a biaxial rotational time correlation function (TCF)-the integral rotational correlation time. Numerous O MRD studies have demonstrated that this correlation time, when averaged over the first hydration shell, is longer than in bulk water by a factor 3-5. This rotational perturbation factor (RPF) has been corroborated by molecular dynamics simulations, which can also reveal the underlying molecular mechanisms. Here, we address several outstanding problems in this area by analyzing an extensive set of molecular dynamics data, including four globular proteins and three water models. The vexed issue of polarity versus topography as the primary determinant of hydration water dynamics is resolved by establishing a protein-invariant exponential dependence of the RPF on a simple confinement index. We conclude that the previously observed correlation of the RPF with surface polarity is a secondary effect of the correlation between polarity and confinement. Water rotation interpolates between a perturbed but bulk-like collective mechanism at low confinement and an exchange-mediated orientational randomization (EMOR) mechanism at high confinement. The EMOR process, which accounts for about half of the RPF, was not recognized in previous simulation studies, where only the early part of the TCF was examined. Based on the analysis of the experimentally relevant TCF over its full time course, we compare simulated and measured RPFs, finding a 30% discrepancy attributable to force field imperfections. We also compute the full O MRD profile, including the low-frequency dispersion produced by buried water molecules. Computing a local RPF for each hydration shell, we find that the perturbation decays exponentially with a decay "length" of 0.3 shells and that the second and higher shells account for a mere 3% of the total perturbation measured by O MRD. The only long-range effect is a weak water alignment in the electric field produced by an electroneutral protein (not screened by counterions), but this effect is negligibly small for O MRD. By contrast, we find that the O TCF is significantly more sensitive to the important short-range perturbations than the other two TCFs examined here.
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http://dx.doi.org/10.1063/1.5026861DOI Listing
June 2018

How amide hydrogens exchange in native proteins.

Proc Natl Acad Sci U S A 2015 Aug 20;112(33):10383-8. Epub 2015 Jul 20.

Department of Biophysical Chemistry, Center for Molecular Protein Science, Lund University, SE-22100 Lund, Sweden

Amide hydrogen exchange (HX) is widely used in protein biophysics even though our ignorance about the HX mechanism makes data interpretation imprecise. Notably, the open exchange-competent conformational state has not been identified. Based on analysis of an ultralong molecular dynamics trajectory of the protein BPTI, we propose that the open (O) states for amides that exchange by subglobal fluctuations are locally distorted conformations with two water molecules directly coordinated to the N-H group. The HX protection factors computed from the relative O-state populations agree well with experiment. The O states of different amides show little or no temporal correlation, even if adjacent residues unfold cooperatively. The mean residence time of the O state is ∼100 ps for all examined amides, so the large variation in measured HX rate must be attributed to the opening frequency. A few amides gain solvent access via tunnels or pores penetrated by water chains including native internal water molecules, but most amides access solvent by more local structural distortions. In either case, we argue that an overcoordinated N-H group is necessary for efficient proton transfer by Grotthuss-type structural diffusion.
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http://dx.doi.org/10.1073/pnas.1506079112DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4547260PMC
August 2015

Analysis of Protein Dynamics Simulations by a Stochastic Point Process Approach.

J Chem Theory Comput 2013 Jun 21;9(6):2838-48. Epub 2013 May 21.

Biophysical Chemistry, Lund University , POB 124, SE-22100 Lund, Sweden.

MD simulations can now explore the complex dynamics of proteins and their associated solvent in atomic detail on a millisecond time scale. Among the phenomena that thereby become amenable to detailed study are intermittent conformational transitions where the protein accesses transient high-energy states that often play key roles in biology. Here, we present a coherent theoretical framework, based on the stochastic theory of stationary point processes, that allows the essential dynamical characteristics of such processes to be efficiently extracted from the MD trajectory without assuming Poisson statistics. Since the complete information content of a point process is contained in the sequence of residence or interevent times, the experimentally relevant survival correlation function can be computed several orders of magnitude more efficiently than with the conventional approach, involving averaging over initial times. We also present a detailed analysis of the statistical and binning errors, of particular importance when MD results are compared with experiment. As an illustration of the general theoretical framework, we use a 1 ms MD trajectory of the protein BPTI to analyze the exchange kinetics of an internal water molecule and the dynamics of the rare conformational fluctuations that govern the rate of this exchange process.
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http://dx.doi.org/10.1021/ct400161uDOI Listing
June 2013

Transient access to the protein interior: simulation versus NMR.

J Am Chem Soc 2013 Jun 29;135(23):8735-48. Epub 2013 May 29.

Biophysical Chemistry, Lund University, P.O. Box 124, SE-22100 Lund, Sweden.

Many proteins rely on rare structural fluctuations for their function, whereby solvent and other small molecules gain transient access to internal cavities. In magnetic relaxation dispersion (MRD) experiments, water molecules buried in such cavities are used as intrinsic probes of the intermittent protein motions that govern their exchange with external solvent. While this has allowed a detailed characterization of exchange kinetics for several proteins, little is known about the exchange mechanism. Here, we use a millisecond all-atom MD trajectory produced by Shaw et al. (Science2010, 330, 341) to characterize water exchange from the four internal hydration sites in the protein bovine pancreatic trypsin inhibitor. Using a recently developed stochastic point process approach, we compute the survival correlation function probed by MRD experiments as well as other quantities designed to validate the exchange-mediated orientational randomization (EMOR) model used to interpret the MRD data. The EMOR model is found to be quantitatively accurate, and the simulation reproduces the experimental mean survival times for all four sites with activation energy discrepancies in the range 0-3 kBT. On the other hand, the simulated hydration sites are somewhat too flexible, and the water flip barrier is underestimated by up to 6 kBT. The simulation reveals that water molecules gain access to the internal sites by a transient aqueduct mechanism, migrating as single-file water chains through transient (<5 ns) tunnels or pores. The present study illustrates the power of state-of-the-art molecular dynamics simulations in validating and extending experimental results.
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http://dx.doi.org/10.1021/ja403405dDOI Listing
June 2013

Can misfolded proteins be beneficial? The HAMLET case.

Ann Med 2009 ;41(3):162-76

Department of Microbiology, Immunology and Glycobiology (MIG), Institute of Laboratory Medicine, Lund University, Sölvegatan 23, Lund, Sweden.

By changing the three-dimensional structure, a protein can attain new functions, distinct from those of the native protein. Amyloid-forming proteins are one example, in which conformational change may lead to fibril formation and, in many cases, neurodegenerative disease. We have proposed that partial unfolding provides a mechanism to generate new and useful functional variants from a given polypeptide chain. Here we present HAMLET (Human Alpha-lactalbumin Made LEthal to Tumor cells) as an example where partial unfolding and the incorporation of cofactor create a complex with new, beneficial properties. Native alpha-lactalbumin functions as a substrate specifier in lactose synthesis, but when partially unfolded the protein binds oleic acid and forms the tumoricidal HAMLET complex. When the properties of HAMLET were first described they were surprising, as protein folding intermediates and especially amyloid-forming protein intermediates had been regarded as toxic conformations, but since then structural studies have supported functional diversity arising from a change in fold. The properties of HAMLET suggest a mechanism of structure-function variation, which might help the limited number of human protein genes to generate sufficient structural diversity to meet the diverse functional demands of complex organisms.
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http://dx.doi.org/10.1080/07853890802502614DOI Listing
July 2009