The first crystal structure of a macromolecular assembly under high pressure: CpMV at 330 MPa

Differential Responses in the Core, Active Site and Peripheral Regions of Cytochrome c Peroxidase to Extreme Pressure and Temperature

In consideration of life in extreme environments, the effects of hydrostatic pressure on proteins at the atomic level have drawn substantial interest. Large deviations of temperature and pressure from ambient conditions can shift the free energy landscape of proteins to reveal otherwise lowly populated structural states and even promote unfolding. We report the crystal structure of the heme-containing peroxidase, cytochrome c peroxidase (CcP) at 1.5 and 3.0 kbar and make comparisons to structures determined at 1.0 bar and cryo-temperatures (100 K). Pressure produces anisotropic changes in CcP, but compressibility plateaus after 1.5 kbar. CcP responds to pressure with volume declines at the periphery of the protein where B-factors are relatively high but maintains nearly intransient core structure, hydrogen bonding interactions and active site channels. Changes in active-site solvation and heme ligation reveal pressure sensitivity to protein-ligand interactions and a potential docking site for the substrate peroxide. Compression at the surface affects neither alternate side-chain conformers nor B-factors. Thus, packing in the core, which resembles a crystalline solid, limits motion and protects the active site, whereas looser packing at the surface preserves side-chain dynamics. These data demonstrate that conformational dynamics and packing densities are not fully correlated in proteins and that encapsulation of cofactors by the polypeptide can provide a precisely structured environment resistant to change across a wide range of physical conditions.

Exploring the structural dynamics of proteins by pressure perturbation using macromolecular crystallography

High pressure is a convenient thermodynamic parameter to probe the dynamics of proteins as it is intimately related to volume which is essential for protein function. To be biologically active, a protein fluctuates between different substates. Pressure perturbation can promote some hidden substates by modifying the population between them. High pressure macromolecular crystallography (HPMX) is a perfect tool to capture and to characterize such substates at a molecular level providing new insights on protein dynamics. The present chapter describes the use of the diamond anvil cell to perform HPMX experiments. It also provides tips on sample preparation and optimal data collection as well as on efficient analysis of the resulting high-pressure structures.

Equilibria between conformational states of the Ras oncogene protein revealed by high pressure crystallography

In this work, we experimentally investigate the allosteric transitions between conformational states on the Ras oncogene protein using high pressure crystallography. Ras protein is a small GTPase involved in central regulatory processes occurring in multiple conformational states. Ras acts as a molecular switch between active GTP-bound, and inactive GDP-bound states, controlling essential signal transduction pathways. An allosteric network of interactions between the effector binding regions and the membrane interacting regions is involved in Ras cycling. The conformational states which coexist simultaneously in solution possess higher Gibbs free energy than the ground state. Equilibria between these states can be shifted by applying pressure favouring conformations with lower partial molar volume, and has been previously analyzed by high-pressure NMR spectroscopy. High-pressure macromolecular crystallography (HPMX) is a powerful tool perfectly complementary to high-pressure NMR, allowing characterization at the molecular level with a high resolution the different allosteric states involved in the Ras cycling. We observe a transition above 300 MPa in the crystal leading to more stable conformers. Thus, we compare the crystallographic structures of Ras(wt)·Mg²⁺·GppNHp and Ras(D33K)·Mg²⁺·GppNHp at various high hydrostatic pressures. This gives insight into per-residue descriptions of the structural plasticity involved in allosteric equilibria between conformers. We have mapped out at atomic resolution the different segments of Ras protein which remain in the ground-state conformation or undergo structural changes, adopting excited-energy conformations corresponding to transient intermediate states. Such in crystallo phase transitions induced by pressure open the possibility to finely explore the structural determinants related to switching between Ras allosteric sub-states without any mutations nor exogenous partners.

The equilibria between structural states induced by pressure within the crystal structure of Ras are illustrated with different colors corresponding to different Ras substates.

Determinants of neuroglobin plasticity highlighted by joint coarse-grained simulations and high pressure crystallography

Investigating the effect of pressure sheds light on the dynamics and plasticity of proteins, intrinsically correlated to functional efficiency. Here we detail the structural response to pressure of neuroglobin (Ngb), a hexacoordinate globin likely to be involved in neuroprotection. In murine Ngb, reversible coordination is achieved by repositioning the heme more deeply into a large internal cavity, the “heme sliding mechanism”. Combining high pressure crystallography and coarse-grain simulations on wild type Ngb as well as two mutants, one (V101F) with unaffected and another (F106W) with decreased affinity for CO, we show that Ngb hinges around a rigid mechanical nucleus of five hydrophobic residues (V68, I72, V109, L113, Y137) during its conformational transition induced by gaseous ligand, that the intrinsic flexibility of the F-G loop appears essential to drive the heme sliding mechanism, and that residue Val 101 may act as a sensor of the interaction disruption between the heme and the distal histidine.

Reduction of lattice disorder in protein crystals by high-pressure cryocooling

High-pressure cryocooling (HPC) has been developed as a technique for reducing the damage that frequently occurs when macromolecular crystals are cryocooled at ambient pressure. Crystals are typically pressurized at around 200 MPa and then cooled to liquid nitrogen temperature under pressure; this process reduces the need for penetrating cryoprotectants, as well as the damage due to cryocooling, but does not improve the diffraction quality of the as-grown crystals. Here it is reported that HPC using a pressure above 300 MPa can reduce lattice disorder, in the form of high mosaicity and/or nonmerohedral twinning, in crystals of three different proteins, namely human glutaminase C, the GTP pyrophosphokinase YjbM and the uncharacterized protein lpg1496. Pressure lower than 250 MPa does not induce this transformation, even with a prolonged pressurization time. These results indicate that HPC at elevated pressures can be a useful tool for improving crystal packing and hence the quality of the diffraction data collected from pressurized crystals.

Cavities and Excited States in Proteins

Protein cavities or voids are observed as defects in atomic packing. Cavities have long been suggested to play important roles in protein dynamics and function, but the underlying origin and mechanism remains elusive. Here, recent studies about the cavities characterized by high-pressure NMR spectroscopy have been reviewed. Analysis of the pressure-dependent chemical shifts showed both linear and nonlinear response of proteins to pressure. The linear response corresponded to compression within the native ensemble, while the nonlinear response indicated the involvement of low-lying excited states that were different from the native state. The finding of non-linear pressure shifts in various proteins suggested that the existence of the low-lying excited states was common for globular proteins. However, the absolute nonlinear coefficient values varied significantly from protein to protein, and showed a good correlation with the density of cavities. Extensive studies on hen lysozyme as a model system showed that the cavity hydration and water penetration into the interior of proteins was an origin of the conformational transition to the excited states. The importance of cavities for protein function and evolution has also been explained. In addition to these "equilibrium" cavities, there are also "transient" cavities formed in the interior of the protein structure, as manifested by the ring flip motions of aromatic rings. The significance of transient cavities, reflecting an intrinsic dynamic nature within the native state, has also been discussed.

Functional Sub-states by High-pressure Macromolecular Crystallography

At the molecular level, high-pressure perturbation is of particular interest for biological studies as it allows trapping conformational substates. Moreover, within the context of high-pressure adaptation of deep-sea organisms, it allows to decipher the molecular determinants of piezophily. To provide an accurate description of structural changes produced by pressure in a macromolecular system, developments have been made to adapt macromolecular crystallography to high-pressure studies. The present chapter is an overview of results obtained so far using high-pressure macromolecular techniques, from nucleic acids to virus capsid through monomeric as well as multimeric proteins.

High Pressure Macromolecular Crystallography

X-ray crystallography is a powerful tool for the high resolution structural study of biomacromolecules. One of the most important beneficial feature of the method is the possibility of the direct observation of hydration water structure. In recent years, significant development in high-pressure macromolecular crystallography (HPMX) using a diamond anvil cell (DAC) has been performed in combination with shorter wavelength X-ray of synchrotron radiation. The number of protein structures determined by HPMX at pressure ranging from several hundred MPa to 1 GPa is gradually growing. In this chapter, we describe DAC with its usage, and then an example of HPMX study on hydration structure of 3-isopropylmalate dehydrogenase (IPMDH).

High-pressure protein crystallography of hen egg-white lysozyme

Crystal structures of hen egg-white lysozyme (HEWL) determined under pressures ranging from ambient pressure to 950 MPa are presented. From 0.1 to 710 MPa, the molecular and internal cavity volumes are monotonically compressed. However, from 710 to 890 MPa the internal cavity volume remains almost constant. Moreover, as the pressure increases to 950 MPa, the tetragonal crystal of HEWL undergoes a phase transition from P43212 to P43. Under high pressure, the crystal structure of the enzyme undergoes several local and global changes accompanied by changes in hydration structure. For example, water molecules penetrate into an internal cavity neighbouring the active site and induce an alternate conformation of one of the catalytic residues, Glu35. These phenomena have not been detected by conventional X-ray crystal structure analysis and might play an important role in the catalytic activity of HEWL.

Pressure-induced molten globule state of human acetylcholinesterase: structural and dynamical changes monitored by neutron scattering

We used neutron scattering to study the effects of high hydrostatic pressure on the structure and dynamics of human acetylcholinesterase (hAChE).

High hydrostatic pressure: a probing tool and a necessary parameter in biophysical chemistry

High pressures extending up to several thousands of atmospheres provide extreme conditions for biological organisms to survive. Recent studies are investigating the survival mechanisms and biological function of microorganisms under natural and laboratory conditions extending into the GigaPascal range, with applications to understanding the Earth's deep biosphere and food technology. High pressure has also emerged as a useful tool and physical parameter for probing changes in the structure and functional properties of biologically important macromolecules and polymers encountered in soft matter science. Here we highlight some areas of current interest in high pressure biophysics and physical chemistry that are emerging at the frontier of this cross-disciplinary field.

Functional relevance of the internal hydrophobic cavity of urate oxidase

Urate oxidase from Aspergillus flavus is a 135 kDa homo-tetramer which has a hydrophobic cavity buried within each monomer and located close to its active site. Crystallographic studies under moderate gas pressure and high hydrostatic pressure have shown that both gas presence and high pressure would rigidify the cavity leading to an inhibition of the catalytic activity. Analysis of the cavity volume variations and functional modifications suggest that the flexibility of the cavity would be an essential parameter for the active site efficiency. This cavity would act as a connecting vessel to give flexibility to the neighboring active site, and its expansion under pure oxygen pressure reveals that it might serve as a transient reservoir on its pathway to the active site.

Effect of pressure on thermal stability of g-quadruplex DNA and double-stranded DNA structures

Pressure is a thermodynamic parameter that can induce structural changes in biomolecules due to a volumetric decrease. Although most proteins are denatured by pressure over 100 MPa because they have the large cavities inside their structures, the double-stranded structure of DNA is stabilized or destabilized only marginally depending on the sequence and salt conditions. The thermal stability of the G-quadruplex DNA structure, an important non-canonical structure that likely impacts gene expression in cells, remarkably decreases with increasing pressure. Volumetric analysis revealed that human telomeric DNA changed by more than 50 cm3 mol-1 during the transition from a random coil to a quadruplex form. This value is approximately ten times larger than that for duplex DNA under similar conditions. The volumetric analysis also suggested that the formation of G-quadruplex DNA involves significant hydration changes. The presence of a cosolute such as poly(ethylene glycol) largely repressed the pressure effect on the stability of G-quadruplex due to alteration in stabilities of the interactions with hydrating water. This review discusses the importance of local perturbations of pressure on DNA structures involved in regulation of gene expression and highlights the potential for application of high-pressure chemistry in nucleic acid-based nanotechnology.

High-pressure macromolecular crystallography and NMR: status, achievements and prospects

Biomacromolecules are thermodynamic entities that exist in general as an equilibrium mixture of the basic folded state and various higher-energy substates including all functionally relevant ones. Under physiological conditions, however, the higher-energy substates are usually undetectable on spectroscopy, as their equilibrium populations are extremely low. Hydrostatic pressure gives a general solution to this problem. As proteins generally have smaller partial molar volumes in higher-energy states than in the basic folded state, pressure can shift the equilibrium toward the former substantially, and allows their direct detection and analysis with X-ray crystallography or NMR spectroscopy at elevated pressures. These techniques are now mature, and their status and selected applications are presented with future prospects.

High-pressure-induced water penetration into 3-isopropylmalate dehydrogenase

Hydrostatic pressure induces structural changes in proteins, including denaturation, the mechanism of which has been attributed to water penetration into the protein interior. In this study, structures of 3-isopropylmalate dehydrogenase (IPMDH) from Shewanella oneidensis MR-1 were determined at about 2 Å resolution under pressures ranging from 0.1 to 650 MPa using a diamond anvil cell (DAC). Although most of the protein cavities are monotonically compressed as the pressure increases, the volume of one particular cavity at the dimer interface increases at pressures over 340 MPa. In parallel with this volume increase, water penetration into the cavity could be observed at pressures over 410 MPa. In addition, the generation of a new cleft on the molecular surface accompanied by water penetration could also be observed at pressures over 580 MPa. These water-penetration phenomena are considered to be initial steps in the pressure-denaturation process of IPMDH.

A new paradigm for macromolecular crystallography beamlines derived from high-pressure methodology and results

Biological structures can now be investigated at high resolution by high-pressure X-ray macromolecular crystallography (HPMX). The number of HPMX studies is growing, with applications to polynucleotides, monomeric and multimeric proteins, complex assemblies and even a virus capsid. Investigations of the effects of pressure perturbation have encompassed elastic compression of the native state, study of proteins from extremophiles and trapping of higher-energy conformers that are often of biological interest; measurements of the compressibility of crystals and macromolecules were also performed. HPMX results were an incentive to investigate short and ultra-short wavelengths for standard biocrystallography. On cryocooled lysozyme crystals it was found that the data collection efficiency using 33 keV photons is increased with respect to 18 keV photons. This conclusion was extended from 33 keV down to 6.5 keV by exploiting previously published data. To be fully exploited, the potential of higher-energy photons requires detectors with a good efficiency. Accordingly, a new paradigm for MX beamlines was suggested, using conventional short and ultra-short wavelengths, aiming at the collection of very high accuracy data on crystals under standard conditions or under high pressure. The main elements of such beamlines are outlined.

Protein dynamics and pressure: what can high pressure tell us about protein structural flexibility?

After a brief overview of NMR and X-ray crystallography studies on protein flexibility under pressure, we summarize the effects of hydrostatic pressure on the native fold of azurin from Pseudomonas aeruginosa as inferred from the variation of the intrinsic phosphorescence lifetime and the acrylamide bimolecular quenching rate constants of the buried Trp residue. The pressure/temperature response of the globular fold and modulation of its dynamical structure is analyzed both in terms of a reduction of internal cavities and of the hydration of the polypeptide. The study of the effect of two single point cavity forming mutations, F110S and I7S, on the unfolding volume change (ΔV(0)) of azurin and on the internal dynamics of the protein fold under pressure demonstrate that the creation of an internal cavity will enhance the plasticity and lower the stability of the globular structure. This article is part of a Special Issue entitled: Protein Dynamics: Experimental and Computational Approaches.

High pressure macromolecular crystallography for structural biology: a review

In recent years, significant progress in high pressure macromolecular crystallography has been observed. It can be attributed both to the developments in experimental techniques, as well as to recognition of importance of high pressure protein studies in biochemistry and biophysics. The number of protein structures determined at pressure up to 1 GPa is growing. The unique advantages of this method can greatly improve the investigation of higher energy conformers of functional significance and our understanding of functionally important conformers, protein folding processes and the structural base of conformational diseases.

Structure-function perturbation and dissociation of tetrameric urate oxidase by high hydrostatic pressure

Structure-function relationships in the tetrameric enzyme urate oxidase were investigated using pressure perturbation. As the active sites are located at the interfaces between monomers, enzyme activity is directly related to the integrity of the tetramer. The effect of hydrostatic pressure on the enzyme was investigated by x-ray crystallography, small-angle x-ray scattering, and fluorescence spectroscopy. Enzymatic activity was also measured under pressure and after decompression. A global model, consistent with all measurements, discloses structural and functional details of the pressure-induced dissociation of the tetramer. Before dissociating, the pressurized protein adopts a conformational substate characterized by an expansion of its substrate binding pocket at the expense of a large neighboring hydrophobic cavity. This substate should be adopted by the enzyme during its catalytic mechanism, where the active site has to accommodate larger intermediates and product. The approach, combining several high-pressure techniques, offers a new (to our knowledge) means of exploring structural and functional properties of transient states relevant to protein mechanisms.

Biocrystallography: past, present, future

The evolution of biocrystallography from the pioneers' time to the present era of global biology is presented in relation to the development of methodological and instrumental advances for molecular sample preparation and structure elucidation over the last 6 decades. The interdisciplinarity of the field that generated cross-fertilization between physics- and biology-focused themes is emphasized. In particular, strategies to circumvent the main bottlenecks of biocrystallography are discussed. They concern (i) the way macromolecular targets are selected, designed, and characterized, (ii) crystallogenesis and how to deal with physical and biological parameters that impact crystallization for growing and optimizing crystals, and (iii) the methods for crystal analysis and 3D structure determination. Milestones that have marked the history of biocrystallography illustrate the discussion. Finally, the future of the field is envisaged. Wide gaps of the structural space need to be filed and membrane proteins as well as intrinsically unstructured proteins still constitute challenging targets. Solving supramolecular assemblies of increasing complexity, developing a "4D biology" for decrypting the kinematic changes in macromolecular structures in action, integrating these structural data in the whole cell organization, and deciphering biomedical implications will represent the new frontiers.

Advances in High-Pressure Biophysics: Status and Prospects of Macromolecular Crystallography

A survey of the main interests of high pressure for molecular biophysics highlights the possibility of exploring the whole conformational space using pressure perturbation. A better understanding of fundamental mechanisms responsible for the effects of high pressure on biomolecules requires high-resolution molecular information. Thanks to recent instrumental and methodological progress taking advantage of the remarkable adaptation of the crystalline state to hydrostatic compression, pressure-perturbed macromolecular crystallography is now a full-fledged technique applicable to a variety of systems, including large assemblies. This versatility is illustrated by selected applications, including DNA fragments, a tetrameric protein, and a viral capsid. Binding of compressed noble gases to proteins is commonly used to solve the phase problem, but standard macromolecular crystallography would also benefit from the transfer of experimental procedures developed for high-pressure studies. Dedicated short-wavelength synchrotron radiation beamlines are unarguably required to fully exploit the various facets of high-pressure macromolecular crystallography.

Pressure-dependent 13C chemical shifts in proteins: origins and applications

Pressure-dependent (13)C chemical shifts have been measured for aliphatic carbons in barnase and Protein G. Up to 200 MPa (2 kbar), most shift changes are linear, demonstrating pressure-independent compressibilities. CH(3), CH(2) and CH carbon shifts change on average by +0.23, -0.09 and -0.18 ppm, respectively, due to a combination of bond shortening and changes in bond angles, the latter matching one explanation for the gamma-gauche effect. In addition, there is a residue-specific component, arising from both local compression and conformational change. To assess the relative magnitudes of these effects, residue-specific shift changes for protein G were converted into structural restraints and used to calculate the change in structure with pressure, using a genetic algorithm to convert shift changes into dihedral angle restraints. The results demonstrate that residual (13)C alpha shifts are dominated by dihedral angle changes and can be used to calculate structural change, whereas (13)C beta shifts retain significant dependence on local compression, making them less useful as structural restraints.

Structural plasticity of the phage P22 tail needle gp26 probed with xenon gas

The tail needle, gp26, is a highly stable homo-trimeric fiber found in the tail apparatus of bacteriophage P22. In the mature virion, gp26 is responsible for plugging the DNA exit channel, and likely plays an important role in penetrating the host cell envelope. In this article, we have determined the 1.98 A resolution crystal structure of gp26 bound to xenon gas. The structure led us to identify a calcium and a chloride ion intimately bound at the interior of alpha-helical core, as well as seven small cavities occupied by xenon atoms. The two ions engage in buried polar interactions with gp26 side chains that provide specificity and register to gp26 helical core, thus enhancing its stability. Conversely, the distribution of xenon accessible cavities correlates well with the flexibility of the fiber observed in solution and in the crystal structure. We suggest that small internal cavities in gp26 between the helical core and the C-terminal tip allow for flexible swinging of the latter, without affecting the overall stability of the protein. The C-terminal tip may be important in scanning the bacterial surface in search of a cell-envelope penetration site, or for recognition of a yet unidentified receptor on the surface of the host.

Pressure-induced changes in the solution structure of the GB1 domain of protein G

The solution structure of the GB1 domain of protein G at a pressure of 2 kbar is presented. The structure was calculated as a change from an energy-minimised low-pressure structure using (1)H chemical shifts. Two separate changes can be characterised: a compression/distortion, which is linear with pressure; and a stabilisation of an alternative folded state. On application of pressure, linear chemical shift changes reveal that the backbone structure changes by about 0.2 A root mean square, and is compressed by about 1% overall. The alpha-helix compresses, particularly at the C-terminal end, and moves toward the beta-sheet, while the beta-sheet is twisted, with the corners closest to the alpha-helix curling up towards it. The largest changes in structure are along the second beta-strand, which becomes more twisted. This strand is where the protein binds to IgG. Curved chemical shift changes with pressure indicate that high pressure also populates an alternative structure with a distortion towards the C-terminal end of the helix, which is likely to be caused by insertion of a water molecule.

Structural change in a B-DNA helix with hydrostatic pressure

Study of the effects of pressure on macromolecular structure improves our understanding of the forces governing structure, provides details on the relevance of cavities and packing in structure, increases our understanding of hydration and provides a basis to understand the biology of high-pressure organisms. A study of DNA, in particular, helps us to understand how pressure can affect gene activity. Here we present the first high-resolution experimental study of B-DNA structure at high pressure, using NMR data acquired at pressures up to 200 MPa (2 kbar). The structure of DNA compresses very little, but is distorted so as to widen the minor groove, and to compress hydrogen bonds, with AT pairs compressing more than GC pairs. The minor groove changes are suggested to lead to a compression of the hydration water in the minor groove.

High-pressure diffraction studies of molecular organic solids. A personal view

This paper discusses the trends in the experimental studies of molecular organic solids at high pressures by diffraction techniques. Crystallization of liquids, crystallization from solutions and solid-state transformations are considered. Special attention is paid to the high-pressure studies of pharmaceuticals and of biomimetics.

Adaptation of the base-paired double-helix molecular architecture to extreme pressure

The behaviour of the d(GGTATACC) oligonucleotide has been investigated by X-ray crystallography at 295 K in the range from ambient pressure to 2 GPa (approximately 20,000 atm). Four 3D-structures of the A-DNA form (at ambient pressure, 0.55, 1.09 and 1.39 GPa) were refined at 1.60 or 1.65 A resolution. In addition to the diffraction pattern of the A-form, the broad meridional streaks previously explained by occluded B-DNA octamers within the channels of the crystalline A-form matrix were observed up to at least 2 GPa. This work highlights an important property of nucleic acids, their capability to withstand very high pressures, while keeping in such conditions a nearly invariant geometry of base pairs that store and carry genetic information. The double-helix base-paired architecture behaves as a molecular spring, which makes it especially adapted to very harsh conditions. These features may have contributed to the emergence of a RNA World at prebiotic stage.

Structural rigidity of a large cavity-containing protein revealed by high-pressure crystallography

Steric constraints, charged interactions and many other forces important to protein structure and function can be explored by mutagenic experiments. Research of this kind has led to a wealth of knowledge about what stabilizes proteins in their folded states. To gain a more complete picture requires that we perturb these structures in a continuous manner, something mutagenesis cannot achieve. With high pressure crystallographic methods it is now possible to explore the detailed properties of proteins while continuously varying thermodynamic parameters. Here, we detail the structural response of the cavity-containing mutant L99A of T4 lysozyme, as well as its pseudo wild-type (WT*) counterpart, to hydrostatic pressure. Surprisingly, the cavity has almost no effect on the pressure response: virtually the same changes are observed in WT* as in L99A under pressure. The cavity is most rigid, while other regions deform substantially. This implies that while some residues may increase the thermodynamic stability of a protein, they may also be structurally irrelevant. As recently shown, the cavity fills with water at pressures above 100 MPa while retaining its overall size. The resultant picture of the protein is one in which conformationally fluctuating side groups provide a liquid-like environment, but which also contribute to the rigidity of the peptide backbone.

Protein under pressure: Molecular dynamics simulation of the arc repressor

Experimental nuclear magnetic resonance results for the Arc Repressor have shown that this dimeric protein dissociates into a molten globule at high pressure. This structural change is accompanied by a modification of the hydrogen-bonding pattern of the intermolecular beta-sheet: it changes its character from intermolecular to intramolecular with respect to the two monomers. Molecular dynamics simulations of the Arc Repressor, as a monomer and a dimer, at elevated pressure have been performed with the aim to study this hypothesis and to identify the major structural and dynamical changes of the protein under such conditions. The monomer appears less stable than the dimer. However, the complete dissociation has not been seen because of the long timescale needed to observe this phenomenon. In fact, the protein structure altered very little when increasing the pressure. It became slightly compressed and the dynamics of the side-chains and the unfolding process slowed down. Increasing both, temperature and pressure, a tendency of conversion of intermolecular into intramolecular hydrogen bonds in the beta-sheet region has been detected, supporting the mentioned hypothesis. Also, the onset of denaturation of the separated chains was observed.

What lies in the future of high-pressure bioscience?

Without being comprehensive in this mini-review, I will address perspectives, some speculative, for the development and use of high pressure to explore biochemical phenomena. This will be illustrated with several examples.

High-pressure macromolecular crystallography (HPMX): status and prospects

Recent technical developments, achievements and prospects of high-pressure (HP) macromolecular crystallography (MX) are reviewed. Technical difficulties associated with this technique have been essentially solved by combining synchrotron radiation of ultra-short wavelength, large-aperture diamond anvil cells and new sample-mounting techniques. The quality of diffraction data collected at HP can now meet standards of conventional MX. The exploitation of the potential of the combination of X-ray diffraction and high-pressure perturbation is progressing well. The ability of pressure to shift the population distribution of conformers in solution, which is exploited in particular by NMR, can also be used in the crystalline state with specific advantages. HPMX has indeed bright prospects, in particular to elucidate the structure of higher-energy conformers that are often of high biological significance. Furthermore, HPMX may be of interest for conventional crystallographic studies, as pressure is a fairly general tool to improve order in pre-existing crystals with minimal perturbation of the native structure.

High pressure macromolecular crystallography: the 140-MPa crystal structure at 2.3 A resolution of urate oxidase, a 135-kDa tetrameric assembly

We report the three-dimensional structure determined by high-pressure macromolecular crystallography (HPMX) of a 135-kDa homo-tetrameric enzyme, urate oxidase from Aspergillus flavus complexed with its potent inhibitor 8-azaxanthin. Urate oxidase crystals are quite sensitive to pressure, as three-dimensional order is lost at about 180 MPa. A highly complete 2.3 A resolution data set was collected at 140 MPa, close to the critical pressure. Crystal structures at atmospheric pressure and at high pressure were refined in the orthorhombic space group I222 with final crystallographic R factors 14.1% and 16.1%, respectively. The effect of pressure on temperature factors, ordered water molecules, hydrogen bond lengths, contacts, buried surface areas as well as cavity volume was investigated. Results suggest that the onset of disruption of the tetrameric assembly by pressure has been captured in the crystalline state.