International audienceMolecular oxygen (O2) is a key player in many fundamental biological processes. However, the combination of the labile nature and poor affinity of O2 often makes this substrate difficult to introduce into crystals at sufficient concentrations to enable protein/O2 interactions to be deciphered in sufficient detail. To overcome this problem, a gas pressure cell has been developed specifically for the `soak-and-freeze' preparation of crystals of O2-dependent biological molecules. The `soak-and-freeze' method uses high pressure to introduce oxygen molecules or krypton atoms (O2 mimics) into crystals which, still under high pressure, are then cryocooled for X-ray data collection. Here, a proof of principle of the gas pressure cell and the methodology developed is demonstrated with crystals of enzymes (lysozyme, thermolysin and urate oxidase) that are known to absorb and bind molecular oxygen and/or krypton. The successful results of these experiments lead to the suggestion that the soak-and-freeze method could be extended to studies involving a wide range of gases of biological, medical and/or environmental interest, including carbon monoxide, ethylene, methane and many others

Deuterated proteins and other bio-derived molecules are important for NMR spectroscopy, neutron reflectometry, small angle neutron scattering, and neutron protein crystallography. In the current study we optimized expression media and cell culture conditions to produce high levels of 3 different deuterated human carbonic anhydrases (hCAs). The labeled hCAs were then characterized and tested for deuterium incorporation by mass spectrometry, temperature stability, and propensity to crystallize. The results show that is possible to get very good yields (>10 mg of pure protein per liter of cell culture under deuterated conditions) and that protein solubility is unaffected at the crystallization concentrations tested. Using unlabeled carbon source and recycled heavy water, we were able to get 65-77% deuterium incorporation, sufficient for most neutron-based techniques, and in a very cost-effective way. For most deuterated proteins characterized in the literature, the solubility and thermal stability is reduced. The data reported here is consistent with these observations and it was clear that there are measurable differences between hydrogenous and deuterated versions of the same protein in T and how they crystallize.

Triose-phosphate isomerase (TIM) catalyses the interconversion of dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. Two catalytic mechanisms have been proposed based on two reaction-intermediate analogues, 2-phosphoglycolate (2PG) and phosphoglycolohydroxamate (PGH), that have been used as mimics of the cis-enediol(ate) intermediate in several studies of TIM. The protonation states that are critical for the mechanistic interpretation of these structures are generally not visible in the X-ray structures. To resolve these questions, it is necessary to determine the hydrogen positions using neutron crystallography. Neutron crystallography requires large crystals and benefits from replacing all hydrogens with deuterium. Leishmania mexicana triosephosphate isomerase was therefore perdeuterated and large crystals with 2PG and PGH were produced. Neutron diffraction data collected from two crystals with different volumes highlighted the importance of crystal volume, as smaller crystals required longer exposures and resulted in overall worse statistics.

Human carbonic anhydrase IX (CA IX) expression is upregulated in hypoxic solid tumours, promoting cell survival and metastasis. This observation has made CA IX a target for the development of CA isoform-selective inhibitors. To enable structural studies of CA IX–inhibitor complexes using X-ray and neutron crystallography, a CA IX surface variant (CA IXSV; the catalytic domain with six surface amino-acid substitutions) has been developed that can be routinely crystallized. Here, the preparation of protiated (H/H), H/D-exchanged (H/D) and deuterated (D/D) CA IXSV for crystallographic studies and their structural comparison are described. Four CA IXSV X-ray crystal structures are compared: two H/H crystal forms, an H/D crystal form and a D/D crystal form. The overall active-site organization in each version is essentially the same, with only minor positional changes in active-site solvent, which may be owing to deuteration and/or resolution differences. Analysis of the crystal contacts and packing reveals different arrangements of CA IXSV compared with previous reports. To our knowledge, this is the first report comparing three different deuterium-labelled crystal structures of the same protein, marking an important step in validating the active-site structure of CA IXSV for neutron protein crystallography.