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Abstract Two related classes of ligand-binding hemec-containing proteins with a high degree of structural homology have been identified and characterized over recent decades: cytochromes P460 (cyts P460), defined by an unusual heme-lysine cross-link, and cytochromesc′-β (cytsc′-β), containing a canonicalc-heme without the lysine cross-link. The shared protein fold of the cyt P460-cytc′-β superfamily can accommodate a variety of heme environments with entirely different reactivities. On the one hand, cyts P460 with polar distal pockets have been shown to oxidize NH₂OH to NO and/or N₂O via proton-coupled electron transfer. On the other hand, cytsc′-β with hydrophobic distal pockets have a proposed gas binding function similar to the unrelated, but more extensively characterized, alpha helical cytochromesc′. Recent studies have also identified ‘halfway house’ proteins (cyts P460 with non-polar heme pockets and cytsc′-β with polar distal heme pockets) with functions yet to be resolved. Here, we review the structural, spectroscopic and enzymatic properties of the cyt P460-cytc′-β superfamily with a view to understanding the structural determinants of their different functional properties. Graphical abstract 

Cytochromes P460 oxidise hydroxylamine within the nitrogen cycle and contain as their active site an unusual catalyticc-type haem where the porphyrin is crosslinked to the protein via a lysine... 

Room-temperature macromolecular crystallography allows protein structures to be determined under close-to-physiological conditions, permits dynamic freedom in protein motions and enables time-resolved studies. In the case of metalloenzymes that are highly sensitive to radiation damage, such room-temperature experiments can present challenges, including increased rates of X-ray reduction of metal centres and site-specific radiation-damage artefacts, as well as in devising appropriate sample-delivery and data-collection methods. It can also be problematic to compare structures measured using different crystal sizes and light sources. In this study, structures of a multifunctional globin, dehaloperoxidase B (DHP-B), obtained using several methods of room-temperature crystallographic structure determination are described and compared. Here, data were measured from large single crystals and multiple microcrystals using neutrons, X-ray free-electron laser pulses, monochromatic synchrotron radiation and polychromatic (Laue) radiation light sources. These approaches span a range of 18 orders of magnitude in measurement time per diffraction pattern and four orders of magnitude in crystal volume. The first room-temperature neutron structures of DHP-B are also presented, allowing the explicit identification of the hydrogen positions. The neutron data proved to be complementary to the serial femtosecond crystallography data, with both methods providing structures free of the effects of X-ray radiation damage when compared with standard cryo-crystallography. Comparison of these room-temperature methods demonstrated the large differences in sample requirements, data-collection time and the potential for radiation damage between them. With regard to the structure and function of DHP-B, despite the results being partly limited by differences in the underlying structures, new information was gained on the protonation states of active-site residues which may guide future studies of DHP-B. 

High-throughput X-ray crystal structures of protein–ligand complexes are critical to pharmaceutical drug development. However, cryocooling of crystals and X-ray radiation damage may distort the observed ligand binding. Serial femtosecond crystallography (SFX) using X-ray free-electron lasers (XFELs) can produce radiation-damage-free room-temperature structures. Ligand-binding studies using SFX have received only modest attention, partly owing to limited beamtime availability and the large quantity of sample that is required per structure determination. Here, a high-throughput approach to determine room-temperature damage-free structures with excellent sample and time efficiency is demonstrated, allowing complexes to be characterized rapidly and without prohibitive sample requirements. This yields high-quality difference density maps allowing unambiguous ligand placement. Crucially, it is demonstrated that ligands similar in size or smaller than those used in fragment-based drug design may be clearly identified in data sets obtained from <1000 diffraction images. This efficiency in both sample and XFEL beamtime opens the door to true high-throughput screening of protein–ligand complexes using SFX.