A Working Group consisting of the co-authors of this paper was established in 2020 to re-evaluate the standard valence geometry used for the validation of nucleic acid structure models in the Protein Data Bank (PDB). This Working Group re-examined the dependence of Cambridge Structural Database (CSD) derived targets on base and sugar type, sugar pucker, and phosphate and glycosidic conformation, before comparing those targets with the geometry of a quality-filtered reference set of nucleic acid crystal structural models held in the PDB. This revealed that the valence bond and angle mean values are close to the CSD targets, but many parameters have highly non-Gaussian or even multimodal distributions. One explanation is the inconsistency of restraints used over time and by different refinement programs. The Working Group recommends a new validation scheme for use by the PDB. For this purpose, we have developed a new three-tier scale for outlier detection-graded as Preferred, Allowed, and Of Concern intervals-based on a combination of quality-curated reference data from the CSD and the PDB. The proposed approach to validation should lead to improved nucleic acid models in (future) PDB-deposited macromolecular structures.

L-Asparaginases hydrolyze L-asparagine to L-aspartate with the release of ammonia. Currently, three completely unrelated structural classes of L-asparaginases are known, further subdivided into five types. In each class, the hydrolysis reaction is thought to proceed via a nucleophilic attack of an activated Thr or Ser residue on the carbonyl Csp2 atom of the substrate amide group. With the possible exception of class 2 L-asparaginases, which function as N-terminal nucleophile (Ntn) hydrolases, the identity of the nucleophilic residue is, or at least has been historically, the subject of some controversy, and even in class 2 this issue may not be so entirely obvious. Structural chemistry has, however, excellent tools to figure out reaction mechanisms, based on the application of Bürgi's structure correlation method (SCM). Its principle allows one to predict the reaction trajectory if sufficient structural (crystallographic) examples of the reagents along the reaction path are known. With respect to the nucleophilic attack on a carbonyl group, the stereochemistry is governed by the nucleophile...electrophile distance and the Bürgi–Dunitz angle, later supplemented with the Flippin–Lodge angle. The latter angle is shown to be a poor parameter and is better replaced by the Herschlag dihedral between the planes of the attacking nucleophile and the attacked electrophile. In structural enzymology, applicability of the SCM principle requires the availability of structural examples of the enzyme in question in complex with the substrate or product of the catalytic reaction. In this work, we applied the SCM concept to the three classes of L-asparaginases, identifying in each case the most probable nucleophilic residue as Thr12 in EcAII (class 1), Thr179 in EcAIII (class 2) and Ser48 in ReAV (class 3). In addition, we applied the SCM analysis to the newly identified group of asparaginases without proper classification, called short-chain asparaginases, providing a basis for their proper affiliation in class 1. Finally, the SCM analysis shows that the chirality of the nucleophilic attack in class 2 asparaginases (pro-R) is opposite to that in all other asparaginases.

Isothermal titration calorimetry (ITC) studies of the enzyme kinetics and substrate specificity of Rhizobium etli Class 3 L-asparaginases, ReAIV (constitutive) and ReAV (inducible), showed that despite highly conserved catalytic site, the two isoforms differ significantly in thermostability, zinc affinity, and biochemical properties. As part of a wider investigation of potential non-natural substrates, acrylamide was tested, revealing a pronounced heat effect with ReAIV but none with ReAV. Crystallographic analysis showed the formation of a Michael adduct between acrylamide and a surface-exposed cysteine 183 in ReAIV, while the catalytic activity toward L-asparagine hydrolysis remained unaffected. These findings highlight the unique and multimodal reactivity of ReAIV, suggesting its potential dual application in the food industry: in selective removal of L-asparagine and in covalent sequestration of acrylamide under mild conditions. The acrylamide modification improved crystal morphology of ReAIV, offering practical advantages for structural studies. Additionally, a covalent modification of the catalytic Ser47 residue was observed in the presented crystal structure. Based on B-factor analysis, literature data, and detection of borate contamination in the laboratory water, this modification was interpreted as an orthoborate ester.

Common bean ( Phaseolus vulgaris ) encodes three class 2 L-asparaginase enzymes: two potassium-dependent enzymes [PvAIII(K)-1 and PvAIII(K)-2] and a potassium-independent enzyme (PvAIII). Here, we present the crystal structure of PvAIII, which displays a rare P 2 space-group symmetry and a unique pseudosymmetric 4 1 -like double-helical packing. The asymmetric unit contains 32 protein chains (16 αβ units labeled A – P ) organized into two right-handed coiled arrangements, each consisting of four PvAIII (αβ) 2 dimers. Detailed analysis of the crystal structure revealed that this unusual packing originates from three factors: (i) the ability of the PvAIII molecules to form extended intermolecular β-sheets, a feature enabled by the PvAIII sequence and secondary structure, (ii) incomplete degradation of the flexible linker remaining at the C-terminus of α subunits of protein chain C after the autoproteolytic cleavage (maturation) of the PvAIII precursor and (iii) intermolecular entanglement between protein chains from the two helices to create `hydrogen-bond linchpins' that connect adjacent protein chains. The K m value of PvAIII for L-asparagine is approximately five times higher than for β-peptides, suggesting that the physiological role of PvAIII may be more related to the removal of toxic β-peptides than to basic L-asparagine metabolism. A comparison of the active sites of PvAIII and PvAIII(K)-1 shows that the proteins have nearly identical residues in the catalytic center, except for Thr219, which is unique to PvAIII. To test whether the residue type at position 219 affects the enzymatic activity of PvAIII, we designed and produced a T219S mutant. The kinetic parameters determined for L-asparagine hydrolysis indicate that the T/S residue type at position 219 does not affect the L-asparaginase activity of PvAIII.

The ReAV enzyme from Rhizobium etli, a representative of Class 3 L-asparaginases, is sequentially and structurally different from other known L-asparaginases. This distinctiveness makes ReAV a candidate for novel antileukemic therapies. ReAV is a homodimeric protein, with each subunit containing a highly specific zinc-binding site created by two cysteines, a lysine, and a water molecule. Two Ser-Lys tandems (Ser48-Lys51, Ser80-Lys263) are located in the close proximity of the metal binding site, with Ser48 hypothesized to be the catalytic nucleophile. To further investigate the catalytic process of ReAV, site-directed mutagenesis was employed to introduce alanine substitutions at residues from the Ser-Lys tandems and at Arg47, located near the Ser48-Lys51 tandem. These mutational studies, along with enzymatic assays and X-ray structure determinations, demonstrated that substitution of each of these highly conserved residues abolished the catalytic activity, confirming their essential role in enzyme mechanism.

Rhizobium etli is a nitrogen-fixing bacterium that encodes two l-asparaginases. The structure of the inducible R. etli asparaginase ReAV has been recently determined to reveal a protein with no similarity to known enzymes with l-asparaginase activity, but showing a curious resemblance to glutaminases and β-lactamases. The uniqueness of the ReAV sequence and 3D structure make the enzyme an interesting candidate as potential replacement for the immunogenic bacterial-type asparaginases that are currently in use for the treatment of acute lymphoblastic leukemia. The detailed catalytic mechanism of ReAV is still unknown; therefore, the enzyme was subjected to mutagenetic experiments to investigate its catalytic apparatus. In this work, we generated two ReAV variants of the conserved Lys138 residue (K138A and K138H) that is involved in zinc coordination in the wild-type protein and studied them kinetically and structurally. We established that the activity of wild-type ReAV and the generated variants is significantly reduced in the presence of Cd2+ cations, which slow down the proteins while improving their apparent substrate affinity. Moreover, the inhibitory effect of Cd2+ is enhanced by the substitutions of Lys138, which disrupt the metal coordination sphere. The proteins with impaired activity but increased affinity were cocrystallized with the L-Asn substrate. Here, we present the crystal structures of wild-type ReAV and its K138A and K138H variants, unambiguously revealing bound l-asparagine in the active site. After careful analysis of the stereochemistry of the nucleophilic attack, we assign the role of the primary nucleophile of ReAV to Ser48. Furthermore, we propose that the reaction catalyzed by ReAV proceeds according to a double-displacement mechanism.

Ultra-high-resolution crystal structures of proteins provide critical insights into protein structure, dynamics, hydrogen bonding, and solvent networks. Crambin, a small hydrophobic storage protein consisting of 46 residues (4.7 kDa), is found in the embryonic tissue of seeds from Crambe abyssinica. This protein is renowned for its ability to crystallize readily, forming some of the best-ordered macromolecular crystals known, which diffract X-rays to the highest sub-atomic resolution recorded for any protein to date.We have previously reported the room temperature structure of crambin, refined to an exceptional resolution of 0.70 Å using SHELXL. That analysis revealed intricate details of the dynamic solvent network, characterized by alternative side chain conformations and shifts in water molecule positions. In this work, we extend our investigation by presenting new structural data collected at cryogenic temperatures: 15K using liquid helium and 100K using liquid nitrogen cooling.We will report the ultra-high-resolution structures at 15K and 100K, providing a comparative analysis of the solvent networks across these different temperature datasets. This comparison aims to deepen our understanding of the solvent and protein dynamics, offering valuable insights into the protein interactions within solvent environments. Our findings underscore the significance of ultrahigh-resolution crystallography in elucidating the complex interplay between proteins and their solvent environments, with potential implications for the broader field of structural biology.

Ultrahigh-resolution structures provide unprecedented details about protein dynamics, hydrogen bonding and solvent networks. The reported 0.70 Å, room-temperature crystal structure of crambin is the highest-resolution ambient-temperature structure of a protein achieved to date. Sufficient data were collected to enable unrestrained refinement of the protein and associated solvent networks using SHELXL. Dynamic solvent networks resulting from alternative side-chain conformations and shifts in water positions are revealed, demonstrating that polypeptide flexibility and formation of clathrate-type structures at hydrophobic surfaces are the key features endowing crambin crystals with extraordinary diffraction power.

The BisI family of restriction endonucleases is unique in requiring multiple methylated or hydroxymethylated cytosine residues within a short recognition sequence (GCNGC), and in cleaving directly within this sequence, rather than at a distance. Here, we report that the number of modified cytosines that are required for cleavage can be tuned by the salt concentration. We present crystal structures of two members of the BisI family, NhoI and Eco15I_Ntd (N-terminal domain of Eco15I), in the absence of DNA and in specific complexes with tetra-methylated GCNGC target DNA. The structures show that NhoI and Eco15I_Ntd sense modified cytosine bases in the context of double-stranded DNA (dsDNA) without base flipping. In the co-crystal structures of NhoI and Eco15I_Ntd with DNA, the internal methyl groups (G5mCNGC) interact with the side chains of an (H/R)(V/I/T/M) di-amino acid motif near the C-terminus of the distal enzyme subunit and arginine residue from the proximal subunit. The external methyl groups (GCNG5mC) interact with the proximal enzyme subunit, mostly through main chain contacts. Surface plasmon resonance analysis for Eco15I_Ntd shows that the internal and external methyl binding pockets contribute about equally to sensing of cytosine methyl groups.

The absence of solvent molecules in high-resolution protein crystal structure models deposited in the Protein Data Bank (PDB) contradicts the fact that, for proteins crystallized from aqueous media, water molecules are always expected to bind to the protein surface, as well as to some sites in the protein interior. An analysis of the contents of the PDB indicated that the expected ratio of the number of water molecules to the number of amino-acid residues exceeds 1.5 in atomic resolution structures, decreasing to 0.25 at around 2.5 Å resolution. Nevertheless, almost 800 protein crystal structures determined at a resolution of 2.5 Å or higher are found in the current release of the PDB without any water molecules, whereas some other depositions have unusually low or high occupancies of modeled solvent. Detailed analysis of these depositions revealed that the lack of solvent molecules might be an indication of problems with either the diffraction data, the refinement protocol, the deposition process or a combination of these factors. It is postulated that problems with solvent structure should be flagged by the PDB and addressed by the depositors.