Alessandro Vergara

An overview of biological macromolecule crystallization.

The elucidation of the three dimensional structure of biological macromolecules has provided an important contribution to our current understanding of many basic mechanisms involved in life processes. This enormous impact largely results from the ability of X-ray crystallography to provide accurate structural details at atomic resolution that are a prerequisite for a deeper insight on the way in which bio-macromolecules interact with each other to build up supramolecular nano-machines capable of performing specialized biological functions. With the advent of high-energy synchrotron sources and the development of sophisticated software to solve X-ray and neutron crystal structures of large molecules, the crystallization step has become even more the bottleneck of a successful structure determination. This review introduces the general aspects of protein crystallization, summarizes conventional and innovative crystallization methods and focuses on the new strategies utilized to improve the success rate of experiments and increase crystal diffraction quality.

High resolution crystal structure of deoxy hemoglobin from Trematomus bernacchii at different pH values: The role of histidine residues in modulating the strength of the root effect

The Root effect is a widespread property in fish hemoglobins (Hbs) that produces a drastic reduction of cooperativity and oxygen‐binding ability at acidic pH. Here, we report the high‐resolution structure of the deoxy form of Hb isolated from the Antarctic fish Trematomus bernacchii (HbTb) crystallized at pH 6.2 and 8.4. The structure at acidic pH has been previously determined at a moderate resolution (Ito et al., J Mol Biol 1995;250:648–658). Our results provide a clear picture of the events occurring upon the pH increase from 6.2 to 8.4, observed within a practically unchanged crystal environment. In particular, at pH 8.4, the interaspartic hydrogen bond at the α1β2 interface is partially broken, suggesting a pKa close to 8.4 for Asp95α. In addition, a detailed survey of the histidine modifications, caused by the change in pH, also indicates that at least three hot regions of the molecule are modified (Eβ helix, Cβ‐tail, CDα corner) and can be considered to be involved at various levels in the release of the Root protons. Most importantly, at the CDα corner, the break of the salt bridge Asp48α–His55α allows us to describe a detailed mechanism that transmits the modification from the CDα corner far to the α heme. More generally, the results shed light on the role played by the histidine residues in modulating the strength of the Root effect and also support the emerging idea that the structural determinants, at least for a part of the Root effect, are specific of each Hb endowed with this property. Proteins 2006.

Interaction of Anticancer Ruthenium Compounds with Proteins: High-Resolution X-ray Structures and Raman Microscopy Studies of the Adduct between Hen Egg White Lysozyme and AziRu

The binding properties of AziRu, a ruthenium(III) complex with high antiproliferative activity, toward a hen egg white lysozyme have been investigated by X-ray crystallography and Raman microscopy. The data provide clear evidence on the mechanism of AziRu-protein adduct formation and of ligand exchange in the crystal state.

Minimal structural requirements for root effect: crystal structure of the cathodic hemoglobin isolated from the antarctic fish Trematomus newnesi.

The cathodic hemoglobin component of the Antarctic fish Trematomus newnesi (HbCTn) is a Root‐effect protein. The interpretation of its functional properties in relation to its sequence is puzzling. Indeed, HbCTn sequence is characterized by an extremely low histidyl content, and in particular by the lack of His146β and His69β, which are believed to be important in Bohr and Root effects, respectively. Furthermore, previous analyses suggested that the local environment of Asp95α, Asp99β, and Asp101β should not be appropriate for the formation of Asp–Asp interactions, which are important for the Root effect. Here, we report the high‐resolution crystal structure of the deoxy form of HbCTn. Our data provide a structural interpretation for the very low oxygen affinity of the protein and insights into the structural determinants of the Root effect protein. The structure demonstrates that the presence of Ile41α and Ser97α at the α1β2 interface does not prevent the formation of the inter‐Asp interactions in HbCTn, as previous studies had suggested. The present data indicate that the hydrogen bond formed between Asp95α and Asp101β, which is stabilized by Asp99β, is per se sufficient to generate the Root effect, and it is the minimal structural requirement needed for the design of Root‐effect Hbs. Proteins 2006.

The mode of action of anticancer gold-based drugs: a structural perspective

The interactions between a few representative gold-based drugs and hen egg white lysozyme were studied by X-ray crystallography. High resolution crystal structures solved for three metallodrug-protein adducts provide valuable insight into the molecular mechanism of these promising metal compounds and the inherent protein metalation processes.

Structure and flexibility in cold-adapted iron superoxide dismutases: The case of the enzyme isolated from Pseudoalteromonas haloplanktis

Superoxide dismutases (SODs) are metalloenzymes catalysing the dismutation of superoxide anion radicals into molecular oxygen and hydrogen peroxide. Here, we present the crystal structure of a cold-adapted Fe-SOD from the Antarctic eubacterium Pseudoalteromonas haloplanktis (PhSOD), and that of its complex with sodium azide. The structures were compared with those of the corresponding homologues having a high sequence identity with PhSOD, such as the mesophilic SOD from Escherichia coli (EcSOD) or Pseudomonas ovalis, and the psychrophilic SOD from Aliivibrio salmonicida (AsSOD). These enzymes shared a large structural similarity, such as a conserved tertiary structure and arrangement of the two monomers, an almost identical total number of inter- and intramolecular hydrogen bonds and salt bridges. However, the two cold-adapted SODs showed an increased flexibility of the active site residues with respect to their mesophilic homologues. Structural information was combined with a characterisation of the chemical and thermal stability performed by CD and fluorescence measurements. Despite of its psychrophilic origin, the denaturation temperature of PhSOD was comparable with that of the mesophilic EcSOD, whereas AsSOD showed a lower denaturation temperature. On the contrary, the values of the denaturant concentration at the transition midpoint were in line with the psychrophilic/mesophilic origin of the proteins. These data provide additional support to the hypothesis that cold-adapted enzymes achieve efficient catalysis at low temperature, by increasing the flexibility of their active site; moreover, our results underline how fine structural modifications can alter enzyme flexibility and/or stability without compromising the overall structure of typical rigid enzymes, such as SODs.

Spectroscopic and crystallographic characterization of a tetrameric hemoglobin oxidation reveals structural features of the functional intermediate relaxed/tense state.

Tetrameric hemoglobins represent the most commonly used model for the description of the basic concepts of protein allostery. The classical stereochemical model assumes a concerted transition of the protein, upon oxygen release, from the relaxed (R) to the tense (T) state. Despite the large amount of data accumulated on the end-points of the transition, scarce structural information is available on the intermediate species along the pathway. Here we report a spectroscopic characterization of the autoxidation process of the Trematomus newnesi major Hb component and the atomic resolution structure (1.25 A) of an intermediate form along the pathway characterized by a different binding and oxidation state of the alpha and beta chains. In contrast to the alpha-heme iron, which binds a CO molecule, the beta iron displays a pentacoordinated oxidized state, which is rare in tetrameric hemoglobins. Interestingly, the information provided by the present analysis is not limited to the characterization of the peculiar oxidation process of Antarctic fish hemoglobins. Indeed, this structure represents the most detailed snapshot of hemoglobin allosteric transition hitherto achieved. Upon ligand release at the beta heme, a cascade of structural events is observed. Notably, several structural features of the tertiary structure of the alpha and beta chains closely resemble those typically observed in the deoxygenated state. The overall quaternary structure also becomes intermediate between the R and the T state. The analysis of the alterations induced by the ligand release provides a clear picture of the temporal sequence of the events associated with the transition. The implications of the present findings have also been discussed in the wider context of tetrameric Hbs.

Correlation between Hemichrome Stability and the Root Effect in Tetrameric Hemoglobins

Oxidation of Hbs leads to the formation of different forms of Fe(III) that are relevant to a range of biochemical and physiological functions. Here we report a combined EPR/x-ray crystallography study performed at acidic pH on six ferric tetrameric Hbs. Five of the Hbs were isolated from the high-Antarctic notothenioid fishes Trematomus bernacchii, Trematomus newnesi, and Gymnodraco acuticeps, and one was isolated from the sub-Antarctic notothenioid Cottoperca gobio. Our EPR analysis reveals that 1), in all of these Hbs, at acidic pH the aquomet form and two hemichromes coexist; and 2), only in the three Hbs that exhibit the Root effect is a significant amount of the pentacoordinate (5C) high-spin Fe(III) form found. The crystal structure at acidic pH of the ferric form of the Root-effect Hb from T. bernacchii is also reported at 1.7 A resolution. This structure reveals a 5C state of the heme iron for both the alpha- and beta-chains within a T quaternary structure. Altogether, the spectroscopic and crystallographic results indicate that the Root effect and hemichrome stability at acidic pH are correlated in tetrameric Hbs. Furthermore, Antarctic fish Hbs exhibit higher peroxidase activity than mammalian and temperate fish Hbs, suggesting that a partial hemichrome state in tetrameric Hbs, unlike in monomeric Hbs, does not remove the need for protection from peroxide attack, in contrast to previous results from monomeric Hbs.

Investigating the ruthenium metalation of proteins: X-ray structure and Raman microspectroscopy of the complex between RNase A and AziRu.

A Raman-assisted crystallographic study on the adduct between AziRu, a Ru(III) complex with high antiproliferative activity, and RNase A is presented. The protein structure is not perturbed significantly by the Ru label. The metal coordinates to ND atoms of His105 or of His119 imidazole rings, losing all of its original ligands but retaining octahedral, although distorted, coordination geometry. The AziRu binding inactivates the enzyme, suggesting that its antitumor action can be exerted by a mechanism of competitive inhibition.

Physical aspects of protein crystal growth investigated with the Advanced Protein Crystallization Facility in reduced-gravity environments.

The physicochemical aspects of protein crystallization in reduced-gravity environments ( micro g) have been investigated with the Advanced Protein Crystallization Facility during six space missions. This review summarizes the results, dealing with the mechanisms of nucleation and crystal growth and with the quality of the crystals that were obtained under reduced gravity as well as under normal gravity on earth. Statistical analyses of the experimental data strongly support the fact that micro g has a positive effect on crystallization and on crystal quality. A comparison of experiments and theories of protein crystallization in reduced-gravity environments is presented. Recommendations for improving the performance of protein crystallization experiments in micro g and on earth are discussed.