Arsenic is a widely distributed environmental toxin whose presence in drinking water poses a threat to >140 million people worldwide. The respiratory enzyme arsenite oxidase from various bacteria catalyses the oxidation of arsenite to arsenate and is being developed as a biosensor for arsenite. The arsenite oxidase from Rhizobium sp. str. NT-26 (a member of the Alphaproteobacteria) is a heterotetramer consisting of a large catalytic subunit (AioA), which contains a molybdenum centre and a 3Fe-4S cluster, and a small subunit (AioB) containing a Rieske 2Fe-2S cluster. Stopped-flow spectroscopy and isothermal titration calorimetry (ITC) have been used to better understand electron transfer through the redox-active centres of the enzyme, which is essential for biosensor development. Results show that oxidation of arsenite at the active site is extremely fast with a rate of >4000s−1 and reduction of the electron acceptor is rate-limiting. An AioB-F108A mutation results in increased activity with the artificial electron acceptor DCPIP and decreased activity with cytochrome c, which in the latter as demonstrated by ITC is not due to an effect on the protein-protein interaction but instead to an effect on electron transfer. These results provide further support that the AioB F108 is important in electron transfer between the Rieske subunit and cytochrome c and its absence in the arsenite oxidases from the Betaproteobacteria may explain the inability of these enzymes to use this electron acceptor.

Many proteins naturally carry metal centers, with a large share of them being in the active sites of several enzymes. Paramagnetic effects are a powerful source of structural information and, therefore, if the native metal is paramagnetic, or it can be functionally substituted with a paramagnetic one, paramagnetic effects can be used to study the metal sites, as well as the overall structure of the protein. One notable example is cobalt(II) substitution for zinc(II) in carbonic anhydrase. In this manuscript we investigate the effects of sodium thiocyanate on the chemical environment of the metal ion of the human carbonic anhydrase II. The electron paramagnetic resonance (EPR) titration of the cobalt(II) protein with thiocyanate shows that the EPR spectrum changes from A-type to C-type on passing from 1:1 to 1:1000-fold ligand excess. This indicates the occurrence of a change in the electronic structure, which may reflect a sizable change in the metal coordination environment in turn caused by a modification of the frozen solvent glass. However, paramagnetic nuclear magnetic resonance (NMR) data indicate that the metal coordination cage remains unperturbed even in 1:1000-fold ligand excess. This result proves that the C-type EPR spectrum observed at large ligand concentration should be ascribed to the low temperature at which EPR measurements are performed, which impacts on the structure of the protein when it is destabilized by a high concentration of a chaotropic agent.

Background: The STEAP1 is a cell-surface antigen over-expressed in prostate cancer, which contributes to tumor progression and aggressiveness. However, the molecular mechanisms underlying STEAP1 and its structural determinants remain elusive. Methods: The fraction capacity of Butyl- and Octyl-Sepharose matrices on LNCaP lysates was evaluated by manipulating the ionic strength of binding and elution phases, followed by a Co-Immunoprecipitation (Co-IP) polishing. Several potential stabilizing additives were assessed, and the melting temperature (Tm) values ranked the best/worst compounds. The secondary structure of STEAP1 was identified by circular dichroism. Results: The STEAP1 was not fully captured with 1.375 M (Butyl), in contrast with interfering heterologous proteins, which were strongly retained and mostly eluted with water. This single step demonstrated higher selectivity of Butyl-Sepharose for host impurities removal from injected crude samples. Co-IP allowed recovering a purified fraction of STEAP1 and contributed to unveil potential physiologically interacting counterparts with the target. A Tm of  55 °C was determined, confirming STEAP1 stability in the purification buffer. A predominant α-helical structure was identified, ensuring the protein’s structural stability. Conclusions: A method for successfully isolating human STEAP1 from LNCaP cells was provided, avoiding the use of detergents to achieve stability, even outside a membrane-mimicking environment.

Nucleotide-processing enzymes are key players in biological processes. They often operate through high substrate specificity for catalysis. How such specificity is achieved is unclear. Here, we dealt with this question by investigating all-α dimeric deoxyuridine triphosphate nucleotidohydrolases (dUTPases). Typically, these dUTPases hydrolyze either dUTP or deoxyuridine diphosphate (dUDP) substrates. However, the dUTPase enzyme DR2231 from Deinococcus radiodurans selectively hydrolyzes dUTP only, and not dUDP. By means of extended classical molecular dynamics simulations and quantum chemical calculations, we show that DR2231 achieves this specificity for dUTP via second-shell basic residues that, together with the two catalytic magnesium ions, contribute to properly orienting the γ-phosphate of dUTP in a prereactive state. This allows a nucleophilic water to be correctly placed and activated in order to perform substrate hydrolysis. We show that this enzymatic mechanism is not viable when dUDP is bound to DR2231. Importantly, in several other dUTPases capable of hydrolyzing either dUTP or dUDP, we detected that active site second-shell basic residues are more in number, anchoring the β-phosphate of the nucleotide substrate too, in contrast to what is observed in DR2231. Thus, strategically located basic second-shell residues mediate precise reactant positioning at the catalytic site, determining substrate specificity in dUTPases and possibly in other structurally similar nucleotide-processing metalloenzymes.Nucleotide-processing enzymes are key players in biological processes. They often operate through high substrate specificity for catalysis. How such specificity is achieved is unclear. Here, we dealt with this question by investigating all-α dimeric deoxyuridine triphosphate nucleotidohydrolases (dUTPases). Typically, these dUTPases hydrolyze either dUTP or deoxyuridine diphosphate (dUDP) substrates. However, the dUTPase enzyme DR2231 from Deinococcus radiodurans selectively hydrolyzes dUTP only, and not dUDP. By means of extended classical molecular dynamics simulations and quantum chemical calculations, we show that DR2231 achieves this specificity for dUTP via second-shell basic residues that, together with the two catalytic magnesium ions, contribute to properly orienting the γ-phosphate of dUTP in a prereactive state. This allows a nucleophilic water to be correctly placed and activated in order to perform substrate hydrolysis. We show that this enzymatic mechanism is not viable when dUDP is bound to DR2231. Importantly, in several other dUTPases capable of hydrolyzing either dUTP or dUDP, we detected that active site second-shell basic residues are more in number, anchoring the β-phosphate of the nucleotide substrate too, in contrast to what is observed in DR2231. Thus, strategically located basic second-shell residues mediate precise reactant positioning at the catalytic site, determining substrate specificity in dUTPases and possibly in other structurally similar nucleotide-processing metalloenzymes.

Reflectins are a family of intrinsically disordered proteins involved in cephalopod camouflage, making them an interesting source for bioinspired optical materials. Understanding reflectin assembly into higher-order structures by standard biophysical methods enables the rational design of new materials, but it is difficult due to their low solubility. To address this challenge, we aim to understand the molecular self-assembly mechanism of reflectin’s basic unit—the protopeptide sequence YMDMSGYQ—as a means to understand reflectin’s assembly phenomena. Protopeptide self-assembly was triggered by different environmental cues, yielding supramolecular hydrogels, and characterized by experimental and theoretical methods. Protopeptide films were also prepared to assess optical properties. Our results support the hypothesis for the protopeptide aggregation model at an atomistic level, led by hydrophilic and hydrophobic interactions mediated by tyrosine residues. Protopeptide-derived films were optically active, presenting diffuse reflectance in the visible region of the light spectrum. Hence, these results contribute to a better understanding of the protopeptide structural assembly, crucial for the design of peptide- and reflectin-based functional materials.

We have performed experiments on the crystallization of two low molecular weight, positively charged proteins, lysozyme and ribonuclease A, using ionic liquids as either crystallization additives or, in particular cases, as precipitating agents. The ionic liquids (ILs) have been ordered according to their salting-in/out ability and the relative position of these ionic liquids in this ranking has been rationalized by considering their hydration properties (positive-negative, hydrophobic-hydrophilic). The ability to screen the effective charge of cationic proteins and aid protein nucleation (salting-out) has been shown to be superior for large polarizable anions with low charge density, negatively hydrated-Cl-, Br-, [SCN](-), methane-[C1SO3](-) and ethanesulfonates [C2SO3](-), than for anions with a relatively stable hydration shell, positively hydrated-lactate [Lac](-), butylsulfonate [C4SO3](-) and acetate [Ac](-). Upon increasing the background salt concentration, where electrostatic interactions are already effectively screened, the ability of the IL ions to stabilize proteins in solution (salting-in) has been shown to increase as the ions are likely to migrate to the non-polar protein surface and lower protein-water interfacial tension. This tendency is enhanced as the focus moves from those ions with positively hydrated hydrophilic compartments (e. g. [Ac](-)) to those with negatively hydrated groups (e. g. [C1SO3](-)) and the prevailing hydrophobic hydration (e. g. [C4SO3](-)). The observed inversion in the relative effect of ILs on protein crystallization with increasing ionic strength of the aqueous media has been interpreted as the differing effects of ion adsorption: charge screening and interfacial tension modification. Moreover, this work can further help in our understanding of the influence of ionic liquids on conformational changes of biomacromolecules in solution. Identification of the specific incorporation sites for choline and acetate ions, localized in two lysozyme crystals grown in pure IL solutions without any buffer or inorganic precipitant, can give us some insight into the role of the ionic liquid ions in protein structure development.