Recombinant expression, purification and preliminary biophysical and structural studies of C-terminal carbohydrate recognition domain from human galectin-4.

Galectin-4 (Gal4), a tandem-repeat type galectin, is expressed in healthy epithelium of the gastrointestinal tract. Altered levels of Gal4 expression are associated with different types of cancer, suggesting its usage as a diagnostic marker as well as target for drug development. The functional data available for this class of proteins suggest that the wide spectrum of cellular activities reported for Gal4 relies on distinct glycan specificity and structural characteristics of its two carbohydrate recognition domains. In the present work, two independent constructs for recombinant expression of the C-terminal domain of human galectin-4 (hGal4-CRD2) were developed. His6-tagged and untagged recombinant proteins were overexpressed in Escherichia coli, and purified by affinity chromatography followed by gel filtration. Correct folding and activity of hGal4-CRD2 were assessed by circular dichroism and fluorescence spectroscopies, respectively. Diffraction quality crystals were obtained by vapor-diffusion sitting drop setup and the crystal structure of CRD2 was solved by molecular replacement techniques at 1.78 Å resolution. Our work describes the development of important experimental tools that will allow further studies in order to correlate structure and binding properties of hGal4-CRD2 and human galectin-4 functional activities.

Structural, biochemical and biophysical characterization of recombinant human fumarate hydratase.

Fumarate hydratases (FHs, fumarases) catalyze the reversible conversion of fumarate into l-malate. FHs are distributed over all organisms and play important roles in energy production, DNA repair and as tumor suppressors. They are very important targets both in the study of human metabolic disorders and as potential therapeutic targets in neglected tropical diseases and tuberculosis. In this study, human FH (HsFH) was characterized by using enzyme kinetics, differential scanning fluorimetry and X-ray crystallography. For the first time, the contribution of both substrates was analyzed simultaneously in a single kinetics assay allowing to quantify the contribution of the reversible reaction for kinetics. The protein was crystallized in the spacegroup C2221 , with unit-cell parameters a = 125.43, b = 148.01, c = 129.76. The structure was solved by molecular replacement and refined at 1.8 Å resolution. In our study, a HEPES molecule was found to interact with HsFH at the C-terminal domain (Domain 3), previously described as involved in allosteric regulation, through a set of interactions that includes Lys 467. HsFH catalytic efficiency is higher when in the presence of HEPES. Mutations at residue 467 have already been implicated in genetic disorders caused by FH deficiency, suggesting that the HEPES-binding site may be important for enzyme kinetics. This study contributes to the understanding of the HsFH structure and how it correlates with mutation, enzymatic deficiency and pathology.

Insights into the structure, function and stability of bordonein-L, the first L-amino acid oxidase from Crotalus durissus terrificus snake venom.

Snake venom L-amino acid oxidases (svLAAOs) are an interesting class of enzymes with important biological activities. Their participation in key metabolic processes, including pathological disorders, suggest that svLAAOs are potential lead compounds in drug discovery. However, their short-term stability defies their applications. This paper describes the stability studies together with functional and structural characterization of the LAAO bordonein-L. It has 498 amino acid residues, one N-glycosylation site and two disulfide bonds, revealed by high-resolution MS/MS. Molecular modeling approach showed its monomer folds into three conserved domains: FAD, substrate and helical domains. Differential scanning fluorimetry showed the enzyme tends to destabilize from neutral to basic pHs and in presence of mono/bivalent ions and it is highly stabilized by acid pHs and its substrates. However, high concentrations of L-amino acids decrease bordonein-L enzyme activity. Dynamic light scattering revealed bordonein-L remains in the dimeric and monodisperse form, so aggregation does not cause the rapidly decrease of enzyme activity. In vitro, the enzyme exhibited cytotoxicity against fibroblast cell line and killed Leishmania amazonensis promastigotes, intensified by substrate addition. Concluding, our results provide biochemistry and biophysical insights to improve LAAOs stability and better approaches to long-term storage. Moreover, our study emphasizes the importance of proper buffers choice mainly in cell-based assays.

Crystal structure and molecular dynamics studies of L-amino acid oxidase from Bothrops atrox.

L-amino acid oxidases (LAAOs) are dimeric flavoproteins that catalyze the deamination of L-amino acid to α-keto acid, producing ammonia and hydrogen peroxide. In this study, we report the crystal structure and molecular dynamics simulations of LAAO from the venom of Bothrops atrox (BatroxLAAO). BatroxLAAO presents several biological and pharmacological properties with promising biomedical applications. BatroxLAAO structure contains the highly conserved structural pattern of LAAOs comprising a FAD-binding domain, substrate-binding domain and helical domain, and a dimeric arrangement that can be stabilized by zinc. Also, molecular dynamics results show an asymmetric behavior, and a direct communication between FAD- and substrate-binding domains of counterpart subunits. These findings shed light on the structural role of dimerization to catalytic mechanism of SV-LAAOs.

Purification, partial characterization and preliminary X-ray diffraction analysis of a mannose-specific lectin from Cymbosema roseum seeds.

A lectin from Cymbosema roseum seeds (CRL) was purified, characterized and crystallized. The best crystals grew in a month and were obtained by the vapour-diffusion method using a precipitant solution consisting of 0.1 M Tris-HCl pH 7.8, 8%(w/v) PEG 3350 and 0.2 M proline at a constant temperature of 293 K. A data set was collected to 1.77 A resolution at a synchrotron-radiation source. CRL crystals are orthorhombic, belonging to space group P2(1)2(1)2(1). Crystallographic refinement and full amino-acid sequence determination are in progress.

Full-length model of the human galectin-4 and insights into dynamics of inter-domain communication.

Galectins are proteins involved in diverse cellular contexts due to their capacity to decipher and respond to the information encoded by ß-galactoside sugars. In particular, human galectin-4, normally expressed in the healthy gastrointestinal tract, displays differential expression in cancerous tissues and is considered a potential drug target for liver and lung cancer. Galectin-4 is a tandem-repeat galectin characterized by two carbohydrate recognition domains connected by a linker-peptide. Despite their relevance to cell function and pathogenesis, structural characterization of full-length tandem-repeat galectins has remained elusive. Here, we investigate galectin-4 using X-ray crystallography, small- and wide-angle X-ray scattering, molecular modelling, molecular dynamics simulations, and differential scanning fluorimetry assays and describe for the first time a structural model for human galectin-4. Our results provide insight into the structural role of the linker-peptide and shed light on the dynamic characteristics of the mechanism of carbohydrate recognition among tandem-repeat galectins.

Development of ML390: A Human DHODH Inhibitor That Induces Differentiation in Acute Myeloid Leukemia.

Homeobox transcription factor A9 (HoxA9) is overexpressed in 70% of patients diagnosed with acute myeloid leukemia (AML), whereas only a small subset of AML patients respond to current differentiation therapies. A cell line overexpressing HoxA9 was derived from the bone marrow of a lysozyme-GFP mouse. In this fashion, GFP served as an endogenous reporter of differentiation, permitting a high-throughput phenotypic screen against the MLPCN library. Two chemical scaffolds were optimized for activity yielding compound ML390, and genetic resistance and sequencing efforts identified dihydroorotate dehydrogenase (DHODH) as the target enzyme. The DHODH inhibitor brequinar works against these leukemic cells as well. The X-ray crystal structure of ML390 bound to DHODH elucidates ML390s binding interactions.

Investigation of the relationship between the structure and function of Ts2, a neurotoxin from Tityus serrulatus venom.

Scorpion toxins targeting voltage-gated sodium (Na(V)) channels are peptides that comprise 60-76 amino acid residues cross-linked by four disulfide bridges. These toxins can be divided in two groups (α and ß toxins), according to their binding properties and mode of action. The scorpion α-toxin Ts2, previously described as a ß-toxin, was purified from the venom of Tityus serrulatus, the most dangerous Brazilian scorpion. In this study, seven mammalian Na(V) channel isoforms (rNa(V)1.2, rNa(V)1.3, rNa(V)1.4, hNa(V)1.5, mNa(V)1.6, rNa(V)1.7 and rNa(V)1.8) and one insect Na(V) channel isoform (DmNa(V)1) were used to investigate the subtype specificity and selectivity of Ts2. The electrophysiology assays showed that Ts2 inhibits rapid inactivation of Na(V)1.2, Na(V)1.3, Na(V)1.5, Na(V)1.6 and Na(V)1.7, but does not affect Na(V)1.4, Na(V)1.8 or DmNa(V)1. Interestingly, Ts2 significantly shifts the voltage dependence of activation of Na(V)1.3 channels. The 3D structure of this toxin was modeled based on the high sequence identity (72%) shared with Ts1, another T. serrulatus toxin. The overall fold of the Ts2 model consists of three ß-strands and one α-helix, and is arranged in a triangular shape forming a cysteine-stabilized α-helix/ß-sheet (CSαß) motif.