Snake venoms are cocktails of enzymes and non‐enzymatic proteins used for both the immobilization and digestion of prey. The most common snake venom enzymes include acetylcholinesterases, l‐amino acid oxidases, serine proteinases, metalloproteinases and phospholipases A2. Higher catalytic efficiency, thermal stability and resistance to proteolysis make these enzymes attractive models for biochemists, enzymologists and structural biologists. Here, we review the structures of these enzymes and describe their structure‐based mechanisms of catalysis and inhibition. Some of the enzymes exist as protein complexes in the venom. Thus we also discuss the functional role of non‐enzymatic subunits and the pharmacological effects of such protein complexes. The structures of inhibitor–enzyme complexes provide ideal platforms for the design of potent inhibitors which are useful in the development of prototypes and lead compounds with potential therapeutic applications.

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Product inhibition of β-glucosidases (BGs) by glucose is considered to be a limiting step in enzymatic technologies for plant-biomass saccharification. Remarkably, some β-glucosidases belonging to the GH1 family exhibit unusual properties, being tolerant to, or even stimulated by, high glucose concentrations. However, the structural basis for the glucose tolerance and stimulation of BGs is still elusive. To address this issue, the first crystal structure of a fungal β-glucosidase stimulated by glucose was solved in native and glucose-complexed forms, revealing that the shape and electrostatic properties of the entrance to the active site, including the +2 subsite, determine glucose tolerance. The aromatic Trp168 and the aliphatic Leu173 are conserved in glucose-tolerant GH1 enzymes and contribute to relieving enzyme inhibition by imposing constraints at the +2 subsite that limit the access of glucose to the -1 subsite. The GH1 family β-glucosidases are tenfold to 1000-fold more glucose tolerant than GH3 BGs, and comparative structural analysis shows a clear correlation between active-site accessibility and glucose tolerance. The active site of GH1 BGs is located in a deep and narrow cavity, which is in contrast to the shallow pocket in the GH3 family BGs. These findings shed light on the molecular basis for glucose tolerance and indicate that GH1 BGs are more suitable than GH3 BGs for biotechnological applications involving plant cell-wall saccharification.

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Winged-helix transcriptional factors play important roles in the control of gene expression in many organisms. In the plant pathogens Xylella fastidiosa and Agrobacterium tumefaciens, the winged-helix protein BigR, a member of the ArsR/SmtB family of metal sensors, regulates transcription of the bigR operon involved in bacterial biofilm growth. Previous studies showed that BigR represses transcription of its own operon through the occupation of the RNA polymerase-binding site; however, the signals that modulate its activity and the biological function of its operon are still poorly understood. Here we show that although BigR is a homodimer similar to metal sensors, it functions as a novel redox switch that derepresses transcription upon oxidation. Crystal structures of reduced and oxidized BigR reveal that formation of a disulfide bridge involving two critical cysteines induces conformational changes in the dimer that remarkably alter the topography of the winged-helix DNA-binding interface, precluding DNA binding. This structural mechanism of DNA association-dissociation is novel among winged-helix factors. Moreover, we demonstrate that the bigR operon is required for hydrogen sulfide detoxification through the action of a sulfur dioxygenase (Blh) and sulfite exporter. As hydrogen sulfide strongly inhibits cytochrome c oxidase, it must be eliminated to allow aerobic growth under low oxygen tension, an environmental condition found in bacterial biofilms, xylem vessels, and root tissues. Accordingly, we show that the bigR operon is critical to sustain bacterial growth under hypoxia. These results suggest that BigR integrates the transcriptional regulation of a sulfur oxidation pathway to an oxidative signal through a thiol-based redox switch.Winged-helix proteins belong to a larger ensemble of helixturn-helix (HTH) 4 factors employed by living organisms to sense and respond to diverse environmental cues (1). For example, in bacterial cells, winged-helix transcriptional factors function as metal sensors, which control metal tolerance (2). In the plant pathogens Xylella fastidiosa and Agrobacterium tumefaciens, the winged-helix repressor protein BigR (biofilm growth-associated repressor) controls the expression of the bigR operon through the recognition of the Ϫ10 region in the operator site, thus blocking transcription of the operon genes (3). bigR operons are evolutionarily conserved in some plantassociated bacteria and human opportunistic pathogens and encode BigR itself, membrane transporters, and Blh, a DUF442-␤-lactamase domain protein related to the mitochondrial sulfur dioxygenase ETHE1 involved in sulfide detoxification in mammals (3, 4). Previous studies have shown that the bigR operon is actively transcribed in Xylella and Agrobacterium biofilms and that mutants lacking BigR can attach more tightly to glass and root surfaces than normal bacteria. Although these results strongly indicated that the bigR operon plays an important role in bacterial biofilm formation or cell adhesion, the biological function ...

Sphingomyelinases D (SMases D) fromLoxosceles spider venom are the principal toxins responsible for the manifestation of dermonecrosis, intravascular hemolysis, and acute renal failure, which can result in death. These enzymes catalyze the hydrolysis of sphingomyelin, resulting in the formation of ceramide 1-phosphate and choline or the hydrolysis of lysophosphatidyl choline, generating the lipid mediator lysophosphatidic acid. This report represents the first crystal structure of a member of the sphingomyelinase D family from Loxosceles laeta (SMase I), which has been determined at 1.75-Å resolution using the "quick cryo-soaking" technique and phases obtained from a single iodine derivative and data collected from a conventional rotating anode x-ray source. SMase I folds as an (␣/␤) 8 barrel, the interfacial and catalytic sites encompass hydrophobic loops and a negatively charged surface. Substrate binding and/or the transition state are stabilized by a Mg 2؉

Cellulases participate in a number of biological events, such as plant cell wall remodelling, nematode parasitism and microbial carbon uptake. Their ability to depolymerize crystalline cellulose is of great biotechnological interest for environmentally compatible production of fuels from lignocellulosic biomass. However, industrial use of cellulases is somewhat limited by both their low catalytic efficiency and stability. In the present study, we conducted a detailed functional and structural characterization of the thermostable BsCel5A (Bacillus subtilis cellulase 5A), which consists of a GH5 (glycoside hydrolase 5) catalytic domain fused to a CBM3 (family 3 carbohydrate-binding module). NMR structural analysis revealed that the Bacillus CBM3 represents a new subfamily, which lacks the classical calcium-binding motif, and variations in NMR frequencies in the presence of cellopentaose showed the importance of polar residues in the carbohydrate interaction. Together with the catalytic domain, the CBM3 forms a large planar surface for cellulose recognition, which conducts the substrate in a proper conformation to the active site and increases enzymatic efficiency. Notably, the manganese ion was demonstrated to have a hyper-stabilizing effect on BsCel5A, and by using deletion constructs and X-ray crystallography we determined that this effect maps to a negatively charged motif located at the opposite face of the catalytic site.

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