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Maurocalcine is a novel toxin isolated from the venom of the chactid scorpion Scorpio maurus palmatus. It is a 33-mer basic peptide cross-linked by three disulfide bridges, which shares 82% sequence identity with imperatoxin A, a scorpion toxin from the venom of Pandinus imperator. Maurocalcine is peculiar in terms of structural properties since it does not possess any consensus motif reported so far in other scorpion toxins. Due to its low concentration in venom (0.5% of the proteins), maurocalcine was chemically synthesized by means of an optimized solid-phase method, and purified after folding/oxidation by using both C18 reversed-phase and ion exchange highpressure liquid chromatographies. The synthetic product (sMCa) was characterized. The half-cystine pairing pattern of sMCa was identified by enzyme-based cleavage and Edman sequencing. The pairings were Cys3-Cys17, Cys10-Cys21, and Cys16-Cys32. In vivo, the sMCa was lethal to mice following intracerebroventricular inoculation (LD 50 , 20 W Wg/mouse). In vitro, electrophysiological experiments based on recordings of single channels incorporated into planar lipid bilayers showed that sMCa potently and reversibly modifies channel gating behavior of the type 1 ryanodine receptor by inducing prominent subconductance behavior.z 2000 Federation of European Biochemical Societies.

The navel orangeworm, Amyelois transitella is a major agricultural pest causing large losses in a variety of tree crops. Control of this insect pest may be achieved by interfering with olfactory pathways to block detection of female-produced sex pheromones and consequently, disrupt mating. The first component of this pathway is the pheromone-binding protein AtraPBP1, which recognizes the pheromone and presents it to the odorant receptor housed in a sensory neuron of the male antennae. Release of the ligand depends on a pH-induced conformational change associated with the acidity of the membrane surface. To characterize this conformational change and to understand how pheromones bind, we have determined the high resolution crystal structures of AtraPBP1 in complex with two main constituents of the sex pheromone, i.e., (11Z,13Z)-hexadecadienal and (11Z,13Z)-hexadecadienol. Comparison with the structure of the unliganded form demonstrates a large ∼90° movement of the C-terminal helix which is observed in other pheromone- or odorant-binding proteins accompanied by an unpredicted 37° displacement of the N-terminal helix. Molecular dynamic trajectories suggest that the conformational change of the α1 helix facilitates the movement of the C-terminal helix.

The primary metabolic route for D-xylose, the second most abundant sugar in nature, is via the pentose phosphate pathway after a two or three step conversion to xylulose-5-phosphate. Xylulose kinase (XK; EC 2.7.1.17) phosphorylates D-xylulose, the last step in this conversion. The apo and xylulose-bound crystal structures of E. coli XK have been determined and show a dimer composed of two domains separated by an open cleft. XK dimerization was directly observed by a cryo-EM reconstruction at 36 Å resolution. Kinetic studies reveal that XK has a weak substrate-independent MgATP-hydrolyzing activity and phosphorylates several sugars and polyols with low catalytic efficiencies. Binding of pentulose and MgATP to form the reactive ternary complex is strongly synergistic. Although the steady-state kinetic mechanism of XK is formally random, a path is preferred in which D-xylulose binds before MgATP. Modeling of MgATP binding to XK and the accompanying conformational change suggests that sugar binding is accompanied by a dramatic hinge bending movement that enhances interactions with MgATP, explaining the observed synergism. A catalytic mechanism is proposed and supported by relevant site-directed mutants.

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