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A lepidopteran insect cell-based expression system has been employed to express three Anopheles gambiae odorant receptors (ORs), OR1 and OR2, which respond to components of human sweat, and OR7, the ortholog of Drosophila's OR83b, the heteromerization partner of all functional ORs in that system. With the aid of epitope tagging and specific antibodies, efficient expression of all ORs was demonstrated and intrinsic properties of the proteins were revealed. Moreover, analysis of the orientation of OR1 and OR2 on the cellular plasma membrane through the use of a novel ‘topology screen’ assay and FACS analysis demonstrates that, as was recently reported for the ORs in Drosophila melanogaster, mosquito ORs also have a topology different than their mammalian counterparts with their N-terminal ends located in the cytoplasm and their C-terminal ends facing outside the cell. These results set the stage for the production of mosquito ORs in quantities that should permit their detailed biochemical and structural characterization and the exploration of their functional properties.

l‐Asparaginase (EC 3.5.1.1, l‐ASNase) catalyses the hydrolysis of l‐Asn, producing l‐Asp and ammonia. This enzyme is an anti‐neoplastic agent; it is used extensively in the chemotherapy of acute lymphoblastic leukaemia. In this study, we describe the use of in vitro directed evolution to create a new enzyme variant with improved thermal stability. A library of enzyme variants was created by a staggered extension process using the genes that code for the l‐ASNases from Erwinia chrysanthemi and Erwinia carotovora. The amino acid sequences of the parental l‐ASNases show 77% identity, but their half‐inactivation temperature (Tm) differs by 10 °C. A thermostable variant of the E. chrysamthemi enzyme was identified that contained a single point mutation (Asp133Val). The Tm of this variant was 55.8 °C, whereas the wild‐type enzyme has a Tm of 46.4 °C. At 50 °C, the half‐life values for the wild‐type and mutant enzymes were 2.7 and 159.7 h, respectively. Analysis of the electrostatic potential of the wild‐type enzyme showed that Asp133 is located at a neutral region on the enzyme surface and makes a significant and unfavourable electrostatic contribution to overall stability. Site‐saturation mutagenesis at position 133 was used to further analyse the contribution of this position on thermostability. Screening of a library of random Asp133 mutants confirmed that this position is indeed involved in thermostability and showed that the Asp133Leu mutation confers optimal thermostability.

A library of Tau class GSTs (glutathione transferases) was constructed by DNA shuffling using the DNA encoding the Glycine max GSTs GmGSTU2-2, GmGSTU4-4 and GmGSTU10-10. The parental GSTs are >88% identical at the sequence level; however, their specificity varies towards different substrates. The DNA library contained chimaeric structures of alternated segments of the parental sequences and point mutations. Chimaeric GST sequences were expressed in Escherichia coli and their enzymatic activities towards CDNB (1-chloro-2,4-dinitrobenzene) and the herbicide fluorodifen (4-nitrophenyl α,α,α-trifluoro-2-nitro-p-tolyl ether) were determined. A chimaeric clone (Sh14) with enhanced CDNB- and fluorodifen-detoxifying activities, and unusual co-operative kinetics towards CDNB and fluorodifen, but not towards GSH, was identified. The structure of Sh14 was determined at 1.75 Å (1 Å=0.1 nm) resolution in complex with S-(p-nitrobenzyl)-glutathione. Analysis of the Sh14 structure showed that a W114C point mutation is responsible for the altered kinetic properties. This was confirmed by the kinetic properties of the Sh14 C114W mutant. It is suggested that the replacement of the bulky tryptophan residue by a smaller amino acid (cysteine) results in conformational changes of the active-site cavity, leading to enhanced catalytic activity of Sh14. Moreover, the structural changes allow the strengthening of the two salt bridges between Glu(66) and Lys(104) at the dimer interface that triggers an allosteric effect and the communication between the hydrophobic sites.

The present study confirmed the marked specificity of DAH and AAK analogues towards EcRs from lepidopteran insects. THQ compounds did not show this specificity, indicating that dipteran-specific ecdysone-agonist-based insecticides based on the THQ mother structure can be developed. The differences in activity of ecdysone agonists in dipteran and lepidopteran ecdysone-reporter-based screening systems are discussed.

The present study demonstrated that species-specific ecdysone-agonist-based insecticides can be developed, but their species specificity is not based on differences in the activation of the ecdysone receptor but rather on unidentified in vivo parameters such as permeability of the cuticle, uptake/excretion by the gut or metabolic detoxification.

Bacterial L-ASNases (L-asparaginases) catalyse the conversion of L-asparagine into L-aspartate and ammonia, and are widely used for the treatment of ALL (acute lymphoblastic leukaemia). In the present paper, we describe an efficient approach, based on protein chemistry and protein engineering studies, for the construction of trypsin-resistant PEGylated L-ASNase from Erwinia carotovora (EcaL-ASNase). Limited proteolysis of EcaL-ASNase with trypsin was found to be associated with a first cleavage of the peptide bond between Lys53 and Gly54, and then a second cleavage at Arg206-Ser207 of the C-terminal fragment, peptide 54-327, showing that the initial recognition sites for trypsin are Lys53 and Arg206. Site-directed mutagenesis of Arg206 to histidine followed by covalent coupling of mPEG-SNHS [methoxypoly(ethylene glycol) succinate N-hydroxysuccinimide ester] to the mutant enzyme resulted in an improved modified form of EcaL-ASNase that retains 82% of the original catalytic activity, exhibits enhanced resistance to trypsin degradation, and has higher thermal stability compared with the wild-type enzyme.