Aflatoxins are polyaromatic mycotoxins that contaminate a range of food crops as a result of fungal growth and contribute to serious health problems in the developing world because of their toxicity and mutagenicity. Although relatively resistant to biotic degradation, aflatoxins can be metabolized by certain species of Actinomycetales. However, the enzymatic basis for their breakdown has not been reported until now. We have identified nine Mycobacterium smegmatis enzymes that utilize the deazaflavin cofactor F420H2 to catalyse the reduction of the α,β-unsaturated ester moiety of aflatoxins, activating the molecules for spontaneous hydrolysis and detoxification. These enzymes belong to two previously uncharacterized F420H2 dependent reductase (FDR-A and -B) families that are distantly related to the flavin mononucleotide (FMN) dependent pyridoxamine 5′-phosphate oxidases (PNPOxs). We have solved crystal structures of an enzyme from each FDR family and show that they, like the PNPOxs, adopt a split barrel protein fold, although the FDRs also possess an extended and highly charged F420H2 binding groove. A general role for these enzymes in xenobiotic metabolism is discussed, including the observation that the nitro-reductase Rv3547 from Mycobacterium tuberculosis that is responsible for the activation of bicyclic nitroimidazole prodrugs belongs to the FDR-A family.

The entire pathway for synthesis of the tyrosine-derived cyanogenic glucoside dhurrin has been transferred from Sorghum bicolor to Arabidopsis thaliana. Here, we document that genetically engineered plants are able to synthesize and store large amounts of new natural products. The presence of dhurrin in the transgenic A. thaliana plants confers resistance to the flea beetle Phyllotreta nemorum, which is a natural pest of other members of the crucifer group, demonstrating the potential utility of cyanogenic glucosides in plant defense.

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Chitinases account for ca. 50% of the soluble proteins in the berries of the grape vine (Vitis vinifera L. Muscat of Alexandria). The other major proteins present are thaumatin-like proteins. Both these groups of proteins persist through the vinification process and cause hazes and sediments in bottled wines. Four chitinases have been purified from Muscat of Alexandria fruit and characterized by both sequence and mass spectral analysis. Their protein sequences were highly similar, and all proteins were modified at their N-terminus. It was confirmed for three of the chitinases that the N-terminal group was a pyroglutamate residue. Comparative sequence analysis of two chitinases from berries and two from wine demonstrated that, despite their reputed resistance to proteolytic degradation, some limited proteolytic processing of these proteins occurs. The peptide fragments of the chitinases account for only 3% of the total soluble protein content of the fruit. Keywords: Pathogenesis related proteins; chitinases; protein sequence; pyroglutamate; proteolytic processing; Vitis vinifera; grapes; grape juice; wine; protein haze

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