An In Silico Target Fishing Approach to Identify Novel Ochratoxin A Hydrolyzing Enzyme.

Ochratoxin A (OTA), a mycotoxin that is of utmost concern in food and feed safety, is produced by fungal species that mainly belong to the Aspergillus and Penicillium genera. The development of mitigation strategies to reduce OTA content along the supply chains is key to ensuring safer production of food and feed. Enzyme-based strategies are among the most promising methods due to their specificity, efficacy, and multi-situ applicability. In particular, some enzymes are already known for hydrolyzing OTA into ochratoxin alpha (OTα) and phenylalanine (Phe), eventually resulting in detoxification action. Therefore, the discovery of novel OTA hydrolyzing enzymes, along with the advancement of an innovative approach for their identification, could provide a broader basis to develop more effective mitigating strategies in the future. In the present study, a hybrid in silico/in vitro workflow coupling virtual screening with enzymatic assays was applied in order to identify novel OTA hydrolyzing enzymes. Among the various hits, porcine carboxypeptidase B was identified for the first time as an effective OTA hydrolyzing enzyme. The successful experimental endorsement of findings of the workflow confirms that the presented strategy is suitable for identifying novel OTA hydrolyzing enzymes, and it might be relevant for the discovery of other mycotoxin- mitigating enzymes.

Generation and identification of variants with improved purification yield of Bowman-Birk protease inhibitors carrying protein binding loops.

Replacing the chymotrypsin inhibitory loop of soybean Bowman-Birk inhibitor (sBBI) with a VEGF binding peptide (BBI-AV) significantly reduces the overall purification yield when BBI-AV is produced as a fusion protein in a Bacillussubtilis expression system. The low purification yield is primarily due to a higher fraction of molecules with incorrect disulfide bond configurations after production and also after disulfide bond shuffling induced by 2-mercaptoethanol. To improve production yields, site-saturation libraries were generated at 39 out of the 66 amino acid residues of BBI-AV. Initial screens were designed to select for variants with higher trypsin inhibitory activities than the parent after treatment with a reducing agent. Secondary screens were developed to select for variants with the highest purification yields, and to also eliminate any false positives. From the screens, it was found that positively charged substitutions in the exposed hydrophobic patch region (sites 27, 29, 40, 50 & 52) are especially productive. In fact, one substitution, F50R, improves the purification yield to nearly the same level as wild-type sBBI. Productive amino acid substitutions were combined to select for the variant with the best overall yield after purification. Several variants were obtained with higher purification yields than even sBBI. The octuple variants, A13I-S25R-M27A-L29P-S31A-A40H-F50K-V52T and A13I-S25K-M27A-L29R-S31E-A40K-F50Q-V52Q, are particularly productive having greater than a five fold increase in final purification yield over the parent.

A Bacillus subtilis fusion protein system to produce soybean Bowman-Birk protease inhibitor.

A fusion protein based expression system was developed in the Gram-positive bacterium Bacillus subtilis to produce the soybean Bowman-Birk protease inhibitor (sBBI). The N-terminus of the mature sBBI was fused to the C-terminus of the 1st cellulose binding domain linker (CBD linker) of the BCE103 cellulase (from an alkalophilic Bacillus sp.). The strong aprE promoter was used to drive the transcription of the fusion gene and the AprE signal sequence was fused to the mature BCE103 cellulase for efficient secretion of the fusion protein into the culture medium. It was necessary to use a B. subtilis strain deficient in nine protease genes in order to reduce the proteolytic degradation of the fusion protein during growth. The fusion protein was produced in shake flasks at concentrations >1g/L. After growth, the sBBI was activated by treatment with 2-mercaptoethanol to allow the disulfide bonds to form correctly. An economical and scalable purification process was developed to purify sBBI based on acid precipitation of the fusion protein followed by acid/heat cleavage of the fusion protein at labile Asp-Pro bonds in the CBD linker. If necessary, non-native amino acids at the N- and C-termini were trimmed off using glutamyl endopeptidase I. After purification, an average of 72 mg of active sBBI were obtained from 1L of culture broth representing an overall yield of 21% based on the amount of sBBI activated before purification.

Monoclonal antibodies against soybean Bowman-Birk inhibitor recognize the protease-reactive loops.

Monoclonal antibodies against soybean Bowman-Birk protease inhibitor (BBI) have been generated and used to detect and quantify BBI in foods, soybean germplasm, and animal tissues and fluids. The purpose of this study was to determine the recognition sites of two monoclonal antibodies to BBI (mAb 238 and mAb 217) in relation to the protease-inhibitory sites of BBI. The results showed that (1) the binding of mAb 238 can be blocked by trypsin and that of mAb 217 by chymotrypsin; (2) the trypsin or chymotrypsin inhibitory activities of BBI are blocked by mAb 238 or mAb 217, respectively; and (3) mAb 238 failed to recognize a tryptic loop mutant BBI variant and mAb 217 was unable to bind a chymotryptic loop mutant BBI variant. These findings demonstrate that the epitopes recognized by mAb 238 and mAb 217 reside, at least in part, in the tryptic and chymotryptic loops of BBI, respectively.