Changing the substrate reactivity of 2-hydroxybiphenyl 3-monooxygenase from Pseudomonas azelaica HBP1 by directed evolution

A monomeric L-aspartase obtained by in vitro selection

By mimicking the partial spatial structure of the dimer of the l-aspartase subunit, the central ten-helix bundle, and an "active site" between the cleft of domain 1 (D1) and domain 3 (D3) from different subunits, we designed l-aspartase variants, in which D1D2 and D2D3 were ligated with a random hexapeptide loop. As expected, we obtained the variant with the highest activity (relative activity is 21.3% of the native enzyme, named as drAsp017) by in vitro selection. The molecular weight of this variant, obtained from size-exclusion column chromatography, is about 81 kDa, which indicates that it is indeed a monomer, whereas native l-aspartase is a tetramer. The activity-reversibility of drAsp017 (10(-7) m) was 80% after incubation for 30 min at 50 degrees C, while native enzyme only retained about 17% under the same conditions. Reactivation of drAsp017 denatured in 4 m guanidine HCl was independent of protein concentration at up to 20 x 10(-8) m at 25 degrees C, whereas the protein concentration of native enzyme strongly affected its reactivation under the above conditions. The sensitivity of drAsp017 (10(-7) m) to effective factors in the fumarate-amination reaction compared with native enzyme was also determined. Half-saturating concentrations of the activator l-aspartate and Mg2+ for drAsp017 (0.8 and 0.5 mm, respectively) are much higher than that of the native enzyme (0.10 and 0.15 mm, respectively). The data show that a monomeric l-aspartase is obtained by in vitro selection. Thus, the conversion of oligomeric proteins into their functional monomers could have important applications.

Family shuffling of a targeted bphA region to engineer biphenyl dioxygenase

In this work we used a new strategy designed to reduce the size of the library that needs to be explored in family shuffling to evolve new biphenyl dioxygenases (BPDOs). Instead of shuffling the whole gene, we have targeted a fragment of bphA that is critical for enzyme specificity. We also describe a new protocol to screen for more potent BPDOs that is based on the detection of catechol metabolites from chlorobiphenyls. Several BphA variants with extended potency to degrade polychlorinated biphenyls (PCBs) were obtained by shuffling critical segments of bphA genes from Burkholderia sp. strain LB400, Comamonas testosteroni B-356, and Rhodococcus globerulus P6. Unlike all parents, these variants exhibited high activity toward 2,2'-, 3,3'-, and 4,4'-dichlorobiphenyls and were able to oxygenate the very persistent 2,6-dichlorobiphenyl. The data showed that the replacement of a short segment (335TFNNIRI341) of LB400 BphA by the corresponding segment (333GINTIRT339) of B-356 BphA or P6 BphA contributes to relax the enzyme toward PCB substrates.

Protein engineering of oxygenases for biocatalysis

Oxygenase enzymes have seen limited practical applications because of their complexity, poor stabilities, and often low catalytic rates. However, their ability to perform difficult chemistry with high selectivity and specificity has kept oxygenases at the forefront of engineering efforts. Growing understanding of structure-function relationships and improved protein engineering methods are paving the way for applications of oxygenases in chemical synthesis and bioremediation.