Fed-batch operation in special microtiter plates: a new method for screening under production conditions.

Batch and fed-batch operation result in completely different physiological conditions for cultivated microorganisms or cells. To close the gap between screening, which is hitherto exclusively performed in batch mode, and fed-batch production processes, a special microtiter plate was developed that allows screening in fed-batch mode. The fed-batch microtiter plate (FB-MTP) enables 44 parallel fed-batch experiments at small scale. A small channel filled with a hydrogel connects a reservoir well with a culture well. The nutrient compound diffuses from the reservoir well through the hydrogel into the culture well. Hence, the feed rate can easily be adjusted to the needs of the cultured microorganisms by changing the geometry of the hydrogel channel and the driving concentration gradient. Any desired compound including liquid nutrients like glycerol can be fed to the culture. In combination with an optical measuring device (BioLector), online monitoring of these 44 fed-batch cultures is possible. Two Escherichia coli strains and a Hansenula polymorpha strain were successfully cultivated in the new FB-MTP. As a positive impact of the fed-batch mode on the used strains, a fourfold increase in product formation was observed for E. coli. For H. polymorpha, the use of fed-batch mode resulted in a strong increase in product formation, whereas no measurable product formation was observed in batch mode. In conclusion, the newly developed fed-batch microtiter plate is a versatile, easy-to-use, disposable system to perform fed-batch cultivations at small scale. Screening cultures in high-throughput under online monitoring are possible similar to cultivations under production conditions.

Rational design of a glycosynthase by the crystal structure of ß-galactosidase from Bacillus circulans (BgaC) and its use for the synthesis of N-acetyllactosamine type 1 glycan structures.

The crystal structure of ß-galactosidase from Bacillus circulans (BgaC) was determined at 1.8Å resolution. The overall structure of BgaC consists of three distinct domains, which are the catalytic domain with a TIM-barrel structure and two all-ß domains (ABDs). The main-chain fold and steric configurations of the acidic and aromatic residues at the active site were very similar to those of Streptococcus pneumoniae ß(1,3)-galactosidase BgaC in complex with galactose. The structure of BgaC was used for the rational design of a glycosynthase. BgaC belongs to the glycoside hydrolase family 35. The essential nucleophilic amino acid residue has been identified as glutamic acid at position 233 by site-directed mutagenesis. Construction of the active site mutant BgaC-Glu233Gly gave rise to a galactosynthase transferring the sugar moiety from α-d-galactopyranosyl fluoride (αGalF) to different ß-linked N-acetylglucosamine acceptor substrates in good yield (40-90%) with a remarkably stable product formation. Enzymatic syntheses with BgaC-Glu233Gly afforded the stereo- and regioselective synthesis of ß1-3-linked key galactosides like galacto-N-biose or lacto-N-biose.