Parallelised online biomass monitoring in shake flasks enables efficient strain and carbon source dependent growth characterisation of Saccharomyces cerevisiae.

BACKGROUND: Baker's yeast, Saccharomyces cerevisiae, as one of the most often used workhorses in biotechnology has been developed into a huge family of application optimised strains in the last decades. Increasing numbers of strains render their characterisation highly challenging, even with the simple methods of growth-based analytics. Here we present a new sensor system for the automated, non-invasive and parallelisable monitoring of biomass in continuously shaken shake flask cultures, called CGQ ("cell growth quantifier"). The CGQ implements a dynamic approach of backscattered light measurement, allowing for efficient and accurate growth-based strain characterisation, as exemplarily demonstrated for the four most commonly used laboratory and industrial yeast strains, BY4741, W303-1A, CEN.PK2-1C and Ethanol Red. RESULTS: Growth experiments revealed distinct carbon source utilisation differences between the investigated S. cerevisiae strains. Phenomena such as diauxic shifts, morphological changes and oxygen limitations were clearly observable in the growth curves. A strictly monotonic non-linear correlation of OD600 and the CGQ's backscattered light intensities was found, with strain-to-strain as well as growth-phase related differences. The CGQ measurements showed high resolution, sensitivity and smoothness even below an OD600 of 0.2 and were furthermore characterised by low background noise and signal drift in combination with high reproducibility. CONCLUSIONS: With the CGQ, shake flask fermentations can be automatically monitored regarding biomass and growth rates with high resolution and parallelisation. This makes the CGQ a valuable tool for growth-based strain characterisation and development. The exceptionally high resolution allows for the identification of distinct metabolic differences and shifts as well as for morphologic changes. Applications that will benefit from that kind of automatized biomass monitoring include, amongst many others, the characterization of deregulated native or integrated heterologous pathways, the fast detection of co-fermentation as well as the realisation of rational and growth-data driven evolutionary engineering approaches.

The three-dimensional structure of VIM-31--a metallo-ß-lactamase from Enterobacter cloacae in its native and oxidized form.

The metallo-ß-lactamase VIM-31 differs from VIM-2 by only two Tyr224His and His252Arg substitutions. Located close to the active site, the Tyr224His substitution is also present in VIM-1, VIM-4, VIM-7 and VIM-12. The VIM-31 variant was reported in 2012 from Enterobacter cloacae and kinetically characterized. It exhibits globally lower catalytic efficiencies than VIM-2. In the present study, we report the three-dimensional structures of VIM-31 in its native (reduced) and oxidized forms. The so-called 'flapping-loop' (loop 1) and loop 3 of VIM-31 were not positioned as in VIM-2 but instead were closer to the active site as in VIM-4, resulting in a narrower active site in VIM-31. Also, the presence of His224 in VIM-31 disrupts hydrogen-bonding networks close to the active site. Moreover, a third zinc-binding site, which also exists in VIM-2 structures, could be identified as a structural explanation for the decreased activity of VIM-MBLs at high zinc concentrations.

Enzyme-substrate complex structures of CYP154C5 shed light on its mode of highly selective steroid hydroxylation.

CYP154C5 from Nocardia farcinica is a bacterial cytochrome P450 monooxygenase active on steroid molecules. The enzyme has recently been shown to exhibit exclusive regioselectivity and stereoselectivity in the conversion of various pregnans and androstans, yielding 16α-hydroxylated steroid products. This makes the enzyme an attractive candidate for industrial application in steroid hormone synthesis. Here, crystal structures of CYP154C5 in complex with four different steroid molecules were solved at resolutions of up to 1.9 Å. These are the first reported P450 structures from the CYP154 family in complex with a substrate. The active site of CYP154C5 forms a flattened hydrophobic channel with two opposing polar regions, perfectly resembling the size and polarity distribution of the steroids and thus resulting in highly specific steroid binding with Kd values in the range 10-100 nM. Key enzyme-substrate interactions were identified that accounted for the exclusive regioselectivity and stereoselectivity of the enzyme. Additionally, comparison of the four CYP154C5-steroid structures revealed distinct structural differences, explaining the observed variations in kinetic data obtained for this P450 with the steroids pregnenolone, dehydroepiandrosterone, progesterone, androstenedione, testosterone and nandrolone. This will facilitate the generation of variants with improved activity or altered selectivity in the future by means of protein engineering.