Transport of sugars via two anomer-specific sites on mannose-phosphotransferase system in Lactococcus cremoris: in vivo study of mechanism, kinetics, and adaptation

Glucose uptake in Lactococcus lactis subsp. cremoris FD1 occurs via the mannose phosphotransferase system (Man-PTS), which is quite unspecific and allows transport of many different sugars and sugar analogues. It was previously shown (Benthin, S., Nielsen, J., Villadsen, J. Biotechnol. Bioeng. 40:137-146, 1992) that the kinetics of in vivo glucose uptake in a glucose-limited chemostat culture is best described by assuming that the glucose transport system has two anomer-specific sites with a relative uptake rate of 36% through the alpha-site. In the present study, the existence of anomer-specific sites on Man-PTS is shown by experiments where alpha-glucose, beta-glucose, mannose, and 2-deoxyglucose are added to glucose-limited chemostat cultures. A quantitative description of the competitive uptake of the involved sugars at the two sites is given. In a mannose-limited chemostat culture, the relative glucose flux via the alpha-site is 50%, corresponding to a change toward the equilibrium composition of mannose (68%). Furthermore, when the feed to a mannose-limited chemostat culture is changed to glucose, the rate of change of relative glucose flux through the alpha-site corresponds to constitutive synthesis of Man-PTS with 36% alpha-site stoichiometry in new cells. When N-acetylglucosamine (73% alpha-anomer at equilibrium) is the limiting substrate, the relative glucose flux through the alpha-site is also 48% to 50%. With a feed of alpha-glucose generated enzymatically from nonmetabolizable sucrose the relative glucose flux through the alpha-site can be as high as 78%. Finally, growth in the presence of nonmetabolizable alpha-methylglucoside leads to formation of cells with a relative glucose flux through the alpha-site of 29% to 30%. The adaptation of the flux distribution between the alpha- and beta-site is tentatively explained by the hypothesis that two integral membrane proteins of Man-PTS are involved in this process.

A biochemically structured model for Saccharomyces cerevisiae

A biochemically structured model for the aerobic growth of Saccharomyces cerevisiae on glucose and ethanol is presented. The model focuses on the pyruvate and acetaldehyde branch points where overflow metabolism occurs when the growth changes from oxidative to oxido-reductive. The model is designed to describe the onset of aerobic alcoholic fermentation during steady-state as well as under dynamical conditions, by triggering an increase in the glycolytic flux using a key signalling component which is assumed to be closely related to acetaldehyde. An investigation of the modelled process dynamics in a continuous cultivation revealed multiple steady states in a region of dilution rates around the transition between oxidative and oxido-reductive growth. A bifurcation analysis using the two external variables, the dilution rate, D, and the inlet concentration of glucose, S(f), as parameters, showed that a fold bifurcation occurs close to the critical dilution rate resulting in multiple steady-states. The region of dilution rates within which multiple steady states may occur depends strongly on the substrate feed concentration. Consequently a single steady state may prevail at low feed concentrations, whereas multiple steady states may occur over a relatively wide range of dilution rates at higher feed concentrations.

Instrumentation of biotechnological processes

Modern bioprocesses are monitored by on-line sensing devices mounted either in situ or externally. In addition to sensor probes, more and more analytical subsystems are being exploited to monitor the state of a bioprocess on-line and in real time. Some of these subsystems deliver signals that are useful for documentation only, other, less delayed systems generate signals useful for closed loop process control. Various conventional and non-conventional monitoring instruments are evaluated; their usefulness, benefits and associated pitfalls are discussed.

On-line detection of acetate formation in Escherichia coli cultures using dissolved oxygen responses to feed transients

Recombinant protein production in Escherichia coli can be significantly reduced by acetate accumulation. It is demonstrated that acetate production can be detected on-line with a standard dissolved oxygen sensor by superimposing short pulses to the substrate feed rate. Assuming that acetate formation is linked to a respiratory limitation, a model for dissolved oxygen responses to transients in substrate feed rate is derived. The model predicts a clear change in the character of the transient response when acetate formation starts. The predicted effect was verified in fed-batch cultivations of E. coli TOPP1 and E. coli BL21(DE3), both before and after induction of recombinant protein production. It was also observed that the critical specific glucose uptake rate, at which acetate formation starts, was significantly decreased after induction. On-line detection of acetate formation with a standard sensor opens up new possibilities for feedback control of substrate feeding.

Glucose uptake kinetics of Saccharomyces cerevisiae monitored with a newly developed FIA

The glucose content of the culture liquid during shift experiments and synchronized cultures of Saccharomyces cerevisiae H1022 (ATCC 32167) was monitored using a greatly improved and highly precise FIA. During shift-up experiments on the dilution rate, an overshoot of the glucose-concentration was observed. The amplitude of the overshoot showed a dependency on the duration of undisturbed cultivation before application of the shift. Mutarotational non-equilibrium was excluded as the cause of the observed overshoot. For the first time glucose measurements of oscillating cultures of Saccharomyces cerevisiae are demonstrated with high accuracy and reproducibility. The data strongly support the proposals by Münch et al. (1992a, b) that faint oscillations in glucose concentration are responsible for the persistence of the synchronization. Analytical subsystems prove to be a powerful tool for investigation of the dynamics of metabolic pathways of microbial organisms. Accurate glucose measurements at low concentrations point out the limits and allow refinements of commonly used models.

Mathematical modeling of proteinase A overproduction by Saccharomyces cerevisiae

A simple, structured model was developed to describe the growth and product formation behavior of two recombinant strains of Saccharomyces cerevisiae (JG176 and JG180), both overproducing extracellular proteinase A. The model parameters were estimated to data from continuous fermentations obtained at steady-state conditions. Model predictions show good agreement with experimental data obtained by batch fermentations. The two concerned organisms are distinguished from each other by the type of promoter on the plasmids controlling the proteinase A expression. The proteinase A transcription is controlled by the natural proteinase A promoter in JG176 and by a tpi promoter in JG180. By means of experiments and simulations, the extracellular product formation from the two strains with different promoter systems was compared in batch and continuous fermentations. The results showed that the proteinase A formation kinetic from JG176 was a combination of growth and nongrowth associated (production in the stationary growth phase), whereas the proteinase A formation from JG180 was truly growth associated (production in the exponential growth phase). In both batch and continuous cultivations JG176 gave the highest product concentrations and volumetric productivities.

Production of proteinase A by Saccharomyces cerevisiae in a cell-recycling fermentation system: experiments and computer simulations

Overproduction of proteinase A by recombinant Saccharomyces cerevisiae was investigated by cultivations in a cell-recycling bioreactor. Membrane filtration was used to separate cells from the broth. Recycling ratios and dilution rates were varied and the effect on enzyme production was studied both experimentally and by computer simulations. Experiments and simulations showed that cell mass and product concentration were enhanced by high ratios of recycling. Additional simulations showed that the proteinase A concentration decreased drastically at high dilution rates and the optimal volumetric productivities were at high dilution rates just below washout and at high ratios of recycling. Cell-recycling fermentation gave much higher volumetric productivities and stable product concentrations in contrast to simple continuous fermentation.

The importance of the binding-protein-dependent Mgl system to the transport of glucose in Escherichia coli growing on low sugar concentrations

Glucose limitation in chemostats derepressed the binding-protein-dependent Mgl transport system, which is strongly repressed during growth in batch culture with high glucose levels. The limitation-induced Mgl activity was higher than that of batch cultures "fully induced" for the Mgl system after growth on glycerol plus fucose. Mgl- strains were impaired compared to Mgl+ bacteria in removing glucose from sugar-limited chemostats and were outcompeted in mixed continuous culture on limiting glucose. The influence of Mgl was not observed on growth with limiting maltose or non-carbohydrates, and thus was specific for glucose, a known substrate of the Mgl system. In the absence of the two glucose-specific membrane components of the phosphoenolpyruvate:sugar phosphotransferase system, non-PTS-dependent growth on glucose was observed in continuous culture, but only under sugar-limited conditions derepressing the Mgl system and not in glucose-rich batches or continuous culture. Hence growth of Escherichia coli on glucose at micromolar concentrations involves a significant contribution of a binding-protein-dependent transport system. The participation of multiple transporters in glucose transport can account for the complex non-hyperbolic dependence of growth-rate on glucose concentration and for discrepancies in studies attempting to describe growth on glucose purely in terms of phosphotransferase kinetics.