50 research outputs found
Highâcellâdensity cultivation of yeasts on disaccharides in oxygenâlimited batch cultures
Atypical Glycolysis in Clostridium Thermocellum
Cofactor specificities of glycolytic enzymes in Clostridium thermocellum were studied with cellobiose-grown cells from batch cultures. Intracellular glucose was phosphorylated by glucokinase using GTP rather than ATP. Although phosphofructokinase typically uses ATP as a phosphoryl donor, we found only pyrophosphate (PPi)-linked activity. Phosphoglycerate kinase used both GDP and ADP as phosphoryl acceptors. In agreement with the absence of a pyruvate kinase sequence in the C. thermocellum genome, no activity of this enzyme could be detected. Also, the annotated pyruvate phosphate dikinase (ppdk) is not crucial for the generation of pyruvate from phosphoenolpyruvate (PEP), as deletion of the ppdk gene did not substantially change cellobiose fermentation. Instead pyruvate formation is likely to proceed via a malate shunt with GDP-linked PEP carboxykinase, NADH-linked malate dehydrogenase, and NADP-linked malic enzyme. High activities of these enzymes were detected in extracts of cellobiose-grown cells. Our results thus show that GTP is consumed while both GTP and ATP are produced in glycolysis of C. thermocellum. The requirement for PP i in this pathway can be satisfied only to a small extent by biosynthetic reactions, in con- trast to what is generally assumed for a PPi-dependent glycolysis in anaerobic heterotrophs. Metabolic network analysis showed that most of the required PPi must be generated via ATP or GTP hydrolysis exclusive of that which happens during biosynthesis. Experimental proof for the necessity of an alternative mechanism of PPi generation was obtained by studying the glycolysis in washed-cell suspensions in which biosynthesis was absent. Under these conditions, cells still fermented cellobiose to ethanol
Pyruvate decarboxylase: An indispensable enzyme for growth of Saccharomyces cerevisiae on glucose
The Exometabolome of Clostridium Thermocellum Reveals Overflow Metabolism at High Cellulose Loading
BackgroundClostridium thermocellum is a model thermophilic organism for the production of biofuels from lignocellulosic substrates. The majority of publications studying the physiology of this organism use substrate concentrations of â€10 g/L. However, industrially relevant concentrations of substrate start at 100 g/L carbohydrate, which corresponds to approximately 150 g/L solids. To gain insight into the physiology of fermentation of high substrate concentrations, we studied the growth on, and utilization of high concentrations of crystalline cellulose varying from 50 to 100 g/L by C. thermocellum.
The Exometabolome of Clostridium Thermocellum Reveals Overflow Metabolism at High Cellulose Loading
BackgroundClostridium thermocellum is a model thermophilic organism for the production of biofuels from lignocellulosic substrates. The majority of publications studying the physiology of this organism use substrate concentrations of â€10 g/L. However, industrially relevant concentrations of substrate start at 100 g/L carbohydrate, which corresponds to approximately 150 g/L solids. To gain insight into the physiology of fermentation of high substrate concentrations, we studied the growth on, and utilization of high concentrations of crystalline cellulose varying from 50 to 100 g/L by C. thermocellum.
An explorative approach to understanding individual differences in driving performance and neurocognition in long-term benzodiazepine users
Objective: Previous research reported cognitive and psychomotor impairments in longâterm users of benzodiazepine receptor agonists (BZRAs). This article explores the role of acute intoxication and clinical complaints. Methods: Neurocognitive and onâroad driving performance of 19 longâterm (â„6 months) regular (â„twice weekly) BZRA users with estimated plasma concentrations, based on selfâreported use, exceeding the therapeutic threshold (CBZRA+), and 31 longâterm regular BZRA users below (CBZRAâ), was compared to that of 76 controls. Results: BZRA users performed worse on tasks of response speed, processing speed, and sustained attention. Age, but not CBZRA or selfâreported clinical complaints, was a significant covariate. Roadâtracking performance was explained by CBZRA only. The CBZRA + group exhibited increased mean standard deviation of lateral position comparable to that at bloodâalcohol concentrations of 0.5 g/L. Conclusions: Functional impairments in longâterm BZRA users are not attributable to selfâreported clinical complaints or estimated BZRA concentrations, except for roadâtracking, which was impaired in CBZRA + users. Limitations to address are the lack of assessment of objective clinical complaints, acute task related stress, and actual BZRA plasma concentrations. In conclusion, the results confirm previous findings that demonstrate inferior performance across several psychomotor and neurocognitive domains in longâterm BZRA users