Abstract
A major challenge in systems biology lies in the integration of processes occurring at different levels, such as transcription, translation, and metabolism, to understand the functioning of a living cell in its environment. We studied the high temperature-induced glycolytic flux increase in Saccharomyces cerevisiae and investigated the regulatory mechanisms underlying this increase. We used glucose-limited chemostat cultures to separate regulatory effects of temperature from effects on growth rate. Growth at increased temperature (38 degrees C versus 30 degrees C) resulted in a strongly increased glycolytic flux, accompanied by a switch from respiration to a partially fermentative metabolism. We observed an increased flux through all enzymes, ranging from 5- to 10-fold. We quantified the contributions of direct temperature effects on enzyme activities, the gene expression cascade and shifts in the metabolic network, to the increased flux through each enzyme. To do this we adapted flux regulation analysis. We show that the direct effect of temperature on enzyme kinetics can be included as a separate term. Together with hierarchical regulation and metabolic regulation, this term explains the total flux change between two steady states. Surprisingly, the effect of the cultivation temperature on enzyme catalytic capacity, both directly through the Arrhenius effect and indirectly through adapted gene expression, is only a moderate contribution to the increased glycolytic flux for most enzymes. The changes in flux are therefore largely caused by changes in the interaction of the enzymes with substrates, products, and effectors.
Highlights
The analysis showed that the increase in glycolytic flux observed at higher ambient temperature is primarily regulated at the metabolic level, whereas contribution of hierarchical regulation and temperature effects is minor
Focusing on a defined pathway, we studied how S. cerevisiae regulates its systems properties when challenged with an increase in ambient temperature
This study is the first example in which regulation analysis was applied to determine the quantitative contribution of the direct temperature effects on enzyme kinetics, the effects of changes in gene expression, and an altered metabolic make-up of the cell to the increased flux through the glycolytic enzymes
Summary
We studied the quantitative effect of increased temperature on the carbon and energy fluxes of Saccharomyces cerevisiae both at the anabolic and catabolic level. Our data were subsequently subjected to regulation analysis [21]. To this aim, we investigated whether the effect of temperature on enzyme rates could be included as a separate term. We investigated whether the effect of temperature on enzyme rates could be included as a separate term For most enzymes, this was the case. The analysis showed that the increase in glycolytic flux observed at higher ambient temperature is primarily regulated at the metabolic level, whereas contribution of hierarchical regulation and temperature effects is minor
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