Switching from a High-Carb to a Low-Carb Diet

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Science  23 Apr 2004:
Vol. 304, Issue 5670, pp. 493
DOI: 10.1126/science.304.5670.493c

Baker's yeast cells are inordinately fond of glucose as a carbon and energy source, and they carry around a set of seven transporters so as to be ready for a meal when glucose levels are anywhere from μM to M. Utilizing glucose depends, in part, on whether oxygen is available and on how abundant the supply of sugar is. Glycolysis, which transforms carbohydrates into pyruvate, is, roughly speaking, faster than aerobic metabolism, which converts pyruvate into carbon dioxide, hence some of the pyruvate spills over into ethanol when large fluxes of glucose are metabolized.

Otterstedt et al. have made a mutant yeast strain containing only a single hexose transporter, a hybrid of Hxt1 and Hxt7. The affinity and capacity of this transporter are such that it restricts glucose influx and avoids overflow into lactate. The end result is that this strain makes full use of the glucose by slow and steady respiration, and achieves a higher biomass than wild-type yeast under the same conditions. Having only one transporter means that at external glucose concentrations that wild-type yeast would perceive as high (and hence shift into glucose repression mode), the mutant strain sees a relatively low influx and behaves as if glucose is limiting. By analyzing profiles of transcripts in wild-type yeast, Kaniak et al. find that the regulation of repressed (primarily via metabolism of entering glucose) and induced (via a pair of membrane-bound glucose sensors) genes involves what appears to be a small set of intermediaries that speak to each other, which serves to maintain coordinate control of these two pathways. — GJC

EMBO Rep. 5, 10.1038/sj.embor.7400132 (2004); Eukaryot. Cell 3, 221 (2004).

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