Sugar transporters yeast

Yeasts contain a large number of different active and passive sugar-transport systems. The first of these to be cloned was the glucose-repressible, high-affinity passive glucose transporter of Saccharomyces cerevisiae, which is encoded by the SNF3 gene... 

There was some evidence for the possible involvement of PolyPs localized in the cell periphery in the uptake and phosphorylation of sugars as energy and phosphate donors (Van Steveninck and Booij, 1964 Hofeler et al, 1987). Later, studies of the mechanisms of transport-associated phosphorylation of 2-deoxy-D-glucose in the yeast Kluyveromyces marxianus (Schuddemat et al, 1989b) and Saccharomyces cerevisiae (Schuddemat et al, 1990) resulted in the conclusion that PolyPs seem to replenish the Pj pool and therefore had an indirect role in sugar transport. 

Carrier-mediated movement of sugars across the plasmalemma of yeasts involves the combination of the sugar with a protein on one side of the plasmalemma, followed by release of the sugar into the cytoplasm on the other side. Such movement is described either as (t) facilitated diffusion, when the movement requires no metabolic energy, or (ii) active transport, which involves the expenditure of metabolic energy. Sugars entering yeast cells by active transport may be accumulated within the cells to a concentration many hundred times the external level. This subject has been reviewed by... 

In sparkling wine production, if the level of free SO2 in the base wine is high, for example, greater than 10 mg/1, then the yeast that conducts the secondary fermentation may mutate. As a result, it becomes sugar transport deficient and may be unable to metabolise sugars efficiently (Lehmann, 1987). In addition, the viability of the inoculum may decrease from 95-100% to 0-80% depending on the concentration of SO2 present in the base wine, and hence commencement of fermentation may be delayed significantly (Lehmann, 1987). A further consequence of yeast mutation is uncertainty as to whether the secondary fermentation will proceed to completion or stick . 

One kind of active transport, namely group translocation, occurs in bacteria (for reviews, see Refs. 218-223), and some workers consider that this also takes place in yeasts (see, for example, Refs. 224 and 225) by this means, uptake of a sugar is directly coupled to its phosphorylation, and the sugar is released into the cytoplasm as a phosphate. 

Fermentation, which differs from respiration in not having an electron transport chain. Yeasts produce ethanol from sugars by fermentation ... 

Fig. 2. —Temperature Dependence of Net Transport of D-Xylose into Rhodotorula glutinis (gracilis) [—O—] (Heller and Coworkers, 1974)212 and of D-Ribose into Pichia fermentans [— —] (Barnett, 1975).20 [Rate of transport = Iog(0 (gg of sugar/mg of dry weight of yeast/min).]...

---