phosphofructokinase fru-1,6-BP ;
Figure 8-7. Transport and metabolism of glucose, fructose, maltose, and sucrose by Saccharomyces cere-visiae. Fructose and glucose are transported by one of several different hexose transporters via facilitated diffusion. Sucrose is hydrolyzed by a secreted invertase. Maltose transport occurs via a proton symport-mediated maltose permease. Once inside the cell, maltose is hydrolyzed by maltase to give free glucose. The accumulated monosaccharides are phosphorylated to glucose-6-phosphate (glu-6-P) and fructose-6-phosphate (fru-6-P) by hexokinases, then to fructose-1,6-bisphosphate (fru-1,6-BP) which feeds directly into the Embden-Meyerhoff-Parnas (EMP) pathway.
via facilitated diffusion. No energy is required and instead the driving force is simply the osmotic gradient. It is also clear that they vary with respect to their substrate affinities. For example, there are hexose transporters that have high affinity for glucose (with affinity constants or Km values <5 ^M) or very low affinity (Km > 1 M). The presence of low-affinity and high-affinity systems provides the cell with the versatility necessary to transport sugars under a wide range of available concentrations. Thus, yeasts are especially suited to grow in environments containing glucose and fructose, since their uptake costs are cheap (in terms of energy expenditure), and they have carriers that function even when substrates are not plentiful (for example, in lean doughs where no exogenous source of sugar is added).
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