|MadSci Network: Biochemistry|
The question really cannot be answered. It depends completely on the previous history of the organism whether S. cerevisiae or anything else. Assuming that the organism can metabolize sucrose to start with, as not all bacteria or fungi can, the enzymes necessary to do so may need to be induced. Same is true for all glycolytic enzymes. If the cell has been growing on something like succinate, then glycolysis is minimal and it won't initially grow on either sucrose or glucose or fructose. All the enzymes have to be induced first. In this case, you wouldn't be able to tell any difference between growth rate on the various sugars. If we assume that the cell is adapted to one or the other sugar, and then measure growth rate and growth yield (mass of cells), I don't think there would be a signficant difference in either rate or yield. You're talking about a very small part of the overall process. If there was any difference in rate, it would be swamped by all the other steps involved. If you limited yourself to asking whether sucrose, fructose or glucose would make phosphoenolpyruvate (PEP) faster than one of the other sugars, then you might find a small difference, but not anything that you'd really be interested in. And any one of the sugars might be the "fastest" or the "most". It depends not only on the sugar, but the level of the membrane transporters for each and the actual amount of enzyme specific to each step that was already in the cell. For example, even though glucose would take a couple steps more that fructose possibly, if the levels of the enzymes specific to glucose versus fructose were much higher, then glucose would likely show a faster increase in PEP. Also, even if they were equal, fructose (and glucose) can be shunted off into other pathways besides glycolysis. If these pathways were active, they might reduce the amount of fructose rapidly and thus decrease the amount available for glycolysis. In other words, it's complicated and depends on the particular organism and that organism's metabolic potential and its history of growth.
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