Let me re-edit my response to more accurately reflect what happens in the body. I guess I think that this question is probably just asking for:
"Yes, too high blood glucose levels inhibit glycolysis and stimulate glycogen and fatty acid synthesis, but moderate levels of glucose will stimulate glycolysis". For that reason, I'm not convinced that you'd get this on MCAT because the answer is not that simple.
Some facts:
- ATP and F-1,6-BP are negative regulators of Pyruvate Kinase. So while excess blood sugar will initially stimulate gylcolysis, it will quickly cause an excess of F-1,6-BP, thereby inhibiting Pyruvate kinase, and thus glycolysis as a whole.
- ATP is also a negative regulator of phophofructokinase (which is the enzyme responsible for forming F-1,6-BP). This is also the major regulatory point of glycolysis and will have an inhibtory effect on glycolysis.
- An increase in glucose will also cause an increase in F-2,6-BP which is the major regulatory molecule of the phosphofructokinase step. This would lead to a
stimulation of glycolysis.
The major regulatory steps in glycolysis are hexokinase/glucokinase, phosphofructokinase and pyruvate kinase. The product of the phosphofructokinase step is fructose-1,6-bisphosphate (F-1,6-BP) which comes from the phosphorylation of F6P. F6P also forms F-2,6-BP in a side reaction of glycolysis which is a
stimulator of F-1,6-BP production. F-1,6-BP is in turn a major negative regulator of the next irreversible step in glycolysis which is the final step: pyruvate kinase (which catalyzes the formation of pyruvate).
So whether or not high blood glucose stimulates or inhibts glycolysis is dependent on
just how high the energy charge of the cell is. Without knowing just how much ATP there is and how much glucose there is, I'm not sure that we can say for sure whether glycolysis will be inhibited or stimulated.
If there was a question on the MCAT that asked, 'Does glucose inhibit glycolysis?' the answer is debateable depending on how you interpret the question. There is no allosteric site for glucose to
directly regulate glycolysis, but it is a high concentration reactant for HK/GK which will drive the first step of glycolysis toward production of G6P. The eventual products of this reaction both stimulate and inhibit glycolysis as we've outlined above.
If I was asked this question on the MCAT and no answer said, "Glucose stimulates glycolysis but only to a point", I would be very hesitant about answering it. The answer I would
guess the MCAT would be looking for might be 'Yes glucose inhibits glycolysis and stimulates glycogen synthesis and fatty acid synthesis.' I would, however, be pretty weary of my answer and not very happy about the question. Hope this helps, (and I hope I just didn't confuse you more.
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