The sugar is present in the grape juice as glucose,
C6H12O6, which is converted to pyruvate
during a long and complicated biochemical pathway called glycolysis.
In normal, aerobic conditions the pyruvate is converted into a form that
can be used by the cell to give energy via two more pathways, the citric
acid cycle and the electron transfer reaction. Under the anaerobic
conditions found in the wine bottle, the pyruvate is then converted by
the yeast to ethanol.
This is a two-step reaction:
(1) The pyruvate decarboxylated to acataldahyde, releasing CO2 gas. This is catalysed by the enzyme pyruvate decarboxylase.
(2) The acetaldehyde is now reduced to produce ethanol. The H+ comes from NADH, a naturally occuring electron carrier. This is a reversible reaction and is catalysed by the enzyme alcohol dehydrogenase in the fowards direction.
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