In 1906, Harden and Young, in a series of classic studies on the fermentation of glucose to ethanol and $CO_2$ by extracts of brewer's yeast, made the observations
1. inorganic phosphate was essential to fermentation; when the supply of phosphate was exhausted, fermentation ceased before all the glucose was used;
2. during fermentation under these conditions, ethanol, $CO_2$, and a sugar phosphate accumulated;
3. when arsenate was substituted for phosphate, no sugar phosphate accumulated, but the fermentation proceeded until all the glucose was converted to ethanol and $CO_2$.
Which enzyme of glycolysis requires inorganic phosphate and, therefore, stops when no phosphate is available?
triose phosphate isomerase
pyruvate kinase
phosphofructokinase-1
glyceraldehyde 3-phosphate dehydrogenase
