Which mineral is required as an activator for enzymes during glycolysis?

Harden and Young's ester is formed during glycolysis from

$ATP$ is not produced during this stage.

Which enzyme is inhibited by high levels of $ATP$?

Glycolysis:
$I$. Causes partial oxidation of glucose (one molecule) to form $2$ molecules of pyruvic acid and $2$ $ATP$ as net gain.
$II$. Takes place in all living cells.
$III$. Uses $2$ $ATP$ at two steps.
$IV$. Scheme was given by Gustav Embden,Otto Meyerhof,and $J$. Parnas.
Choose the correct option containing appropriate statements from the above.

What is the net gain of $ATP$ when each molecule of glucose is converted to two molecules of pyruvic acid?