The turnover number (number of molecules the enzyme can act on per second) for lactase dehydrogenase is 1,000 lactate molecules per second.
In one minute, this becomes:
(1,000 molecules / second) * (60 seconds / minute)
= 60,000 molecules / minute
In 21 minutes, this will be:
60,000 * 21
= 1.26 x 10⁶ molecules of lactate
