Enzyme Activity Calculator
One unit is one micromole of product per minute; the katal is SI and inconveniently enormous.
The formula
unit = 1 micromole of product per minute ; specific activity = units per mg of protein
Units and specific activity measure different things
Total units tell you how much catalysis a preparation delivers; specific activity tells you how much of the preparation is the enzyme you want. Purification raises specific activity while total units can only fall — every step loses some enzyme. A purification table that shows both rising has an arithmetic error in it.
The katal nobody uses
The SI unit is the katal, one mole of substrate per second. It is inconveniently enormous: a typical laboratory preparation might be a few microkatals, and one enzyme unit is 16.67 nanokatals. The older unit has survived because it produces numbers of a size people can hold in their heads.
Where these models stop
Michaelis-Menten assumes a single substrate and a steady state; Langmuir assumes one layer on identical sites with no interaction between them; Stokes-Einstein assumes a hard sphere much larger than the solvent molecules around it. Each is an idealisation that happens to describe real systems well over a useful range, and each fails predictably outside it. Knowing which assumption a number rests on is usually more valuable than the number.