What does concentration of enzyme sites [Et] mean in Kcat=Vmax/[Et]? How to calculate [Et] of a dimer enzyme?

Question

What does concentration of enzyme sites [Et] mean in Kcat=Vmax/[Et]? How to calculate [Et] of a dimer enzyme?

Answer 1

[E] is just the concentration of enzymes in a given volume of solution.  Now if there are two active sites per enzyme, that means there are twice the concentration of enzyme sites.  The catalytic turnover rate, kcat, is how many substrate molecules are catalyzed by the enzyme per unit time.  Vmax is the maximum rate of catalysis (say, at infinite substrate concentration).  So, you can measure concentration of substrate at multiple times, for example, and from that data, determine [E].