The turnover number is defined as the maximum number of substrate molecules that can be converted into product molecules per unit time by an enzyme molecule. The concentration of enzyme active sites is not necessarily equal to the concentration of enzyme molecules, because some enzyme molecules have more than one active site. If the enzyme molecule has one active site, the turnover number is given by

According to the kinetics of Enzyme catalyzed reaction, the enzyme binds substrate and form the enzyme-substrate complex. The rate constant for this complex formation is K₁.

The enzyme complex later dissociates into the product and the enzyme itself ans the rate constant for this reaction is k₂. An enzyme-catalyzed reaction has the maximum velocity called Vmax.

The k₂ is also known as the turnover number which represents how many substrate molecules are converted into the products. The mathematical expression for this reaction is the:

Vmax/ total enzyme concentration

Thus, Vmax/ total enzyme concentration (Et) is correct.