Different definitions of turnover frequency (TOF), catalytic cycle frequency, and catalytic rate constant abound in the literature. The number of catalytic cycles that each enzyme molecule carries out in its lifetime is often called the "total turnover number," and it is taken as a key industrial performance criterion . The TTN is a dimensionless number, defined as the ratio of moles of product generated divided by the moles of biocatalyst used in a reaction . But this is the definition of a first-order rate constant achieved when the enzyme is saturated with substrate. Another definition is based on the work of Rueveni et al. , who based such work on the equation below.