The equation may not be an outcome of "a supportive and acceptable evolutionary pressure" but a thought process; here, "the turnover rate kturn is the reciprocal of the mean turnover time Tturn-the average time it takes a single enzyme to produce a single molecule of product" . This definition notwithstanding, kcat is given as Vmax/[ET], and substitution into Eq. (39) with rearrangement gives: