3.14 : Turnover Number and Catalytic Efficiency

The turnover number of an enzyme is the maximum number of substrate molecules it can transform per unit time. Turnover numbers for most enzymes range from 1 to 1000 molecules per second. Catalase has the known highest turnover number, capable of converting up to 2.8×10⁶ molecules of hydrogen peroxide into water and oxygen per second. Lysozyme has the lowest known turnover number of half a molecule per second.

Chymotrypsin is a pancreatic enzyme that breaks down proteins during digestion. The turnover number of chymotrypsin is 100 molecules per second. If this reaction were to occur uncatalyzed, peptide bonds would take hundreds of years to break in water at neutral pH. Thus, the high turnover number of chymotrypsin helps quick digestion of proteins in the intestine.

The enzyme ribulose 1,5-bisphosphate carboxylase oxygenase or RuBisCO has a very low turnover number of fixing 3 molecules of CO₂ per second and is one of the slowest enzymes. However, the abundance of RuBisCO in nature makes up for the low turnover number. RuBisCO constitutes around 50% of the total protein found in leaves.

An enzyme with a high turnover number may not necessarily be highly efficient. The catalytic efficiency of an enzyme is given by the ratio of turnover number, k_cat, to the affinity_, K_M. In other words, an enzyme should also have a low K_M for the substrate in order to be efficient. The average catalytic efficiency of most enzymes is approximately 10⁵ M^-1s^-1, meaning they are moderately efficient. Few enzymes with catalytic efficiency between 10⁸-10⁹ M^-1s^-1 are superefficient or catalytically perfect.