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.

The turnover number or k_cat indicates how rapidly an enzyme can convert substrate molecules into products.

k_cat is equal to the maximum number of substrate molecules that can be transformed in a given time per enzyme active site.

Turnover numbers of different enzymes range from fewer than one substrate molecule to millions of molecules per second.

To calculate k_cat, the maximum velocity or V_max of an enzyme-catalyzed reaction is divided by the total enzyme concentration.

Both k_cat, the catalytic rate, and K_M, the affinity for a substrate, affect an enzyme's catalytic efficiency—how effectively an enzyme accelerates a given biochemical reaction.

One way to measure catalytic efficiency for a specific substrate is the ratio of k_cat to K_M.

Enzymes with a high k_cat quickly catalyze a substrate's transformation, and those with a low K_M strongly bind their substrates. Therefore, those enzymes with a larger ratio are more efficient.

An enzyme that binds multiple substrates is most catalytically efficient for the substrate with the highest k_cat to K_M ratio.