3.14:
Turnover Number and Catalytic Efficiency
The turnover number or kcat indicates how rapidly an enzyme can convert substrate molecules into products.
kcat 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 kcat, the maximum velocity or Vmax of an enzyme-catalyzed reaction is divided by the total enzyme concentration.
Both kcat, the catalytic rate, and KM, 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 kcat to KM.
Enzymes with a high kcat quickly catalyze a substrate's transformation, and those with a low KM 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 kcat to KM ratio.
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, kcat, to the affinity, KM. In other words, an enzyme should also have a low KM 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.
Suggested Reading
- Smejkal, G. B., & Kakumanu, S. (2019). Enzymes and their turnover numbers. Expert Review of Proteomics, 16(7), 543-544.
- Bar-Even, A., Milo, R., Noor, E., & Tawfik, D. S. (2015). The moderately efficient enzyme: futile encounters and enzyme floppiness. Biochemistry, 54(32), 4969-4977.