The Impossible Enzyme
Enzymes are extremely important biomolecules for their astonishing catalytic properties. These proteins can catalyze reactions increasing reaction rates by up to 7 orders of magnitude. Even though their catalytic activity is extremely superior to their inorganic counterparts, their efficiency is finite. What physical or chemical properties determine the limit of an enzymatic reaction? Is there an absolute limit a perfect enzyme cannot overcome?
The catalytic activity of an enzyme is governed by two main factors: the catalytic efficiency and the substrate diffusion limit. The catalytic efficiency is basically how good an enzyme is at carrying out a chemical reaction. This is dictated by the slowest step in the mechanism. The substrate diffusion limit is the time it takes for the substrates to enter the active site and exit as the product. It is known that there are enzymes with a perfect catalytic efficiency. This means that they are able to perform the reaction as soon as the substrates enter the active site. Therefore, the only limitation of their reaction rate is the substrate diffusion limit.
This diffusion limit is about 109 M-1 s-1, and even though it is a very fast rate, there is experimental evidence of enzymes that apparently are faster than their diffusion limit. Carbonic anhydrase and triose phosphate isomerase, among others, are very fast enzymes that overcome the diffusion limit. Some of these outstanding mechanisms have been explained using buffer chemistry or even quantum tunneling, but others are still a mystery. It used to be believed that the enzymatic diffusion time was the physical limit of enzyme activity but evolution keeps challenging the most basic laws of physics, apparently beyond perfection.
REFERENCES
1. Cleland, W. W. 1974. What Limits the Rate of an Enzyme-Catalyzed Reaction? Accounts of Chemical Research. V8.5.
2. Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., Walter, P. 2008. Chapter 5: DNA Replication, Repair, and Recombination. Molecular Biology of the Cell. S. Masson, E. Jeffcock, M, Anderson, S. Granum, editors. Garland Science/New York, USA. 281–293
3. Laurence, A. M. 2012. Better Biochemistry: The Perfect Enzyme. Sandwalk blogspot. Online. Extracted Sept 27.