I enjoy superlatives, and today’s post features an enzyme that fits neatly into that category: the world’s most important enzyme. The selected enzyme may generate some disagreements—always welcome. My candidate for the most important enzyme in the world is ribulose-1,5-bisphophate carboxylase/oxygenase (EC 188.8.131.52), more commonly referred to as rubisco.
Why is rubisco the world’s most important enzyme? Well, for starters, it is the foundation for all carbon-based life. Rubisco is responsible for the fixation of carbon. Without carbon fixation, life as we know it would not exist.
Rubisco is also the most abundant enzyme in the world. It is present in every photosynthetic organism, from the smallest cyanobacteria and plankton to palm trees and giant sequoias. In a textbook written by Moore et al it is estimated that rubisco makes up 20-25% of the soluble protein in leaves and is produced at a rate of about 1000 kg/second on earth (Moore, R., Clark, W. D., Kingsley, R. S., and Vodopich, D., Botany, Wm. C. Brown, 1995.) Every living human is being supported by about 44 kg of Rubisco. Go plant some more trees!
Here is another interesting tidbit about rubisco: the enzyme continues to function long after the plant containing it has died, thus aiding decomposition.
Rubisco is also a famously inefficient enzyme. One reason for the inefficiency is that the enzyme must be capable of reacting with O2 as well as CO2. In addition, rubisco is not well-adapted to current atmospheric conditions; the current levels of atmospheric CO2 are roughly half of the concentration required for the enzyme to function at its maximal catalytic rate. A possible reason for this is that rubisco first evolved in cyanobacteria about 3 billion years ago when there was little atmospheric O2 and significantly higher levels of CO2. Natural evolutionary pressure has not been strong enough to improve the enzyme further.
The implications of being able to improve rubisco by directed evolution have not escaped the attention of scientists, who have been trying for years to use genetic engineering methods to improve photosynthetic efficiency. So far those efforts have been mostly without success. The inability to reconstitute functioning rubisco in vitro has limited those experiments. And why is rubisco so difficult to reconstitute in active form? Because the catalytically active form of consists of 16 subunits. But a recent in the scientific journal Nature a few years ago indicates a pending solution to this problem. (Nature 463, 197-202 [14 January 2010])
Enter chaperones, proteins that facilitate the correct folding of newly synthesized proteins. According to Dr. Manajit Hayer-Hartlthe at the Max Planck Institute of Biochemistry in Martinsried, Germany, the right chaperones are needed for rubisco to assemble properly. Chaperones in hand, experiments are now underway to improve rubisco by directed evolution. A modified rubisco that can absorb carbon dioxide from the atmosphere more effectively could enhance crop yields, and would also be expected to help counteract rising atmospheric carbon dioxide levels. (1)
(1) For more information, see the Press Release from the Max Planck Institute of Biochemistry.