My favorite enzymes, part 4: RuBisCO
September 3, 2010(Previous installments of this series: part 1; parts 2 and 3.)
The photosynthetic enzyme ribulose-1,5-bisphosphate carboxylase oxygenase — “RuBisCO” to its friends — is essential to life on Earth, yet seemingly unimpressive in its catalytic speed and specificity. As summarized by the website SCICOM, “Turning over ~1011 tonnes of CO2 each year (cf. annual oil consumption ~3 x 109) RuBisCO is the most abundant enzyme on Earth, being about 60% of soluble leaf protein. Providing the only link between pools of inorganic and organic carbon in the biosphere, its importance would be hard to overestimate. Yet, with a turnover rate of 3 sec-1, RuBisCO is one of the slowest enzymes known…. As well as being slow, RuBisCO has another problem. In a reaction which seems totally wasteful, O2 competes with CO2 for the active site. Is this an accident of evolutionary history? Did RuBisCO first evolve when the earth was anaerobic?”
Recent research suggests a possible reason for RuBisCO’s apparent lameness. Guillaume G. B. Tcherkez et al. (PNAS 103: 7246-51, 2006) propose that the transition state for CO2 addition is such that the more selective the enzyme is for CO2 over O2, the more slowly the reaction occurs. High selectivity is achieved by “a transition state for CO2 addition in which the CO2 moiety closely resembles a carboxylate group”; however, “increasing structural similarity between the carboxylation transition state and its carboxyketone product … causes the carboxyketone intermediate to bind so tightly that its cleavage to products is slowed.” The authors conclude, “We assert that all RuBisCOs may be nearly perfectly adapted to the differing CO2, O2, and thermal conditions in their subcellular environments, optimizing this compromise between CO2/O2 specificity and the maximum rate of catalytic turnover.”
So maybe RuBisCO isn’t so suboptimal after all! It makes my favorite-enzymes list for its unwavering commitment to photosynthesis and its patient tolerance of the criticisms that it is sluggish and promiscuous.
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