RuBisCO: Natures Most Horrible Mistake

No I’m not making up words. RuBisCO is the arguably most important enzyme on the planet, its function is the first step in the capturing of carbon dioxide in plants in order to make biological matter, ergo it is where all the work of photosynthesis gets done, and that no exaggeration because it is so inefficient and its fidelity so poor that the single greatest efficiency loss in photosynthesis is here!

RuBisCO is so painfully slow, garbing only a few carbon dioxide molecules per second that plants have to produce incredible amounts of this enzyme making it quite possibly the most common protein by mass on the planet! In some plants up to 50% of their protein mass is RuBisCO! Its fidelity is such that it will capture oxygen instead of carbon dioxide undergo a energy losing process called photorespiration in which substrates are broken down and energy must be spent simply to repair the substrates, this happens at least 1/4 of the time!

Plants have evolved rather hair-brain ways of trying to improve the efficiency and fidelity of RuBisCO. Take C4 plants for example which capture CO2 by a completely different pathway, only to pump the capture CO2 deeper inside the leaf, away from oxygen and then release the CO2 in concentration around RuBisCO. This process is energy wasteful but worth it for the gain in efficiency and fidelity of RuBisCO. The only problem is dear god why not just cut RuBisCO out all together and use the CO2 captured by a faster more efficient pathway!

Evolution is not a smart process. I remember my bachelor degree days when I was making a Best-Fit Algorithm in MatLab, the algorithm would search through 12 dimensions of scalar and vectored data, picking points at random and asking how well its results fit the data, and then repeating around points that had had good results. Often the program would get stuck around localized mimima: areas where it could not make a better fit or get out of but was still off from the true answer. I relies evolution works like this: it finds answers at random through mutation and those answers may be far from the best possible but the mutations required to find better solutions are so unlikely, so out of range, that it gets stuck optimizing a poor solution rather then finding a better one. This is what I think RuBisCO is, an optimized poor solution that biology is stuck utilizing. This is why C4 plants have their hair-brain work around because to develop a completely alternate pathway would require so many new enzymatic steps that evolutionarily its too unlikely to happen and be competitive off the bat, hence the half-way pathway ‘dump it back on RuBisCO’ solution.

There is significant biotechnological work focused on genetic engineering a more efficient and accurate RuBisCO but so far they have not made significant progress, I don’t think they will, I think the enzyme has been optimized as best it can for each plant. The more they increase the rate of reaction the more they reduce the fidelity and increase photorespiration, and vis versa.

Why did biology get stuck with this pathetic enzyme? Well if you look at it from the fact when photosythesis evolved 3.5 billion years ago carbon dioxide concentrations in atmosphere were thousands of times greater then today and oxygen concentrations were non-existent, back then RuBisCO was probably the best photosynthetic pathway around and plants got stuck in that pathway to this day despite how inadequate RuBisCO is for today’s atmosphere. As a result our planets total carriage capacity is held down by a simple enzyme with a silly name.

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