The claim that Rubisco is poorly designed or unintelligently designed was appearing in textbooks in the 1990s. The idea has been picked up recently in a News & Views piece by John Ellis. He writes that Rubisco “is a relic of a bygone age” and his essay has the title: “Tackling unintelligent design”.
“Rubisco is the most important enyzme on the planet – virtually all the organic carbon in the biosphere derives ultimately from the carbon dioxide that this enzyme fixes from the atmosphere. But Rubisco is also one of the most inefficient enzymes on the planet. It evolved when the atmospheric composition was different from that of today, and its failure to adapt significantly to the modern atmosphere limits agricultural productivity.”
Over the past decade, the pendulum has swung away from the idea that Rubisco is unintelligent design. [snip]
[After drawing attention to some significant papers, the blog continues: ]
Design theorists have drawn attention to three additional considerations:
1. A single-factor analysis of Rubisco is inadequate. The parameters considered to conclude the enzyme is poorly designed and inefficient are very limited. We should note that our perceptions of intelligent design are typically subjective, and most claims for poor design do not stand up to the test of time – further research leads to a greater appreciation of design (a good example being mammalian eye design). Furthermore, unintelligent design of architectures we deem sub-optimal should not be regarded as the only possible hypothesis. Multiple factors are likely to be relevant as chemosynthetic carbon fixation also makes use of Rubisco. It is employed by organisms living at hydrothermal vents and cold hydrocarbon seeps.
2. Photorespiration, the consumption of oxygen to produce a sugar that ultimately forms carbon dioxide during a series of reactions, may not be a mark of inefficiency, but the process may be useful to the plant. The null hypothesis for Design theorists is that processes have functionality. This hypothesis is not without some support: the process of photosynthesis is not just to capture CO2 and release oxygen because nitrate assimilation in plant shoots depends on photorespiration, as Rachmilevitch et al (2004) have shown.
3. Ecological considerations should be included in the analysis. If design is relevant to understanding the way plants work, we should consider not only the benefits to the organism (which limits the horizon for those with a Darwinian perspective) but also the biosphere as a whole. Rubisco’s ability to capture CO2 increases with increasing CO2 content in the atmosphere, so its efficiency rises in a CO2-rich atmosphere. However, increasing oxygen levels in the atmosphere will reduce Rubisco’s ability to capture carbon. So a negative feedback mechanism exists to regulate the relative concentrations of oxygen and carbon dioxide in the atmosphere. This is another example of design affecting the Earth’s ecology – for more on this, go here.
Ellis was commenting on a paper by Liu et al. that reports on work to produce a Rubisco in vitro. In order to do this, the authors required two chaperone proteins, ATP and addition of 18 protein subunits (taken from a cyanobacterial Rubisco) to be introduced in the correct sequence to get yields of the enzyme. It is hoped that this procedure can be used to produce mutated versions that can be screened for improved effectiveness. It’s all very interesting, but the biggest mystery is why people who expend so much intellectual energy on improving this remarkable molecule can live with the thought that “Rubisco is a superb example of unintelligent design for the modern world”. Maybe research funds would be better spent exploring avenues identified using the presumption that this enzyme is optimally designed.