2.1.4   Properties of Rubisco

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Photosynthetic carbon fixation in air is constrained by the kinetic properties of Rubisco (reviewed by Woodrow and Berry 1988; see also Case study 1.1). Rubisco is a huge protein and despite selection pressure over evolutionary history remains an inefficient catalyst (Andrews and Lorimer 1987). Plants compensate by investing large amounts of nitrogen in Rubisco which then comprises more than 50% of leaf protein in C3 plants. On a global scale, this investment equates to around 10 kg of nitrogen per person!

Something like 1000 million years of evolution has still not resulted in a ‘better’ Rubisco. Such a highly conserved catalytic protein is an outcome of thermodynamic and mechanistic difficulties inherent to this reaction. Rubisco first evolved when the earth’s atmosphere was rich in CO2, but virtually devoid of O2. With the advent of oxygenic photo-synthesis, increased partial pressure of atmospheric O2 and subsequent appearance of land plants, one key deficiency of this enzyme became apparent. Rubisco would not only catalyse fixation of CO2 but would also permit incorporation of O2 into RuBP to produce a molecule of 3-PGA and a molecule of 3-phosphoglycolate (Figure 2.10). Indeed, CO2 and O2 actually compete directly for access to the same (CO2) binding sites! So feeble is Rubisco’s ability to distinguish between these two substrates that in air (20% O2) approximately one molecule of O2 is fixed for every three molecules of CO2.

Fixation of O2 and subsequent photorespiration (Section 2.3) is an energy-consuming process, due to competition between O2 and CO2 for RuBP, plus the energy cost of converting phosphoglycolate to a form which can be recycled in the PCR cycle (Section 2.3). This energy cost is increased at higher temperatures because O2 competes more effectively with CO2 at the active site of Rubisco (Lorimer and Andrews 1981). Such sensitivity to temperature × O2 explains why CO2 enrichment, which reduces photorespiration, has a proportionally larger effect upon net carbon gain at higher temperatures than at lower temperatures (Section 13.3.3).

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Figure 2.2 Mechanisms underlying CO2 fixation by Rubisco have changed very little during evolution but Rubisco efficiency has improved. The enzyme in more 'highly evolved' species such as C3 angiosperms is able to fix more CO2 and less O2 in air, reducing photorespiratory energy costs. A measure of this is the relative specificity of Rubisco for CO2, shown here for a range of photosynthetic organisms. (Based on Andrews and Lorimer 1987)

Notwithstanding a meagre catalytic effectiveness in presentday Rubisco, more efficient variants would still have had a selective advantage, and especially during those times in the earth’s geological history when atmospheric CO2 concentration was decreasing. Indeed there has been some improvement (Figure 2.2) such that specificity towards CO2 over O2 has sharpened significantly. Recently evolved angiosperms show a relative specificity almost twice that of 'older' organisms such as photosynthetic bacteria.

Despite such improvement, Rubisco remains seemingly maladapted to its cardinal role in global carbon uptake, and in response to selection pressure for more efficient variants of CO2 assimilation, vascular plants have evolved with photosynthetic mechanisms that alleviate an inefficient Rubisco. One key feature of such devices is a mechanism to increase CO2 concentration at active sites within photosynthetic tissues. Some of these photosynthetic pathways are dealt with below.

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