What is Rubisco?
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 and regulates the rate limiting step in the photosynthetic assimilation of carbon.
Does Rubisco's slow turnover rate reflect the inherent difficulty of the task which it accomplishes? Or is it a very inefficient enzyme? Has it travelled only a short way along its evolutionary path? 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?
Our desire to understand Rubisco comes partly because of its inherent interest, but also because it might be a prime target for genetic engineering. In a world where the population of the poorest nations still increases exponentially, speeding up the slowest step in the photoassimilation of carbon might be a worthy aim. Our purpose in these pages is to introduce the enzyme's mechanism and to give some insight into the nature of the task which Rubisco accomplishes. Is Rubisco open to significant improvements in its design, or is it already optimised for carrying out an extremely difficult task? Before we turn to the mechanism, we need an overview of Rubisco's structure.
A brief look at Rubisco's structure
Rubisco from higher plants and most photosynthetic microorganisms consists of eight large L chains (56 kd) and eight small S chains (14 kd) giving an L8S8 octo-dimer. We call this the form I protein and we find it in proteobacteria, cyanobacteria, and plastids. Form II Rubisco which consists of L2 dimers, was until recently was known only from proteobacteria. Unlike the simpler bacterial gene, genes for the L8S8 molecule from higher plants are present in two different genomes. The gene for the L subunit is part of the chloroplast genome, whereas the S subunits are coded by the nuclear genome. S subunits are translated as preproteins containing ~50 extra residues at the amino end of the polypeptide chain. This acts as a signal to direct the S subunit into the chloroplast, where after passage of the preprotein through the membranes it is cleaved off. Once inside the chloroplast, the chaperone protein Rubisco Binding protein asssembles the mature L and S subunits into competent units. (Rubisco binding protein, present in large amounts in plants, is a member of the chaperonin family. It shows about 50% amino acid sequence identity with GroEL -the classic chaperone protein from Escherichia coli.)
All competent Rubiscos have at least two L chains, and the active site lies at their interface. Understanding the mechanism gives us valuable insight into why this should be the case. However, before we turn to look at the mechanism, it is well worth a first look at the structure of Rubisco, you may like to look at Schreudr.kin (click here for help if you are not familiar with .kin files), otherwise continue with the tutorial. You will need an hour or so to work through it fully - but it is divided into sectons to which you can easily return.
|Introduction to 'Explore the mechanism'|