Rubisco (1RLC) - The carboxylation step of the Calvin cycle, (where carbon dioxide is joined to the 5 carbon sugar ribulose 1,5-bisphosphate), is catalyzed by the enzyme ribulose 1,5-bisphosphate carboxylase oxygenase (better known as "Rubisco"). Rubisco has some quite interesting features worth noting. First of all, as enzymes go this one has a very low affinity for its substrate, CO2. That means that it has a very high Km for CO2. In order to overcome this problem cells simply make LOTS of this enzyme to boost the reaction rate. As a result, Rubisco is the most abundant protein on earth! Imagine a (small) interstellar visitor landing here, taking some samples and going away thinking that the earth was dominated by Rubisco! Another interesting problem with this enzyme is that it not only uses CO2 as a substrate, but it can also use O2 in place of CO2 (hence the long name). This other reaction turns out to be quite wasteful of energy and plants have evolved fascinating tricks to reduce this waste (e.g. C4 photosynthesis).
Rubisco is located in the stroma of chloroplasts and consists of 16 subunits - 8 large ones and 8 small ones. Pictured below are two views of one pair of Rubisco subunits from tobacco (Nicotiana tabaccum). In higher plants the large subunit (blue in the left image) is encoded by the chloroplast genome while the small subunit (green in the left image) is encoded in the nucleus. In the right image the numerous alpha helices (pink) and less numerous beta sheets (yellow) are highlighted. The next time you eat a green vegetable remember that many of the proteins that your digestive proteases act on will be Rubisco!
8/22/04 Copyright (C) 2004,
Jonathan Monroe, firstname.lastname@example.org.
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