Rubisco is a protein; the single most important enzyme known to man.
The carbon atom is found everywhere, as part of simple or complex structures. There are two carbon-containing forms that are most important to us, and particularly to Rubisco. The first is as an inorganic, highly oxidised compound found in air (eg. CO2). The second is found in the organic form, and referred to as an organic compound if it contains solely hydrogen and carbon atoms.
The carbon that makes up highly oxidised structures is difficult to get hold of and needs to be fixed in order to become of use. The process by which this occurs is called carbon fixation. Plants are responsible for fixing the carbon atoms into compounds rich in carbon/carbon double bonds and hydrogen atoms; they do this using energy from the sun.
Rubisco has the important role of turning inorganic carbon – present in the air – into its organic forms.
(It is a bi-functional enzyme that catalyses the carboxylation and the oxygenation of ribulose biphosphate).
Rubisco attaches the carbon atom (derived from CO2) to the ribulose biphosphate chain (a 5-carbon sugar). The addition of the extra carbon consequently lengthens the chains, which is in turn split by Rubisco in order to form two smaller identical ones. They each contain 3 carbon atoms and are known as phosphoglycerates.
93% of the time, the new sugar chain made (phosphoglycerate) is recycled and used to produce more ribulose biphosphate, as the carbon fixing cycle must be maintained. However every 6th molecule made is used either to make sucrose so that the plant can be fed, or to make starch for energy storage.
The Specificity of Rubisco:
The physiology and chemical properties of a diatomic oxygen molecule are very similar to that of a carbon dioxide molecule. The barely noticeable structural difference is that the size of an oxygen molecule is slightly smaller. This means that a carbon dioxide molecule will not be able to bind to a protein made complementary in shape to oxygen – such as haemoglobin – because it will not fit. However it does mean that an oxygen molecule can bind to an enzyme made specifically for carbon dioxide, and fit comfortably. This is unfortunately what happens with Rubisco and oxygen; the O2 will bind to the enzyme’s site designed for C02. This produces a faulty oxygenated molecule.
The Efficiency of Rubisco:
Rubisco works at a very slow pace. To give you some idea, whereas the average enzyme can produce 1000 molecules per second, Rubisco will fix 3 carbon atoms in the same time period.
Luckily, the plant can make up for this inefficiency by holding vast quantities of the Rubisco enzyme in the stroma of chloroplast organelles.
It is clear why Rubisco is the most common enzyme, as well as the most important one. Without Rubisco, the Calvin cycle (the metabolic pathway that is photosynthesis) could not begin and the level of carbon dioxide in the air would be ever increasing, and at a much higher rate than that caused by global warming.