Carbon is essential to life. All of our molecular machines are built around
a central scaffolding of organic carbon. Unfortunately, carbon in the earth and atmosphere is
locked in highly oxidized forms, such as carbonate minerals and carbon dioxide gas. In order
to be useful, this oxidized carbon must be "fixed" into more organic forms, rich
in carbon-carbon bonds and decorated with hydrogen atoms. Powered by the energy of sunlight,
plants perform this central task of carbon fixation.
Inside plant cells, the enzyme ribulose bisphosphate carboxylase/oxygenase (rubisco) forms
the bridge between life and the lifeless, creating organic carbon from the inorganic carbon
dioxide in the air. Rubisco takes carbon dioxide and attaches it to ribulose bisphosphate,
a short sugar chain with five carbon atoms. Rubisco then clips the lengthened chain into two
identical phosphoglycerate pieces, each with three carbon atoms. Phosphoglycerates are
familiar molecules in the cell, and many pathways are available to use it. Most of the
phosphoglycerate made by rubisco is recycled to build more ribulose bisphosphate, which is
needed to feed the carbon-fixing cycle. But one out of every six molecules is skimmed off
and used to make sucrose (table sugar) to feed the rest of the plant, or stored away in
the form of starch for later use.
Slow and Steady
In spite of its central role, rubisco is remarkably inefficient.
As enzymes go, it is painfully slow. Typical enzymes can process a thousand molecules per
second, but rubisco fixes only about three carbon dioxide molecules per second. Plant
cells compensate for this slow rate by building lots of the enzyme. Chloroplasts are
filled with rubisco, which comprises half of the protein. This makes rubisco the
most plentiful single enzyme on the Earth.
Rubisco also shows an embarrassing lack of specificity. Unfortunately, oxygen molecules
and carbon dioxide molecules are similar in shape and chemical properties. In proteins
that bind oxygen, like myoglobin, carbon dioxide is easily excluded because carbon dioxide
is slightly larger. But in rubisco, an oxygen molecule can bind comfortably in the site
designed to bind to carbon dioxide. Rubisco then attaches the oxygen to the sugar chain,
forming a faulty oxygenated product. The plant cell must then perform a costly series of
salvage reactions to correct the mistake.
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