Geoscience Reference
In-Depth Information
the formation of organic matter from CO
2
is promoted by the enzyme
Rubisco, which we met earlier in the chapter. Indeed, Rubisco is the
most abundant enzyme on the planet. hile it is found among oxygen-
producing cyanobacteria and plants, it is also found among a whole
suite of anoxygenic photosynthetic bacteria (mainly the “purple bacte-
ria”) as well as all sorts of other bacterial species that also convert CO
2
to organic matter.
One of the fascinating things about Rubisco is that despite being the
most abundant enzyme on Earth, it isn't terribly good at what it does,
at least when used in oxygenic phototrophic organisms. We can iden-
tify a number of difficulties. First, Rubisco has an extremely slow turn-
over rate of about 0.2 to 0.3 seconds, making it one of the slowest en-
zymes known; this partly explains why there is so much of it around.
Second, its affinity for CO
2
is rather low, although biology has pro-
vided some “fixes” for this challenge, as we shall see. Lastly, and I think
the most interesting, Rubisco is in constant competition with itself. As
we have discussed, Rubisco fixes CO
2
into organic compounds, and this
is known as carboxylase activity (the “c” in Rubisco). It also conducts a
rather stupefying oxygenase activity (the “o” in Rubisco) whereby oxy-
gen reacts with an intermediate compound, eventually forming CO
2
again, and essentially undoing the carbon-ixation process. Thus, as if
it can't make up its mind, Rubisco is characterized by competing reac-
tions. This isn't trivial, because the oxygenase activity of Rubisco is es-
timated to reduce net rates of carbon fixation by 25% to 40% in plants
like rice, wheat, and soybeans.
The ratio between the carboxylase activity (the favored reaction) and
the oxygenase activity (the unfavored reaction) depends on the ratio of
CO
2
to O
2
at the Rubisco active site. As you might imagine, the higher
the CO
2
to O
2
ratio, the more favored is the carboxylase activity. The
sensitivity of Rubisco to oxygen also depends on the exact type of
Rubisco present. Thus, Rubiscos used by anaerobic organisms, like an-
oxygenic phototrophs, tend to have high oxygenase activities for the
same amount of CO
2
, but this is of no consequence to these organisms
as they rarely see any oxygen. This observation, however, provides a pos-
sible window into understanding how oxygenic phototrophs came to use
Rubisco as their carbon-ixing enzyme. The story goes something like
this. Rubisco is an ancient carbon-ixing enzyme that evolved long before
