Geoscience Reference
In-Depth Information
photosynthesis, however, is that it produces the highly oxidizing form of
P680 in PSII, which can extract electrons from water. This is the main
innovation of oxygenic photosynthesis.
So, where does chlorophyll come from? As it turns out, chlorophyll
is not a particularly weird molecule at all. It is related structurally and
chemically to a variety of other very common molecules (so-called por-
phyrin molecules) widely used in all sorts of cellular enzymes, includ-
ing the heme in hemoglobin. Chlorophyll is also closely related to the
bacteriochlorophylls used in anoxygenic photosynthetic organisms. In-
deed, the synthesis pathways of chlorophyll
a
and the various bacterio-
chlorophylls are very similar and diverge mainly in the very last steps.
Therefore, the biochemical distance is small between chlorophyll, bac-
teriochlorophyll, and even the common porphyrins used in our cells.
The question, though, is which came first.
Most biochemists would agree that porphyrins, as a general class of
molecules, evolved before chlorophyll and bacteriochlorophyll. These
early porphyrin molecules would have helped promote the biochemis-
try of the earliest life on Earth. If we now consider the photosynthetic
pigments chlorophyll and bacteriochlorophyll, we might guess that chlo-
rophyll evolved irst—that is, if we base this assessment on the formation
pathways of these molecules. This idea was presented by Sam Granick
in 1965 (known as the Granick hypothesis) and was based on the idea
that chlorophyll
a
forms in only one step from its immediate precursor,
a molecule called chlorophyllide
a
, whereas several steps are needed
to form bacteriochlorophyll
a
from the exact same precursor molecule.
Thus, chlorophyll
a
is easier to make.
We can look at this problem, however, from another angle. In this era
of genomics, evolutionary histories can be constructed directly from the
sequences of DNA within organisms. We saw an elegant application of
this at the end of the last chapter, where David and Alm explored the
evolutionary history of genes that conduct a wide variety of different
microbial metabolisms. Using a related approach, Jin Xiong and Carl
Bauer from Indiana University explored the evolutionary history of
oxygenic and anoxygenic photosynthetic organisms. As noted above,
chlorophyll and bacteriochlorophyll are formed by pretty much the
same biochemical pathway up to the very last stages of biosynthesis.
Therefore, genes can be identified from both oxygenic and anoxygenic
