Geoscience Reference
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hinted at above, for comparing the structures of different proteins and
have hypothesized that the four-membered manganese cluster may have
derived from two-membered manganese clusters found in some other
classes of proteins. Indeed, when they compared the oxygen-evolving
complex with the protein Mn catalase, the similarities in structure were
found to be reasonably good.
Manganese catalase is a protein used to convert hydrogen peroxide,
H
2
O
2
, into water and oxygen. It is a detoxifying protein, since hydro-
gen peroxide is a harmful compound to organisms. As first suggested
by Bob Blankenship and Hyman Hartman, and picked up again by
Raymond and Blankenship, the manganese cluster from Mn catalase
was incorporated into an anoxygenic phototrophic bacterium some-
time after the evolution of chlorophyll synthesis. This assembly would
have generated the first oxygen-producing organism. This organism,
however, would have conducted a two electron transfer from peroxide
to form O
2
, rather than the more challenging four electron transfer from
two water molecules to form oxygen, which we see today in plants and
cyanobacteria. At some point, there was a duplication of the two-Mn
cluster to form a four-Mn cluster, and this eventually allowed the evolu-
tion of oxygenic photosynthesis as we know it today.
Not everyone is happy with this hypothesis, with the main criticism
being that hydrogen peroxide was probably pretty rare before the evo-
lution of oxygen production. John Allen and William Martin have an-
other idea and argue that the precursor oxygenic phototroph obtained
its electrons from the photooxidation of manganese ions (in the chemi-
cal form Mn
2+
) freely dissolved in ancient anoxic seas. The electrons
obtained this way would have entered PSII, been transferred to PSI,
and finally to CO
2
. This is similar to modern oxygenic phototrophs but
with the difference that the source of electrons was found outside of the
cell. In this model, the Mn ions eventually moved into the cell and be-
came held in place within PSII, forming the Mn complex. In the final
stage, H
2
O became the ultimate source of electrons.
We will wrap up our discussions on the evolution of oxygenic photo-
synthesis by looking at the final stage of the process, in which electrons
are combined with CO
2
to make organic matter. This step is accom-
plished through a cycle of biochemical reactions known as the Calvin-
Bassham-Benson cycle (or more commonly, just the Calvin cycle), where
