University of Illinois crop sciences and Institute for Genomic Biology professor Gustavo Caetano-Anollés and his colleagues identified an oxygen-generating enzyme that likely was a key contributor to the rise of molecular oxygen on earth. Photo by L. Brian Stauffer
turning point in the history of life occurred 2 billion to 3 billion
years ago with the unprecedented appearance and dramatic rise of
molecular oxygen. Now researchers report they have identified an enzyme
that was the first—or among the first—to generate molecular oxygen
new findings, reported in the journal Structure, build on more than a
dozen previous studies that aim to track the molecular evolution of life
by looking for evidence of that history in present-day protein
structures. These studies, led by University of Illinois crop sciences
and Institute for Genomic Biology professor Gustavo Caetano-Anollés,
focus on structurally and functionally distinct regions of
proteins—called folds—that are part of the universal toolkit of living
folds are much more stable than the sequences of amino acids that
compose them, Caetano-Anollés said. Mutations or other changes in
sequence often occur without disrupting fold structure or function. This
makes folds much more reliable markers of long-term evolutionary
patterns, he said.
the new study, Caetano-Anollés, working with colleagues in China and
Korea, tackled an ancient mystery: Why did some of the earliest
organisms begin to generate oxygen, and why?
is a consensus from earth scientists that about 2.4 billion years ago
there was a big spike in oxygen on Earth,” Caetano-Anollés said. They
generally agree that this rise in oxygen, called the Great Oxygenation
Event, was tied to the emergence of photosynthetic organisms.
the problem now comes with the following question,” he said. “Oxygen is
toxic, so why would a living organism generate oxygen? Something must
have triggered this.”
researchers looked for answers in the “molecular fossils” that still
reside in living cells. They analyzed protein folds in nearly a thousand
organisms representing every domain of life to assemble a timeline of
protein history. Their timeline for this study was limited to
single-fold proteins (which the researchers believe are the most
ancient), and was calibrated using microbial fossils that appeared in
the geologic record at specific dates.
analysis revealed that the most ancient reaction of aerobic metabolism
involved synthesis of pyridoxal (the active form of vitamin B6, which is
essential to the activity of many protein enzymes) and occurred about
2.9 billion years ago. An oxygen-generating enzyme, manganese catalase,
appeared at the same time.
recent studies also suggest that aerobic (oxygen-based) respiration
began on Earth 300 to 400 million years before the Great Oxidation
Event, Caetano-Anollés said. This would make sense, since oxygen
production was probably going on for a while before the spike in oxygen
convert hydrogen peroxide to water and oxygen. The researchers
hypothesize that primordial organisms “discovered” this enzyme when
trying to cope with an abundance of hydrogen peroxide in the
environment. Some geochemists believe that hydrogen peroxide was
abundant at this time as a result of intensive solar radiation on
glaciers that covered much of Earth.
the glacial melt waters you would have a high concentration of hydrogen
peroxide and that would be gradually exposing a number of the primitive
organisms (alive at that time),” Caetano-Anollés said. The appearance
of manganese catalase, an enzyme that degrades hydrogen peroxide and
generates oxygen as a byproduct, makes it a likely “molecular culprit
for the rise of oxygen on the planet,” he said.
research team included scientists from the Korea Research Institute of
Bioscience and Biotechnology; Huazhong Agricultural University, China;
and Shandong University of Technology, China.