New Worlds: Solving a biological mystery
How do organisms living with arsenate pick and choose the right substance?
Scientist at work (illustrative) Photo: Marretao22/Wikimedia Commons
Some unassuming bacteria found themselves recently at the center of a scientific
controversy: A group claimed that these microorganisms, which live in an
environment rich in the arsenic-based compound arsenate, could take up that
arsenate and use it – instead of the phosphate on which all known life on Earth
depends. The claim, since disproved, raised another question: How do organisms
living with arsenate pick and choose the right substance?
is nearly indistinguishable from phosphate. Prof. Dan Tawfik of the Weizmann
Institute of Science’s department of biological chemistry says, “Phosphate forms
highly stable bonds in DNA and other key biological compounds, while bonds to
arsenate are quickly broken. But how does a microorganism surrounded by arsenate
distinguish between two molecules that are almost the same size and have
identical shapes and ionic properties?”
To investigate, Tawfik – along with
postdoctoral fellow Dr. Mikael Elias, doctoral student Alon Wellner and lab
assistant Korina Goldin, in collaboration with Tobias Erb and Julia Vorholt of
ETH Zurich – looked at a protein in bacteria that takes up phosphate. This
protein, called PBP (phosphate-binding protein), sits near the bacteria’s outer
membrane, where it latches onto phosphates and passes them on to pumps that
transport them into the cell.
In research that recently appeared in
Nature, the team compared the activity of several different PBPs – some from
bacteria like E. coli that are sensitive to arsenate and others, like those from
the arsenic-rich environment, which are tolerant of the chemical.
the PBPs in the ordinary bacterium were about 500 times more likely to bind
phosphate over arsenate, in the arsenic-tolerant bacterium that factor jumped to
around 5,000. In other words, to cope with their toxic environment, the bacteria
evolved a mechanism of extreme selectivity to ensure their supply of phosphate
while keeping the arsenate out.
Elias then compared phosphate and
arsenate binding by crystallizing PBPs along with one of the two compounds. But
the initial comparison suggested that when arsenate bound to the protein, it did
so in just the same way as phosphate. Elias suspected that the key might lie in
a single, highly unusual bond between a hydrogen atom in the protein and the
This bond had been previously noted but ignored, as phosphate
binding occurred with or without it.
To see the difference, the team had
to stretch the limits of crystallization technology, getting the resolution to
less than one angstrom – fine enough to identify individual hydrogen atoms and
compare their bonds. Only then were they able to identify a single disparity:
The angles of that unusual hydrogen bond were different.
Inside a tight
cavity within the PBP structure, phosphate binds at a “textbook angle,”
according to Elias. The slightly larger arsenate molecule, on the other hand,
gets pushed up against the hydrogen and bonds at unnatural, distorted angles.
Tawfik thinks that the angle is likely to lead to repulsion between the molecule
and other atoms in the cavity, preventing the PBP from passing arsenate into the
Tawfik concluded: “These findings may go beyond the
solving of a biological mystery.
Because phosphates are scarce in many
environments, there is quite a bit of interest in understanding how this crucial
resource is taken up by organisms. This first observation of a PBP
discrimination mechanism is an exciting demonstration of the exquisite fine
tuning that enables proteins to distinguish between two nearly identical
GENDER AND NEWS
Women who read negative news remember it
better than men do and have stronger stress responses in subsequent stress
tests, according to new research published recently in the journal PLOS
Scientists from the University of Montreal exposed groups of men and
women to a succession of headlines drawn from recent newspaper articles. One
group viewed only “neutral” news, while the other group was shown news perceived
as “negative.” After reading the news, participants performed a standard
psychological stress test. Researchers monitored participants’ stress levels
during the process by measuring levels of cortisol, a stress hormone, in the
The authors found that, though reading negative news did not
increase stress for any of their subjects, women exposed to negative news had
higher cortisol levels after the psychological stress test exposure than did men
who saw the same negative news, and also higher than either men or women who saw
“neutral” news. In addition, one day later the women were more likely than the
men to remember and experience emotional responses to the negative news they had
viewed the previous day, according to the results of this study.
previous studies have assessed the role of continuous exposure to mass media as
a stress factor, this is the first research that questions the effects of
exposure to negative news coverage on stress reactivity and later recall of the
news. The results of this study suggest that gender differences underlying the
processes of stress and memory may play a role in how we react to negative news
in the media.