CHAMPAIGN,
Ill. – New insight into how nature handles some fundamental processes
is guiding researchers in the design of tailor-made proteins for
applications such as artificial photosynthetic centers, long-range
electron transfers, and fuel-cell catalysts for energy conversion.
Tuning redox potentials of a protein for energy conversions. A combination of water-repelling hydrophobicity (shown in red sphere) and hydrogen bonding interactions (shown in dotted orange lines) can fine-tune the redox potential of copper ion (shown in blue) in azurin in a wide range. | Graphic courtes Yi Lu
From
rusting iron to forest fires to the beating of a human heart,
oxidation-reduction reactions, which transfer electrons from one atom
to another, are at the heart of many chemical and biological processes.
Each process requires a particular redox potential, just as different
electronic devices can require their own special battery.
How nature fine-tunes these potentials over a broad range with little
change to the protein’s electron-transfer properties or efficiency has
largely remained a mystery.
Now, a team led by University of Illinois chemistry
professor Yi Lu has unearthed nature’s secret, and has utilized it to
their advantage. The researchers describe their work in a paper to
appear in the Nov. 5 issue of the journal Nature.
“We show that two important interactions, hydrophobicity (water
repelling) and hydrogen bonding, are capable of fine-tuning the
reduction potential of a particular class of copper-containing proteins
called cupredoxins,” Lu said. “We extended the range both above and
below what had previously been found in nature.”
Lu,
graduate student and lead author Nicholas M. Marshall, and their
collaborators also show that the effects of hydrophobicity and hydrogen
bonding are additive, which offers additional control and extends the
range of redox (short for oxidation-reduction reaction) potentials
beyond what nature, by itself, provides.
Previously, to cover a wide potential range, scientists had to use
several different redox agents in conjunction. This made it difficult,
if not impossible, to tune the redox potentials without changing other
electron transfer properties or the efficiency.
Also, stable, water-soluble redox agents are rare, Lu said, and those
that are available have a limited potential range. “Consequently, there
is a huge demand for efficient, water-soluble redox agents with a wide
potential range for environmentally friendly aqueous or biochemical
studies,” he said.
To unlock nature’s secret, Lu’s team studied the behavior of the
cupredoxin, azurin. Cupredoxins are redox-active copper proteins that
play crucial roles in many important processes, such as photosynthesis
and cell signaling. Cupredoxins use a single redox-active center, whose
reduction potential is tunable without compromising the structure and
electron transfer properties of the protein.
The researchers found that two interactions – hydrophobicity and
hydrogen bonding – can selectively raise or lower azurin’s redox
potential. The interactions occur not in the metalloprotein’s
innermost, primary core, but in a secondary sphere that surrounds the
primary core.
Increasing the hydrophobicity in the secondary sphere can significantly
increase the redox potential, the researchers report. The more this
secondary region repels water, the more the overall charge on the
copper ion becomes destabilized and the higher the potential becomes.
The
effect of the hydrogen bonding interaction is subtler than the effect
of hydrophobicity, Lu said. Hydrogen bonding can either increase or
decrease electron densities around a residue that binds the copper ion
in azurin, making the copper ion either easier or harder to reduce and
thus slightly changing the redox potential.
“This was nature’s secret,” Lu said. “That by adjusting the
hydrophobicity and the hydrogen bonding, we can raise or lower the
redox potential, without changing the protein’s electron-transfer
properties or decreasing the protein’s efficiency.”
The result is a tailor-made redox agent that can be set with a very
high potential, a very low potential, or with a potential somewhere in
between.
“This unprecedented level of control over an electron-transfer protein
was achieved by mapping out the major interactions,” Lu said, “an
approach that may apply to other redox proteins of interest, as well.”
Lu is affiliated with the university’s Beckman Institute, the departments of biochemistry, of bioengineering, and of materials science and engineering, the Frederick Seitz Materials Research Laboratory, and the Center of Biophysics and Computational Biology. The National Science Foundation and the National Institutes of Health funded the work.
