Los Alamos National Laboratory

Clean Air and Abundant Fuel

To reduce carbon emissions from the exhaust stacks of coal- and gas-fired power plants, we might enlist the help of an enzyme found within our blood. The enzyme, called carbonic anhydrase, combines carbon dioxide with water to form bicarbonate ions and protons. If it were used to perform that same chemistry in a power plant's exhaust stream, the resulting bicarbonate could be converted into calcium carbonate, a solid that could be safely stored away or used in making everyday materials like plastic or cement.

The same carbon conversion reaction could also make carbonic anhydrase valuable for growing algae capable of making oil for fuel. The algae-to-fuel conversion process has already been solved, but growing the algae is prohibitively time consuming. However, these algae take up bicarbonate directly, which means that if we could add carbonic anhydrase to algae ponds, the enzyme would rapidly convert carbon dioxide from the atmosphere into bicarbonate—feeding the algae and greatly accelerating their growth.

Los Alamos bioenergy scientist Zoë Fisher is working to design a high-performance version of the enzyme for both settings: exhaust stacks and algae ponds.

To do that, she needs a clear picture of the molecule she's working with. Carbonic anhydrase is a single molecule made up of more than 3000 atoms, but the conversion from carbon dioxide to bicarbonate takes place within just a small pocket of the structure known as the "active site." Inside the pocket, a zinc atom is bound to hydroxide, making ZnOH–. If a carbon dioxide molecule (CO2) ventures too close to the pocket, the hydroxide "attacks" it to form ZnHCO3–, zinc bound to a bicarbonate ion. Then a water molecule from the surrounding fluid displaces the bicarbonate ion (HCO3–), leaving ZnH2O in the active site and setting the bicarbonate ion adrift, to be stored as calcium carbonate or fed to algae.

Picture of study area.

Carbonic anhydrase, as revealed here by Fisher's neutron crystallography experiment, is a single molecule made up of more than 3000 atoms (green "ball" at right). Its active site (exploded view) contains six amino acids (yellow molecules) that force six water molecules (red and white "elbows") to assume the shape of the "water wire" shown. Improving the way the water wire generates electrical forces to push protons out of the active site could lead to important applications in the energy industry.

But in order to convert another CO2 molecule, the active site must be reset. That is, the ZnH2O must be changed back to ZnOH– through the loss of a proton (H+). The speed of the reset step is known to limit the rate at which the entire enzymatic reaction can occur, but many details about this step—especially how the proton is transferred away—have only recently come to light, thanks largely to Fisher's successful application of neutron crystallography to visualize the active site.

Neutron crystallography is a specialized technique for making an image of a complex molecule. Unlike the more conventional x-ray crystallography technique, neutron crystallography allows you to see hydrogen, the smallest atom. The exact location of the hydrogen atoms turns out to be critical because in carbonic anhydrase, they appear in a row of six water molecules (H2O) that are held in a precise arrangement by the amino acids found in the enzyme's active site. It is this "water wire" that is responsible for resetting the enzyme by channeling the unwanted proton down the "wire" and away from the active site. Because water molecules are polar—positively charged at one end and negatively charged at the other end—their exact orientation determines the arrangement of electrical forces that steer the positively charged proton away.

Fisher and collaborators at the University of Florida were able to use the Protein Crystallography Station at the Los Alamos Neutron Science Center—currently the only instrument capable of revealing the exact positions of hydrogen atoms in enzymes—to see the structure of the active site, including the water wire (see figure below). With this information in hand, Fisher can redesign carbonic anhydrase by swapping out amino acids in the active site, thus changing the orientation of the water molecules in the water wire. The resulting alteration in electrical forces should make for a more effective proton transport channel.

The enzyme will also need to be tailored to each specific application. To function in a power plant's exhaust stack, the enzyme must be able to withstand high temperatures. This stability can be achieved by redesigning the structural "scaffold" of the enzyme (the green "ball" on the right in the figure), rather than the functional active site. Fisher's colleague Csaba Kiss from Advanced Measurement Science has developed a unique approach to do just that. It could take two or three years, but Fisher and Kiss are extremely confident that it will work.

To serve as a growth accelerant for algae, the enzyme will have to operate at a higher pH range (acid-base scale) than it experiences in its natural environment—human blood—which is always very close to pH 7.4 (neutral is pH 7). Algae, on the other hand, tend to push the surrounding water to pH 9 or above, where carbonic anhydrase does not work very well. (Nor should it have to: as Fisher points out, "If your blood were this basic, you'd be dead for sure.") To overcome this limitation, she'll need to select different amino acids for the active site, to make the water wire better suited to the higher pH.

If these efforts are successful, and if reasonable economies of scale are achieved, we could see substantial advances in clean electricity and home-grown fuel production in just a few years. And while you might expect that this kind of near-term energy security doesn't come cheap, consider this: the new algae-based fuel could sell for as little as a dollar per gallon.

Craig Tyler
(contributions from Brian Fishbine)

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