News Release

Report: Proteins can be engineered as widely adaptable 'bioelectronic' sensors

Peer-Reviewed Publication

Duke University

DURHAM, N.C. -- Biochemists have developed a technology that will enable proteins to be engineered as sensitive, specific "bioelectronic" sensors for a vast array of chemicals. These engineered proteins, when attached to electrodes, can detect a specific chemical in a complex mixture and produce an electric signal reflecting its identity and concentration.

The researchers already have demonstrated that they can engineer proteins to detect glucose in blood serum and the sugar maltose in beer – showing that such proteins can pick specific molecules out of complex mixtures without interference or fouling from other constituents.

The scientists, led by Duke University Medical Center biochemist Homme Hellinga, reported the new approach to biosensors in the Aug. 31 issue of Science. Their research is sponsored by the Office of Naval Research and the National Institutes of Health. Besides Hellinga, other authors are David Benson, currently at Wayne State University; David Conrad, Duke; Robert de Lorimier, Duke; and Scott Trammell, now with the Naval Research Laboratory in Washington.

"Since these engineered proteins are robust and potentially miniaturizable, we believe they will provide a basis for a vast array of chemical sensors," Hellinga said. "For medical applications, you could imagine a multitude of sensors on a tiny chip that physicians could use at the patient's bedside to immediately determine from a drop of blood the concentrations of drugs, or metabolites such as glucose.

Anesthesiologists could use such biosensors to instantly measure during surgery the concentration of anesthetic or key metabolites such as epinephrine in a patient's body, rather than having to rely on the less accurate monitoring of vital signs. Thus, with these biosensors, in many cases you would no longer need expensive chemical laboratories and time-consuming clinical analysis."

Also, said Hellinga, an implantable glucose sensor would enable constant monitoring of blood glucose in people with diabetes and could also provide a long-term sensor as the basis for an artificial pancreas.

In other applications, Hellinga foresees use of the biosensors to monitor pollutants and chemical and bio-warfare agents. He emphasized the adaptability of the system. "These engineered proteins are based on proteins that bacteria use to sense their chemical environment, and since there are perhaps hundreds or thousands that exist, they provide a basis for a vast array of chemical sensors," Hellinga said. "Using powerful computational design tools that we have developed, it is possible to engineer these candidates to dramatically alter their specificity and sensitivity."

In contrast, said Hellinga, other approaches to biosensors are more complex and less robust, depending on enzymatic reactions that involve measuring the output of chemical reactions and replenishing consumed chemicals. Also, such biosensors have largely depended on natural proteins, limiting their adaptability. "Our engineered proteins can be thought of as solid-state entities that include both a biological component and an electronic component," he said

In the Science paper, Hellinga and his colleagues described how they started with natural bacterial proteins called "bacterial periplasmic binding proteins." These proteins constitute a large "superfamily" of proteins on the bacterial surface that the organisms use to sense food sources such as sugars and to avoid toxic chemicals.

Besides their broad variability, the major advantage of such proteins is that the protein's chemical-sensing active site is "allosterically" coupled to the domain that sends a signal to the bacterial metabolic pathways. Such internal signals trigger actions such as moving toward a food source. Allosteric coupling means that the two domains are separated on the protein, and thus one can be altered even drastically without affecting the other. In particular, the bacterial protein acts like a hinge, such that when a chemical plugs into the active site, the hinge closes, switching on the distant metabolic signal.

"Since hinge-bending motions are easy to understand, they are easy to manipulate," said Hellinga. In one experiment, for example, the scientists altered a bacterial maltose binding protein, tethering to it a metal ruthenium group that would produce a voltage when its conformation was altered. Thus, when they coated a gold electrode with the proteins and added maltose, that sugar altered the proteins' conformation, producing an electric current proportionate to the maltose concentration.

To demonstrate the generality of their "hinge-bending" mechanism, the scientists created additional bioelectronic sensors using other proteins that responded specifically to glucose and glutamine. And as a demonstration of the extreme adaptability of their system, they radically redesigned the maltose binding protein active site so that it acted as a zinc detector.

Finally, to show that their biosensors could function in complex mixtures without being "confused" by related chemicals or fouled by contaminants, the scientists showed that their bioelectronic sensors could specifically measure maltose concentrations in beer and glucose concentrations in human blood serum.

"We believe that these experiments in real-life mixtures dramatically demonstrated that the chemistry of these systems is very robust and specific, and does not suffer fouling by the multitude of other substances in such mixtures," Hellinga said.

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Note to editors: Homme Hellinga can be contacted at 919-681-5885, e-mail hwh@biochem.duke.edu. A photo of Hellinga will be available Wednesday at http://photo1.dukenews.duke.edu/pages/Duke_News_Service/hellinga.jpg.


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