The biochemical world's workaholic is the enzyme. Enzymes are molecules in cells that lead short, active and brutal lives. They restlessly catalyze their neighbors, cleaving and assembling proteins and metabolizing compounds. After a few hours of furious activity, they are what chemists call "destabilized," or spent.
This sad fact of nature, said Pacific Northwest National Laboratory Fellow Jay Grate, has limited the possibilities of harnessing enzymes as catalytic tools outside the cell, in uses that range from biosensing to toxic waste cleanup.
But Grate and PNNL Senior Scientist Jungbae Kim have corrected for the enzyme's quick-burn proclivity. Their idea for creating a long-lived and active "Methuselah enzyme" has been to build a barrier between the enzyme and its certain degradation in a harsh environment. The trick, they have found, is to protectively "cage" the enzyme, while leaving the active region--the part that actually does the catalyzing--accessible for chemical reactions of interest.
And it works, as Kim recently reported to the national meeting of the American Chemical Society. They call their caged enzyme SEN, a single-enzyme nanoparticle, where the "cage" is just a few nanometers thick. The SEN's cage is ingeniously synthesized on the surface of the enzyme molecule using organic and inorganic materials that Kim and Grate add to the mix. The organic-inorganic composite protects the catalyst, enabling it to remain active for months instead of hours.
"Converting free enzymes into these novel enzyme-containing nanoparticles can result in significantly more stable catalytic activity," Grate said.
The duo, working in the Environmental Molecular Sciences Laboratory (EMSL), experimented with a common protein-splitting enzyme called alpha-chymotrypsin. To the enzyme's surface they affixed vinyl groups, which formed anchors for an outgrowth of polymer threads from the enzyme. A second polymerization step cross-linked silicate chains, forming a basketball-netlike structure a few nanometers thick.
SENs appear in electron microscopic images as hollow enzyme-containing blobs about eight nanometers across. Kim and Grate found that by using less reactive forms of vinyl they could vary the nano-netting's thickness by half. Thick or thin, the porous netting preserves the enzyme's shape and activity. SENs also are amenable to storage; they have been refrigerated for five months, losing little of their activity.
Among the uses they note for SENs is to break down toxic waste, in which a single treatment could last months. Stabilized enzymes also are a prerequisite for many types of biosensors. And they may be of interest for coating surfaces, with applications ranging from medicine (protecting implants from protein plaques) to shipping (keeping barnacles off hulls). PNNL is investigating several other applications in the environmental and life sciences.
While the PNNL team has thus far caged a single enzyme, Kim said, "The principal concept can be used with many water-soluble enzymes."
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