Hot-Blooded Proteins: Heat-Loving Enzymes Stay Cool under Stress

By Wu, Corinna | Science News, May 9, 1998 | Go to article overview

Hot-Blooded Proteins: Heat-Loving Enzymes Stay Cool under Stress


Wu, Corinna, Science News


As the saying goes, if you can't take the heat, get out of the kitchen. That ultimatum doesn't apply universally, however. Some creatures not only take the heat, they thrive in it. In recent years, scientists have discovered many such organisms--ranging from microbes to fuzzy, colorful worms--living comfortably in boiling-hot geysers or in steam vents on the ocean floor.

How do these beings keep themselves, from getting cooked? Part of the answer: lies in the structure of their proteins, which don't unravel even in temperatures approaching 100 [degrees] Celsius, which would cause most proteins to fall apart.

Scientists have been hard-pressed to figure out exactly what makes these proteins so heat-resistant. "No one really understands the molecular basis of thermal stability," says Frances H. Arnold of the California Institute of Technology in Pasadena. "People who have stared at protein sequences from nature for years will now readily admit that there are no general rules for stabilizing proteins."

Even without any rules to guide them, researchers have had great success in synthesizing thermophilic, or heat-loving proteins. They use several techniques to either change the structure of existing proteins or design new ones.

Proteins, especially enzymes, that can tolerate heat have lots of potential industrial applications because high temperatures speed reactions and enhance solubility. Such enzymes can purify waste-water, help laundry detergents work better, and aid the synthesis of drugs, for example. Heat-stable enzymes also have a long shelf life, a characteristic that can reduce the ultimate cost of a chemical process.

Enzymes that withstand the assault of high heat--or acidity, alkalinity, and salt--could improve upon many of the inorganic catalysts now used in chemical manufacturing. Those catalysts work in harsh conditions that would destroy most enzymes, but they also tend to be less specific than enzymes and therefore produce more unwanted by-products. Enzymes also do their job in water, whereas the catalysts used in many standard industrial processes require toxic organic solvents.

In trying to understand heat tolerance, some scientists have come to the conclusion that the earliest proteins worked in high heat and that only after millions of years of evolution have proteins acquired the ability to function in cooler conditions. As Arnold states in the March 3 Proceedings of the National Academy of Sciences (PNAS), "Perhaps we should be wondering why mesophilic [moderate-temperature-loving] enzymes are so unstable, rather than why thermophilic ones are so stable."

A protein is essentially a long strand of amino acids that twists and curls upon itself into a specific three-dimensional structure. The amino acids interact with each other at every turn, forming bonds and fitting neatly into the spaces created by the folds. Adding heat provides energy to break the bonds and thus unravels most proteins. For thermophilic proteins, though, even temperatures near 100 [degrees] C don't provide enough energy to break those bonds.

Scientists do have a few clues about what makes a protein thermophilic, says Bertus Van den Burg of the University of Groningen in the Netherlands. For example, the amino acid proline adds stiffness to its section of the protein chain and thus some rigidity to a protein's overall structure. Also, amino acids that contain sulfur can form disulfide bonds with each other--again, locking the chain into place. Finally, charged amino acids can attract each other, forming a bond known as a salt bridge. Nevertheless, these features don't guarantee heat resistance, and many thermophilic proteins don't possess any of them.

Using elementary ideas of this sort, Van den Burg and his colleagues recently modified an enzyme from the soil bacterium Bacillus stearothermophilus to make it hyperthermophilic. They found that changing just eight amino acids in its sequence boosted the enzyme's heat tolerance from about 86 [degrees] C to 100 [degrees] C. …

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