Enzyme helps microorganism thrive in heat.

Beneath southern Italy's sparkling Mediterranean waters dwell some bizarre life-forms. Beside thermal seafloor vents, a primitive microorganism, the bacterialike archaeon, basks amid hot springs. Percolating up from Earth's interior, hot, sulfurous water spews forth from seafloor cracks, providing a rich environment for these organisms.

These tiny creatures use sulfur the way we use oxygen and thrive in tremendous heat.

Among them one finds Pyrococcus furiosus, which enjoys temperatures around 100oC. Since discovering them, scientists have puzzled over how the microorganisms can stand the heat. What special proteins or enzymes enable them to thrive in this hostile environment? And what keeps their key biological molecules from collapsing or breaking up at temperatures that would destroy those of conventional bacteria?

Delving into this question, Michael K. Chan, a chemist at the California Institute of Technology in Pasadena, and his colleagues report their determination of the structure of one of the bacterium's unusual enzymes, aldehyde ferredoxin oxidoreductase (AOR). Playing a fundamental role in the transfer of energy within the speck-sized organism, the enzyme helps it to flourish at extreme temperatures.

Using X-ray crystallography, the researchers found that the protein consists of two big pieces joined together, each containing a cluster of iron and sulfur atoms. Each piece also has a region containing tungsten and molybdenum, the team reports in the March 10 Science.

"Since most organisms can't tolerate this kind of heat, we're interested in how life can exist at such high temperatures, as well as the problem of how proteins remain stable and function in such heat," says coauthor Douglas C. Rees, a Caltech chemist. "We also want to know exactly what role tungsten and molybdenum play in this protein. It's not yet clear why they're there or exactly how they're used."

The Caltech team has also observed some unusual features of the AOR enzyme. Compared to other proteins, it has relatively little exposed surface area and an abundance of ions and atoms buried in its core. In addition, the enzyme's tungsten and molybdenum portions appear to help the organism use carbon, nitrogen, and sulfur more efficiently than it otherwise could.

"By analyzing many of these proteins, we hope to find why they're stable at high temperatures," says Rees. "This could be useful for engineering other proteins and have a technological impact."