Scientists Describe Structure Of An Enzyme That Uses Iron To Make Hydrogen

With funding from the National Science Foundation (NSF), biochemists John Peters and Lance Seefeldt have made a detailed description of the structure of an enzyme known as CpI, found in the soil microorganism Clostridium pasteurianum. The results of their research on the structure and function of CpI are published in the December 4 issue of the journal Science.

CpI is a hydrogenase, a type of enzyme used by microorganisms to make molecular hydrogen (H2), or through reversing the reaction, break down H2 into protons and electrons. Peters and Seefeldt's study suggests a mechanism by which CpI uses atoms of iron as a means to catalyze the production of H2.

"The iron-only hydrogenase is basically a means to get rid of unwanted electrons," said Kamal Shukla, NSF program manager. "Clostridium pasteurianum uses CpI to convert protons and electrons into (H2), a waste product."

What is a waste product to some organisms might be an incredibly useful product for others. According to Peters, a better understanding of this enzymatic process interests not only biologists and biochemists, but also researchers of alternative energy sources.

"Hydrogen is often mentioned as a future fuel source because it is a renewable and clean-burning energy carrier," said Peters. "The biological production of hydrogen, then, represents a tremendous reserve of energy that we may tap through our understanding of the mechanisms that have evolved in nature."

Attaining the three-dimensional structure of enzymes like CpI may be the first step in tapping into that resource. Peters and Seefeldt depict CpI as a collection of 20 iron atoms arranged in clusters around a mushroom shaped framework. Electrons move in through the `stem' of the mushroom in a series of reactions between the iron clusters that pass electrons, like a molecular bucket brigade, towards the `cap' of the mushroom. The `cap' contains the active site of the enzyme, where the final reaction takes place.

At the active site, more clusters of iron atoms introduce electrons, two at a time, to two protons stripped from a single molecule of water. As newly formed molecules of hydrogen leave the enzyme, they make room for more electrons and protons to take their spot, providing the energy for the next reaction to take place.

"Hopefully, through knowing the structure of the iron-only hydrogenase, protein engineers can work on methods to increase the stability of the enzyme," said Peters. "Once in industrial use, such an efficient source of clean energy is likely to be both economically and environmentally significant."