By Sean Henahan, Access Excellence

Seattle, WA (2/14/97 Scientists for the first time have synthesized a working model of the enzyme cytochrome c oxidase, the "cellular furnace" providing the key heat and power in the chemistry of life.

Cytochrome c oxidase performs the neat chemical trick of breaking up tightly bound oxygen molecules and combining them with hydrogen atoms to make water - all within temperatures, pressures and pH compatible with life. The energy released by the enzyme is used to charge up the cell's biological batteries and to generate heat.

Stanford researchers led by James P. Collman, the George A. and Hilda M. Daubert Professor of Chemistry, have synthesized a compound that resembles cytochrome c oxidase both structurally and functionally.

The researchers built the functional model to test the current understanding of how this vital biochemical process works. The structure of the enzyme was determined only in the last year, using X-ray crystallography.

The research also provided new insights that might help make another long-term scientific dream into a reality - the direct extraction of oxygen from the air without requiring energy-intensive liquefaction.

Cytochrome c oxidase is found in mitochondria within the cells of animals, plants and aerobic bacteria. Virtually all organisms that use oxygen as an energy source rely on mitochondria for this.

The enzyme's chemically active region is made up of two hemes and two copper complexes. A heme is a ring-like chemical structure with an iron atom at its center. Hemes are also found in hemoglobin, which carries oxygen in blood, and myoglobin, which acts as an oxygen store for muscle s. Heme is the pigment that gives both blood and muscle tissue their red color.

At the active site of cytochrome c oxidase, one of the hemes and one of the copper complexes form a molecular pocket. Oxygen molecules enter this pocket, bounce around and then attach to the iron atom, and possibly to the copper atom as well.

The metal atoms have a second role as well. They act collectively as a kind of capacitor, storing up an excess charge of four extra electrons. Once the enzyme has the oxygen molecule securely in its grip, it zaps the molecule with its stored electrons. At the same time, positively charged hydrogen ions in the surrounding solution attach to the oxygen atoms. Initially, the hydrogen and oxygen combine to form some very toxic compounds, namely superoxide and hydrogen peroxide. Finally they form water molecules that are released.

"It's a good thing that the enzyme holds onto these intermediate compounds so tightly. They are very nasty actors," Collman said. "For example, Lou Gehrig's disease is associated with the failure of the system that protects the body from superoxide."

As far as anyone knows, neither peroxide nor superoxide leak out of cytochrome c oxidase. The superoxide implicated in Lou Gehrig's disease is probably a byproduct of the hemoglobin-based oxygen transport process, he said.

The net result of each oxygen-to-water conversion is to push four surplus hydrogen ions through a membrane. This creates a region of elevated positive electrical charge within the mitochondria that causes the hydrogen ions to flow back through the membrane at a different location.

The resulting ion flow provides the energy that a second piece of molecular machinery needs to transform adenosine diphosphate (ADP) into adenosine triphosphate (ATP). ATP acts as an energy source for many cellular processes. An active cell needs about two million ATP molecules per second to function.

The model compound that Collman's group synthesized recreates several of the most important fea tures of cytochrome c oxidase's active region. To get a compound that works, however, they had to replace the key iron atom with cobalt (Their model does not include the second iron atom.)

The researchers currently are developing second- and third-generation models that may function without this substitution.

The researchers let the compound soak into a graphite electrode. The electrode was then placed into a flask containing water, dissolved oxygen and electrolytic salts. An electric current serves as the source of electrons.

By spinning the electrode, the scientists can control the rate at which oxygen flows onto its surface. Their tests show that the model compound successfully converts the oxygen molecules into water.

Both the analog model and the real thing enzyme work by the four electron process. Both use two different metal centers to trap the oxygen molecules and force them to accept additional electrons. Both function at biological pH. Neither leaks hydrogen peroxide.

The work is reported in the Feb. 14, 1997 issue of the journal Science.