Explaining Nature's Catalysts

Enzymes are aptly called the mediators of life. Without these essential proteins, vital chemical reactions would occur far too slowly, if at all. Living things could not grow, digest food, store energy, transmit messages across nerve cells or reproduce. Like laboratory or industrial catalysts, enzymes trigger and speed up chemical reactions without themselves being affected or altered by them. But enzymes can cause these reactions to take place up to a billion times faster than catalysts used in the laboratory or chemical plant.

Active Sites. Just how enzymes work their magic--without the aid of the high temperatures and pressures used in industry--has long been a subject of intense scientific study. The mystery may at last be closer to solution. Last week, at an international chemistry conference in Riga, the capital of Soviet Latvia. Biochemist Daniel Koshland Jr. of the University of California reported that he may have identified the elusive mechanism used by enzymes in chemical reactions. He also revealed hard experimental evidence to back his ideas.

Most scientists agree that the 1,000 or so known enzymes owe their prowess to their so-called "active sites," small areas that apparently latch on to specific molecules and guide them together to produce a chemical reaction. But this explanation fails to account for the remarkable speedup that the enzyme contributes to the process.

Koshland's theory seems to provide the answer to the enigma. The reason that enzymes are so effective, he suggests, is that they hold a molecule's constituent atoms at the proper orientation for joining. By this "orbital steering," he explains, enzymes align the outermost electrons spinning around each atom so that they can readily be shared with other atoms. Such electron sharing is at the heart of all chemical reactions.

Holder Molecules. To confirm his hypothesis, Koshland and two young Berkeley colleagues--Dan R. Storm and Kenneth Neet--devised an experiment of classical simplicity. It involved the common chemicals ethyl alcohol and acetic acid, which, in the presence of a catalyst, quickly combine to form an aromatic compound called ethyl acetate (without a catalyst, the reaction occurs very slowly). Instead of using standard catalysts, however, the scientists chemically locked both molecules into various synthetic ring-shaped, larger molecules called "holders," which are in effect chemical vises. After many attempts, they hit upon a holder that gripped the ethyl alcohol and acetic acid so that they were properly oriented for combination; thus when the molecules met, their reacting oxygen and carbon atoms were always at the correct angle for electron snaring--and the reaction took place a million times faster. By "steering" the individual atoms, Koshland concluded, the holder molecule had apparently duplicated the exact role of an enzyme.

Koshland points out that his orbital-steering theory needs more research. He must still find out, for instance, whether such atomic aligning plays a part in all reactions requiring enzymes or other catalysts. But once chemists fully understand enzymatic action, they may be able to produce far more efficient catalysts for industry. Koshland's work could also have a direct effect on man. It might some day lead to improved treatment for diseases that involve enzyme abnormalities, including cystic fibrosis and muscular dystrophy.

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