Researchers explain how enzymes use quantum tunneling to speed up reactions.

The bizarre, unpredictable world of quantum mechanics would appear unlikely to govern everyday biological processes. However, enzymes—protein catalysts that allow chemical reactions to take place millions of times faster than their normal rate—use a phenomenon called quantum tunneling to transfer protons or electrons to or from a reactant. Until now, nobody knew just how they did it.

An interdisciplinary group of UK researchers from the University of Manchester and the University of Bristol examined a single step of a reaction where an enzyme, aromatic amine dehydrogenase, extracts a proton from a substrate called tryptamine, a natural compound related to the neurotransmitter serotonin. The researchers created a computer model of the enzyme and simulated the process. They found that, contrary to what was previously believed, it is not long-range motions of the enzyme, but rather motions close to the substrate, that promote tunneling.

“Our present understanding of the physical basis of enzyme catalysis is still unable to explain the many orders of magnitude by which a reaction is ‘speeded up’ by enzymes, nor why attempts to create artificial enzymes have so far been disappointing,” said study co-author David Leys of the University of Manchester via e-mail. “Our work reveals that not only active site structure, but also motions are an essential part of the enzyme’s repertoire.”

This new discovery, announced in the April 14 issue of Science could have serious implications for medicine, as it may allow scientists to develop drugs that can target or mimic enzymes.

Chemical reactions often face a problem: An energy barrier stands between the reactants and products. In the case of this simple reaction, a proton needs to gain enough energy to leave the reactant molecule in order for the reaction to occur. Quantum tunneling allows the reaction to proceed through the energy barrier, rather than having to climb it.

“While classical theory states that enzymes speed up the reaction by lowering the energy barrier, quantum tunneling allows the reaction to occur by tunneling through the barrier,” Leys said. “As such, the reaction can occur at greater speeds than if the particle would have to reach energies high enough to surmount the barrier.”

According to study co-author Adrian Mulholland, from the University of Bristol, researchers have previously shown that quantum tunneling accounts for proton transfer using deuterium—a form of hydrogen with both a proton and a neutron in its nucleus, instead of standard hydrogen, which has only a proton in its nucleus. When deuterium is involved, the reaction slows down drastically, because tunneling is much less likely with the larger, two-particle nucleus that more closely approximates a classical particle.

The experiment shows that enzymes move a few of their atoms that reside very near the substrate, and as these get closer to the substrate molecule, the possibility of proton tunneling increases.

“Relatively subtle, short-range motions at the active site affect the crucial distance between the groups between which the proton is exchanged, Mulholland said via e-mail, “And so, [they] can promote, or drive, tunneling.”

Originally published April 17, 2006

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