By David N. Leff
FT. LAUDERDALE, Fla. * An enzyme called Cytochrome P450 has the job of detoxifying compounds in the human body.
Biochemist Gregory Petsko calls P450 "a biological blow torch, because it carries out a chemical reaction that we chemists only know how to carry out in the lab at very high temperatures, by burning things in oxygen."
"Cytochrome P450," he added, "can use body heat and oxygen to do the same chemistry in water. That seems almost like magic to us."
Petsko, who directs basic medical science research at Brandeis University, in Waltham, Mass., reported to the symposium session on catalysis here Tuesday morning on "Structural enzymology in four dimensions."
"What we tried to do," he told BioWorld Today, "was to follow that enzyme reaction in time, in the fourth dimension. As we looked in time at this reaction, we obtained three dimensional structures along the reaction pathway of the significant enzyme intermediates. And those structures teach us how CP450 is able to do its unique chemistry that we can't do, except by burning things at 700, 900 and 1,000 degrees."
Petsko compared this dichotomy to soil bacteria, which fix nitrogen on plant roots at ambient temperatures, whereas industrial ammonia plants require elevated temperatures, pressure and costs to fix atmospheric nitrogen. "If we understood that process in chemical detail," he said, "maybe we could too."
Petsko pointed out that "Most plastics, such as styrene, for example, involve Cytochrome P450-like reactions. There are so many reactions that we do in a very complicated, expensive way that P450s do easily and quickly."
The presentation here covered only Petsko's unpublished work.
"Nobody," he told BioWorld Today, "had ever been able to look at the structures of these enzyme intermediates before. Nobody even knew for sure that they were there. Even the model compound studies didn't tell you what they looked like. We now know all of those things."
Biochemist Ilm Schlichting, a former post-doc fellow in Petsko's lab is now a group leader at the Max Planck Institute for Molecular Physiology, in Dortmund, Germany. She described her research on "medically relevant enzymes, such as thymidylic kinase, which phosphorylates AZT.
"When you put AZT, which is a pro-drug, into a cell," Schlichting explained, "then it gets phosphorylated by several enzymes. Then the monophosphate forms, and that's no problem. But then the monophosphate form becomes phosphorylated by thymidinate phosphate * and that is the bottleneck, because this ATP monophosphate accumulates in the cell and is toxic to it.
"We found out recently," she said, "that it's not the problem of binding, but it's the catalysis. So by really knowing the mechanism of this reaction, that's the only way to tackle this ATP monophosphate phosphorylation step."
"AZT goes in and kills viruses that have infected cells. But it can't do that until it's converted to the proper drug by cellular enzymes.
"So to make a better drug, less toxic than AZT, one that can be used at a lower dose, we must understand how the cellular enzymes process molecules like AZT. So this is the first step toward that."
Petsko described Schlichting's contribution to the catalysis field "as determination of the correct reaction mechanism, and the correct transition-state structure, for, in fact, a whole class of enzymes. This now provides the blueprint for going in and making transition-state analog products and improving other drugs.
"But," he added, "there's something else that it does. Many drugs are in fact transitions to analogs. And some of the enzymes you might want to inhibit for killing cancer cells would be enzymes like this; that are involved in controlling the level of nucleotides that the cell needs to make DNA.
"But you can't make the transit ion from substrate to product, unless you know what the transition state is. The point is," Petsko said, "you can make analogs * molecules that look like the transition state * that have the same stereo chemistry, and charge distribution. Those things, if they really do look like the transition state, should be terrific inhibitors. They should be terrific drugs. They should bind tightly and should be specific." *