Anticancer enzyme imaged.

A computer image reveals that this liver enzyme's 434 amino acids arrange in twin sections. Weak interactions help hold the two sections together. In each section, the amino-acid chain spirals (blue barrels) and zigzags (yellow stripes), forming two docking sites: one for a detoxifying molecule called glutathione (pink) and one for a toxin (not shown). A side chain (red) of one amino acid, tyrosine, chemically alters the glutathione in such a way that the sulfur in the glutathione reacts and links with the toxin to disarm it, says biochemist Richard N. Armstrong of the University of Maryland in College Park.

Each of the dozen or so glutathione S-transferases, as these enzymes are called, has evolved a slightly different amino acid sequence at its toxin docking site, which alters the site's shape slightly, says Armstrong. Thus, the enzymes provide protection against a wide variety of cancer-causing substances that enter the body.

Armstrong worked with researchers at the Center for Advanced Research in Biotechnology in Rockville, Md., to obtain the first three-dimensional picture of this type of enzyme. They describe the X-ray diffraction studies, computer analysis, and the resulting high-resolution structure of the enzyme in the Oct. 27 Biochemistry.