In the 1960s and 1970s, scientists observed that plants used one pathway, known as arogenate dehydrogenase (ADH), to make tyrosine. They found that the legume family (i.e., peas, beans, and peanuts), however, had uniquely added a second, called prephenate dehydrogenase (PDH), previously known to be only in microbes. Two years ago, part of this team discovered the genes responsible for making tyrosine, learning that before peanuts and peas evolved into separate lineages, the legumes had evolved PDH enzymes from their existing ADH ones. These sister enzymes are very similar, so only a small number of changes could account for how ADH enzymes evolved into PDH ones. But there were still too many changes to test. The international collaborating teams purified the PDH enzyme of the soybean legume and determined its three-dimensional (3D) structure, which revealed that only a couple of mutations had occurred at the site where the chemical reactions take place. Instead of dozens of possible mutations, there were only two. Thus, changing a single amino acid in the center of the enzyme largely converted the soybean PDH enzyme back into its ancestor ADH enzyme, and this crucial switch also worked in reverse and for enzymes from multiple species. The scientists believe the legume PDH insensitivity to tyrosine could help to produce more tyrosine, and its useful derivatives, in systems like yeast or engineered plants.
Schenck, C. A., et al. “Molecular Basis of the Evolution of Alternative Tyrosine Biosynthetic Routes in Plants,” Nat. Chem. Biol. 13, 1029–1035 (2017). [DOI:10.1038/nchembio.2414].
Instruments and Facilities Used: Structural Biology Center (SBC)–CAT beam 19-ID of the Advanced Photon Source at Argonne National Laboratory.