The structure of an AtFAP protein, superimposed on a cultured Arabidopsis thaliana seedling.
Image courtesy of Gordon Louie

Following CHI to Its Perfect Form

Joe Noel calls chalcone isomerase, or CHI, the “perfect enzyme” because of its extreme speed.

Noel, an HHMI investigator at the Salk Institute, wanted to know how CHI evolved to catalytic perfection. He discovered that CHI likely evolved from a noncatalytic protein that played a completely different role in plants, and that CHI and its noncatalytic relatives coexist today.

CHI is crucial to production of flavonoids, which customize plants to varied terrestrial environments. Flavonoids provide UV sunscreens, for example, and colors that lure pollinators.

 Noel and colleagues combed through the genome of Arabidopsis and found that CHI shares genetic similarities with three members of the FAP protein family.

To learn how the two protein families were related, the scientists introduced FAP genes into E. coli bacteria, and then studied the expressed FAP proteins with x-ray crystallography. Unexpectedly, the researchers saw pockets in FAPs that held tightly to fatty-acid molecules from the bacteria. In a similar way, CHI grabs chalcone to turn it into plant flavonoid.

Tracking FAP proteins in Arabidopsis, the team found that they play a role in fatty acid biosynthesis and regulation in chloroplasts. Even algae, the modern-day example of early plant ancestors, rely on FAPs. Based on these findings, Noel posits that FAPs are the immediate precursor to CHI. The work was published May 13, 2012, in Nature.

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