Abstract
Free chlorophylls should be degraded immediately to detoxify their phototoxicity. In the chlorophyll degradation pathway, RCCR catalyzes the reduction of the C20/C1 double bond of RCC to produce pFCC. Recently we determined the crystal structure of Arabidopsis thaliana RCCR, in which RCCR subunit folds in a characteristic α/β/α sandwich observed in FDBR family.
Here we have determined the crystal structures of RCC-bound RCCR and its F218V mutant, by which the stereoisomer of pFCC at the C1 position is produced. These structures demonstrate that RCC is bound on the pocket between the β-sheet and the C-terminal α-helices of RCCR. The substrate binding mode is similar to that of FDBR, but the substrate appears to loosely bind on RCCR. It is reasonable because the reaction intermediate of RCCR is estimated to be more bulky than RCC. Glu-154 and Asp-291 are nearby the reduction site of RCC, suggesting that these residues function as acid catalysts. Structural comparison between wild-type RCCR and F218V mutant suggests that the catalyst attacking the RCC C1 position is different between them, reflecting the chirality of pFCC produced.
