How Bilin Gets Made♦

♦ See referenced article, J. Biol. Chem. 2010, 285, 1000–1007

Bilins are biological pigments that are used in light-related reactions in plants, algae, and cyanobacteria. These tetrapyrrole molecules are created from the precursor biliverdin IXα (BV) by a class of enzymes known as ferredoxin-dependent bilin reductases (FDBRs). Phycocyanobilin:ferredoxin oxidoreductase (PcyA) is the best characterized FDBR member, and unlike other FDBRs that catalyze a two-electron reduction, PcyA sequentially reduces the D-ring exo- and A-ring endo-vinyl groups of BV to yield its product, phycocyanobilin. To better understand the catalytic mechanisms of PcyA, Yoshinori Hagiwara and colleagues developed three high resolution crystal structures of PcyA-bilin complexes that lack exo-vinyl reduction activity; these include a bilin-bound PcyA E76Q mutant as well as wild-type PcyA in complex with either the reaction intermediate 18EtBV or biliverdin XIIIα (BV13), a synthetic substrate that lacks an exo-vinyl group. Although the overall folds and substrate binding sites of all three complexes were similar to the wild-type enzyme-substrate complex, the orientation of the Glu-76 side chain was rotated farther away from the bilin D-ring than wild-type. In contrast, the local structure around the A-ring, whose reduction is not affected, was nearly identical in all cases. These studies provide strong support for the catalytic role of Glu-76 in exo-vinyl reduction and also reveal insights into the regiospecificity of PcyA in bilin reduction.

Close-up views of the PcyA-BV (A) and PcyA-18EtBV (B) complexes, highlighting the conformational change to Glu-76 following D-ring reduction. (The Glu-76 of A is superimposed in gray in B.)