Biliverdin Amides Reveal Roles for Propionate Side Chains in Bilin Reductase Recognition and in Holophytochrome Assembly and Photoconversion
作者:Lixia Shang、Nathan C. Rockwell、Shelley S. Martin、J. Clark Lagarias
DOI:10.1021/bi100756x
日期:2010.7.27
Linear tetrapyrroles (bilins) perform important antioxidant and light-harvesting functions in cells from bacteria to humans. To explore the role of the propionate moieties in bilin metabolism, we report the semisynthesis of mono- and diamides of biliverdin IX alpha and those of its non-natural XIII alpha isomer. Initially, these were examined as substrates of two types of NADPH-dependent biliverdin reductase, BVR and BvdR, and of the representative ferredoxin-dependent bilin reductase, phycocyanobilin:ferredoxin oxidoreductase (PcyA). Our studies indicate that the NADPH-dependent biliverdin reductases are less accommodating to amidation of the propionic acid side chains of biliverdin IX alpha than PcyA, which does not require free carboxylic acid side chains to yield its phytobilin product, phycocyanobilin. Bilin amides were also assembled with BV-type and phytobilin-type apophytochromes, demonstrating a role for the 8-propionate in the formation of the spectroscopically native P-r dark states of these biliprotein photosensors. Neither ionizable propionate side chain proved to be essential to primary photoisomerization for both classes of phytochromes, but an unsubstituted 12-propionate was required for full photointerconversion of phytobilin-type phytochrome Cph1. Taken together, these studies provide insight into the roles of the ionizable propionate side chains in substrate discrimination by two bilin reductase families while further underscoring the mechanistic differences between the photoconversions of BV-type and phytobilin-type phytochromes.