Biosynthesis of phycobilins. 15,16-Dihydrobiliverdin IX alpha is a partially reduced intermediate in the formation of phycobilins from biliverdin IX alpha.

S.I Beale,J Cornejo

doi:10.1016/s0021-9258(18)54577-4

Abstract

A partially purified protein fraction from the phycocyanin-containing unicellular rhodophyte, Cyanidium caldarium, reductively transforms biliverdin IX alpha to a violet colored bilin in the presence of NADPH, ferredoxin, and ferredoxin-NADP+ reductase. This bilin has a violin-like absorption spectrum with maxima at 335 and 560 nm in methanolic HCl and at 337, 567, and 603-604 nm in CHCl3. The bilin has been determined to be 15,16-dihydrobiliverdin IX alpha by comparative spectrophotometry and 1H NMR spectroscopy. This product of biliverdin IX alpha reduction is converted enzymatically to phycobilins by further reduction. A general biosynthetic pathway is proposed which accounts for the formation of the phycobilins from biliverdin IX alpha by a two-step reduction process followed by isomerization.

Highlights

A partially purified protein fraction from the phycocyanin-containing unicellular rhodophyte, Cyanidium caldariumr,eductively transformsbiliverdin I X a to a violet colored bilin in the presence of NADPH, ferredoxin, and ferredoxin-NADP+ reductase
Biosynthesis of the open-chain tetrapyrrole chromophores NADPH and partially reconstituteednzyme systems containof phycobiliproteins proceeds by conversion of protoheme to ing protein fractions derived from C. caldarium cell extract, biliverdin IXa, catalyzedby heme oxygenase, followed by was converted to two violet colored pigments in addition to reduction of biliverdin IXa to form free phycobilins, which the bluecolored 3(2)-phycocyanobilin (Beale and Cornejo, are covalently ligated to apophycobiliproteins to form 1991b)
The Blue Sepharose-bound fraction catalyzed the conver- 15,16-Dihydrobiliverdin IXa has been identified as a prodsion of biliverdin IXa to 3(Z)-phycoerythrobilin and 3(2)- uct of biliverdin IXa reduction catalyzed by protein fractions phycocyanobilin (Fig. 2)

Summary

Biosynthesis of Phycobilins

15,16-DIHYDROBILIVERDIN I X a IS A PARTIALLYREDUCEDINTERMEDIATEIN PHYCOBILINSFROMBILIVERDIN IXa*. A partially purified protein fraction from the phycocyanin-containing unicellular rhodophyte, Cyanidium caldariumr,eductively transformsbiliverdin I X a to a violet colored bilin in the presence of NADPH, ferredoxin, and ferredoxin-NADP+ reductase. This bilin has a violin-like absorption spectrum with maxima at 335 and 560 nm in methanolic HCI and at 337, 5 6 7 , and 603-604 nm in CHC13. Tuted enzyme system has been characterized as 15,16dihydrobiliverdin I X a by comparative spectrophotometry and ‘H NMR spectroscopy. This bilin appears to be a partially reduced product of biliverdin reduction and is enzymatically reduced further and converted to thephycobilins. Extraction of enzymes and pigments, fractionation of enzyme extract, purification and characterizatioonf pigments, and ‘H NMR spectroscopy were done asdescribed (Beale and Chen,1983; Beale and Cornejo,1991a,1991b).Specific incubationconditions, HPLC’ chromatography procedures, and other experimental details are described more fully under “Results.”

Production of Additional Bilins by Partially Reconstituted

Biliverdin IXa'

DISCUSSION

Biliverdin IXu