OLIGOMERIC STRUCTURE, ENZYME-KINETICS, AND SUBSTRATE-SPECIFICITY OF THE PHYCOCYANIN ALPHA-SUBUNIT PHYCOCYANOBILIN LYASE

Phycobiliproteins carry linear tetrapyrrole chromophores (bilins) thio ether linked to specific cysteine residues. The process of bilin attac hment to apoprotein in vivo has been characterized for only one bilin attachment site on one phycobiliprotein, that on the alpha subunit of phycocyanin (alpha(PC)). In the cyanobacterium Synechococcus sp. PCC 7 002, the addition of phycocyanobilin to apo-alpha(PC) is catalyzed by the protein products of the cpcE and cpcF genes. We have purified and further characterized the recombinant CpcE and CpcF proteins. CpcE and CpcF form an enzymatically active 1:1 complex (Cp -cEF), stable to si ze exclusion chromatography CpcEF causes a reduction in alpha(PC) fluo rescence and strongly affects its absorption spectrum but has no effec t on the beta subunit. The CpcEF bilin addition activity exhibits simp le Michaelis-Menten kinetics with respect to the apo-alpha(PC) and to bilin. CpcEF also catalyzes the addition of phycoerythrobilin to apo-a lpha(PC); phycoerythrobilin is thought to be on the biosynthetic pathw ay of phycocyanobilin. CpcEF shows a preference for phycocyanobilin re lative to phycoerythrobilin, both in binding affinity and in the rate of catalysis, sufficient to account for selective attachment of phycoc yanobilin to apo-alpha(PC).