Voh 183, No. 3, 1992 March 31, 1992 BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS Pages ] 209-I 2"15 RECONSTITUTION OF APOMYOGLOBIN WITH EXTENDED BILIVERDINS Marcelo Femfmdez, Rosalia B. Frydman, Sara Bad and Benjamin Frydman Facultad de Farmacia y Bioquimica, Universidad de Buenos Aires, Junin 956, Buenos Aires, Argentina Received February 3, 1992 SUMMARY: An analysis of the reconstitution of biliverdins with extended conformations and horse heart apomyoglobin was carded out. Biliverdins with the 5Z-syn, lOZ-syn, 15Z-anti and 5Z-anti, lOZ-syn, 15Z-anti conformations, as well as biliverdins with the Z,Z,Z, all-syn conformation recombined with apomyoglobin. In every case the P enantiomers were bound in excess to the M enantiomers, with exception of the 5-syn, l O-syn,15-anti biliverdin where the M enantiomer bound preferentially to the protein. Biliverdins with an anti conformation at the C-10 meso bridge did not recombine with the protein. It was concluded that the presence of a syn conformation at the C-10 methine conferred to the biliverdin the necessary helicity to fit into the apomyoglobin heme pocket. This regioselectivity is of importance in view of the well known analogy between the ligand domains of myoglobin and the C-phycocyanins. ®1992 Academic Press, Inc. Bile pigments interact with proteins in two different ways. In the phycobiliproteins as well as in phytochrome they are covalently bound to the proteins by one or two thioether linkages and the protein enforces extended (or stretched) conformations on the bilitrienes as a result of electrostatic interactions between the propionate groups of the ligand and the basic residues of the proteins (1, 2). In the insect biliproteins known as insecticyanin (3) and biliverdin binding protein (BBP)(4), the biliverdin prosthetic groups are not covalently bound to the protein but interact with the latter through electrostatic and hydrophobic forces. In these biliproteins the biliverdins retain their energetically favored helical conformations. In the case of the phycobiliproteins, the biliverdins are derived from biliverdin IX t~ while in the insect biliproteins they are of the IX y type. It has been shown that apomyoglobin and the o~ and [3 subunits of C-phycocyanin share a similar "globin fold"; although their primary and secondary structures are quite different (5). This remarkable similarity led Falk and coworkers to analyze the recombination of sperm whale apomyoglobin with a number of helical biliverdins (6, 7). They found that apomyoglobin recombined with the biliverdins as long as the latter carded one or more propionate residues which allow them to anchor on the basic residues (His 97 and Arg 45) located at the entrance of the heme pocket. Their results led them to conclude that the apomyoglobin pocket is indifferent with respect to the steric and functional patterns present in the biliverdins (7). Since in the phycobiliproteins the biliverdins have extended conformations, recombination studies of apomyoglobin with extended 1209 0006-291X/92 $1.50 Copyright © 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.