Proteins with covalently bound heme and tetrapyrrole prosthetic groups are present in all organisms and have important roles in electron transport, photochemical reactions, and metabolic processes. However, knowledge of the chemical structure and stereochemistry of these prosthetic groups in their polypeptide-bound forms is very incomplete. The well-characterized biliproteins of red algae and cyanobacteria represent an abundant source of material for the study of the structure and mechanism of attachment of tetrapyrrole prosthetic groups. Five different tetrapyrroles are known to be covalently linked to the polypeptide chains of various biliproteins, and in only one case has the structure been rigorously established. Previous approaches to the study of the tetrapyrrole prosthetic groups have involved detachment of the prosthetic groups from the polypeptide. These studies have led to inclusive and conflicting proposals for the structure of polypeptide-linked bilins. The approach to be used in this study involves the degradation of conjugated proteins to simpler peptide-pigments without disturbing the native covalent linkages and purification of bilin peptides by mild procedures such as high performance liquid chromatography. The structures of the purified bilin-peptides will then be determined directly without further chemical manipulation by high resolution NMR and supported by studies on appropriate synthetic compounds. Peptdies bearing phycocyanobilin, phycoerythrobilin, and phycourobilin groups will be examined. Preliminary studies of the structural parameters governing bilin attachment to polypeptide in vivo will be conducted. The approach to be utilized in this investigation has general applicability to the study of unusual aspects of protein structure introduced by post-translational modification.
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