35] Sβ-(Bilin)cystein derivatives: Structures, spectroscopic properties, and quantitation

Abstract

This chapter describes structures, spectroscopic properties, and quantitation of Sβ-(Bilin)cysteine derivatives. Open-chain tetrapyrroles, commonly called “bile pigments” or “bilins,” occur as covalently bound prosthetic groups in several important types of proteins. The most abundant of such proteins are the diverse phycobiliproteins which serve as light-harvesting photosynthetic accessory proteins in cyanobacteria (blue-green algae), and in three groups of eukaryotic algae,the red algae, the cryptomonads, and certain dinoflagellates. In the phycobiliproteins and phytochrome, the bilins are linked to the polypeptide chain through a thioether bond to cysteinyl residues. Several different bilins function as covalently attached prosthetic groups in biliproteins. Polypeptide-linked bilins have distinctive visible absorption spectra, dependent on the chemical nature of the bilin, which can be exploited to determine the number and type of bilins present. Since the absorption spectra of bilins are strongly conformation dependent, complete denaturation of the proteins is a prerequisite to spectroscopic determination of bilin composition. Polypeptide-bound bilins are most stable at pH 3, or below.