Structure of the covalently bound flavin of Chlorobium cytochrome

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Abstract Cytochrome c 553 (Chlorobium thiosulfatophilum), is known to contain both a cytochrome moiety and a flavin component, which is not extracted by denaturation with trichloroacetic acid. In the present study a peptic flavin peptide was isolated and characterized. This confirms the hypothesis that the flavin is covalently linked to the polypeptide chain. Several lines of evidence suggest that the flavin is attached via its 8α position to the sulfur of a cysteinyl residue. These include a hypsochromic shift of the near ultraviolet maximum relative to riboflavin, an increase in fluorescence on oxidation with performic acid (from that given by an equimolar concentration of riboflavin), with a further hypsochromic shift of the near ultraviolet maximum, and the presence of cysteine after acid hydrolyis but not after aminopeptidase M digestion. All of these properties have previously been observed in 8α-S- cysteinylriboflavin and its peptides. The cysteinyl flavin bond survives mild acid hydrolysis if the sulfur moiety is first oxidized to the sulfone prior to acid treatment. It is thus possible to obtain an amino acyl derivative by this method and show its identity to 8α-S- cysteinylsulfone-2′-5′-anhydroriboflavin obtained via synthetic methods. The flavin occurs as a dinucleotide, presumably FAD, and from the many properties in common with cysteinylriboflavin and its peptides it is concluded that the flavin structure of Chlorobium cytochrome c 553 is 8α-S- cysteinyl-FAD thioether. The amino acid sequence around the flavin site was found to be:

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