Determination of the number of thioether-linked cysteine residues in cytochromes c and phycobiliproteins

Purification, amino-acid sequence and some properties of the ferredoxin isolated from Bacillus acidocaldarius

Ferredoxin was isolated from the aerobic, thermophilic and acidophilic bacterium Bacillus acidocaldarius and its sequence of 78 amino acids completely determined by automated Edman degradation of the protein and of peptides derived from chemical cleavage between aspartic acid and proline and from enzymatic digestions. The optical spectrum of the oxidized protein has a broad maximum around 400 nm. The ferredoxin is thermostable: its absorbance begins to decrease only at incubation over 71 degrees C. The number of iron and inorganic sulphur atoms per molecule was determined to be 5.3 and 5.0, respectively. The calculated molar extinction coefficient was 23 000 M-1 X cm-1, the molecular mass of the apoferredoxin 8 872 Da. Contrary to all expectations, the sequence of B. acidocaldarius ferredoxin shows very little homology to that of B. stearothermophilus but closely resembles that of Thermus thermophilus.

Structures and apoprotein linkages of phycoerythrobilin and phycocyanobilin

Phycoerythrobilin and phycocyanobilin are covalently attached to the apoproteins of phycoerythrins and phycocyanins. One linkage consists of an ester bond between the hydroxy group of a serine residue and the propionate side chain on one of the inner pyrrole rings (probably ring C). The other linkage is a labile thioether bond between a cysteine residue and the two-carbon side chain on pyrrole ring A. This side chain and both of the alpha-positions of the ring A are in the reduced state. This constitutes an important structural revision, since, in the structures currently accepted for the phycobilins, the two-carbon side chain on ring A is depicted as an ethylidene grouping and this has been regarded not only as a very characteristic feature of the phycobilins, but also as a probable structural feature of the chromophore of phytochrome, largely on the basis of other analogies with the phycobilins. The ethylidene-containing structures apply instead to artefact forms of the pigments released from the apoproteins by treatment with hot methanol. Cleavage of the ring-A linkage involves an elimination reaction releasing the cysteine residue and generating a double bond in the ring-A side chain. During cleavage in methanol the direction of the elimination is towards the ring, generating the ethylidene double bond. Since this is linked to the conjugated system, the methanol-released pigments differ spectrally from the native phycobilins. During acid-catalysed release of the pigments, the elimination apparently goes in the opposite direction, generating a double bond at the outer position of the side chain. Since this double bond is not linked to the conjugated system, the acid-released pigments remain spectrally identical with their protein-bound counterparts.

The protein-chromophore bond in B phycoerythrin from Porphyridium cruentum. Radiosulfur labeling experiments

Red algae of the species Porphyridium cruentum were grown in a minimum sulfate medium containing 35SO42-. 35S-labeled phycoerythrin was extracted. B Phycoerythrin, b phycoerythrin and R phycocyanin could be separated from other proteins by using a carrier-free electrophoresis on columns. The final ratio A545/A280 of B phycoerythrin thus obtained was greater than or equal to 5. 35S-labeled B phycoerythrin was digested proteolytically with trypsin and pepsin. The resulting 35S-containing bilipeptides were separated by isoelectric focusing. Zones of enhanced chromophore concentration always showed an enhanced radioactivity. Peptide fractions with a low molar ratio sulfur/chromophore (1.1-1.8) were purified to remove sucrose and the carrier ampholyte. A modified, optimized Edman degradation followed. A butylacetate-soluble, red Edman product was obtained that contained most of the chromophore and the bulk of the radioactivity. This product was purified by two-dimensional thin-layer chromatography. The main spot of the chromatogram was subjected to acidic hydrolysis. The major part of the radioactivity in the hydrolysate cochromatographed with cysteine. That proves cysteine to be the binding amino acid in all cases investigated.