| Popis: |
The effects of modification of apocrustacyanin with the tyrosine-specific reagent, tetanitromethane, on the ability of the protein to reconstitute with astaxanthin is investigated. Modification of an average three to six tyrosine residues per apoprotein molecule destroys the specific carotenoid binding responsible for the bathochromic spectral shift of the native carotenoprotein. The reaction is accompanied by intermolecular cross-linking of the protein, giving polymeric, polydisperse material which associates with carotenoid micelles. The loss of the carotenoid binding in this case may be due to general protein unfolding rather than alteration of specific tyrosine residues at the ligand binding sites. At lower levels of tyrosine modification (two to three tyrosine/subunit) intermolecular cross-linking is still evident but the monomeric and cross-linked dimeric apoprotein have reduced affinity for astaxanthin. Treatment of the separate apoprotein, A 2 , with low levels of tetranitromethane modifying less than three tyrosine on average/subunit results, partly, in the formation of intermolecular cross-linked dimers and lesser amounts of higher molecular sized oligomers. The remaining apoprotein monomer (and the cross-linked dimers) shows a reduced ability to bind astaxanthin. It is concluded that tyrosine residues may be of importance for the interaction between carotenoid and apoprotein or for maintaining the integrity of the carotenoid binding sites. |
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