Evidence for a cysteine-histidine thioether bridge in functional units of molluscan haemocyanins and location of the disulfide bridges in functional units d and g of the beta(c)-haemocyanin of Helix pomatia
European Journal of Biochemistry vol:248 issue:3 pages:879-888
In functional units d and g from the beta(C)-haemocyanin of the gastropod Helix pomatia, aminoacid analysis in the presence of dimethyl sulfoxide showed the occurrence of seven cysteine residues. Titration with 5,5'-dithiobis(2-nitrobenzoic acid), however, did not reveal any free thiol group. Pepsinolysis at pH 2.0 followed by amino acid analysis and partial sequencing of the cysteine-containing peptides showed that six cysteine residues are involved in the formation of three disulfide bridges at corresponding positions. The results indicated that the remaining cysteine residue is linked by a thioether bridge to a histidine residue located two positions further in the sequence (H. pomatia d Cys60 His62; H. pomatia g Cys66 His68). This residue corresponds to one of the three histidine residues considered to be involved in the coordination of the copper A atom of the dinuclear copper group of the functional units.