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Homology comparison of turran, murine and E. coli adenosine deaminases revealed a single conserved cysteine residue. On the basis of many previous reports indicating that there is an essential Cys residue involved in enzymaticactivity, this conserved Cys was proposed to be the essential one involved in the catalytic activity of the enzyrre. Usingsite-directed mutagenesis and steady state kinetics, the role of the single conserved Cys residue was tested. However, results presented here indicate that this conserved Cys is not required for enzyme activity, strongly suggesting that cysteine should not be required for the enzymatic activity of adenosine deaminase. The preponder ance of previously reported data indicating the essentiality of the Cys residue for enzymatic activity are discussed.