Abstract: A simple mutant of the natural asparagine oxygenase comprises at least the amino acids 13 to 318 of the natural asparagine oxygenase AsnO. In this protein according to the present invention, comprising at least the amino acids 13-318 of AsnO D241N, the amino acid residue at position 241 of the natural asparagine oxygenase AsnO is exchanged from aspartate (D) to asparagine (N). The protein according to the present invention, comprising at least the amino acids 13-318 of AsnO D241N, is produced by means of a directed mutagenesis from the AsnO wild type, cloning of this expression plasmid into a vector, transformation of the vector plasmid construction into a host organism and expression of the recombinant protein. The protein according to the present invention is suitable for the chemoenzymatic and enantioselective production of L-threo-hydroxyaspartate from L-aspartate. The protein is substrate-specific and converts quantitatively L-aspartate into L-threo-hydroxyaspartate.