A mutant hydroxylase with increased activity and greater substrate specificity towards phenylacetyl-7-ADCA derivatives for the production of phenylacetyl-7-HACA derivatives, which carries one or more amino acid modification at residue positions when compared with certain wild type hydroxylase from certain groups of residues. Also disclosed is a process for the preparation of deacetyl cephalosporanic acid from the corresponding deacetoxy cephalosporanic acid that includes an enzyme of the present invention. Also provided is a method for the production and processing of such enzymes.