7β-hydroxysteroid dehydrogenase mutants and process for the preparation of ursodeoxycholic acid

A nucleic acid molecule comprising a nucleotide sequence encoding a 7β-HSDH mutant, which catalyzes at least the stereospecific enzymatic reduction of a 7-ketosteroid to the corresponding 7-hydroxysteroid, wherein the mutant has at least 90% sequence identity to SEQ ID NO:2 and at least one mutation in the sequence motif VMVGRRE corresponding to positions 36 to 42 of SEQ ID NO:2, wherein the mutation is selected from the group consisting of:

• a) the single mutation G39X1;
• b) the single mutation R40X2;
• c) the double mutation G39X1 R40X2
• d) the double mutation R40X2 R41X3;
• e) the double mutation G39X1 R41X3;
• and
• f) the triple mutation G39X1 R40X2 R41X3;
• wherein each of X1, X2 and X3 can be the same or different and stands for mutated amino acid residues.