One-pot multienzymatic synthesis of 12-ketoursodeoxycholic acid: subtle cofactor specificities rule the reaction equilibria of five biocatalysts working in a row

The hydroxysteroid dehydrogenases (HSDHs)-catalyzed one-pot enzymatic synthesis of 12-ketoursodeoxycholic acid (3a,7b-dihydroxy-12- oxo-5b-cholanoic acid), a key intermediate for the synthesis of ursodeoxycholic acid, from cholic acid has been investigated. This goal has been achieved by alternating oxidative and reductive steps in a one- pot system employing HSDHs with different cofactor specificity, namely NADH-dependent HSDHs in the oxidative step and an NADPH-dependent 7b-HSDH in the reductive one. Coupled in situ regeneration systems have been exploited not only to allow the use of catalytic amounts of the cofactors, but also to provide the necessary driving force to opposite reac- tions (i.e., oxidation and reduction) acting on differ- ent sites of the substrate molecule. Biocatalysts suita- ble for the set-up of this process have been selected and their kinetic behaviour in respect of the reac- tions of interest has been evaluated. Finally, the pro- cess has been studied employing the enzymes both in free and compartmentalized form.