# Recent Advances in Biocatalysis for Drug Synthesis
## 2 Biocatalysis for Drug Synthesis
### Synthetic Enzyme Cascades
AAE3, acyl activating enzyme 3; Ack, acetate kinase; ADK, adenylate kinase; DERA, deoxyribose 5-phosphate aldolase; GOase, galactose oxidase; GPPS, geranyl pyrophosphate synthase; HRP, horseradish peroxidase; IDI, isopentenyl diphosphate isomerase; IPK, isopentenyl kinase; IRED, imime reductase; KRED, ketoreductase; McAc, malonate decarboxylase \(\alpha\) subunit; MTR kinase, 5-S-methylthorinibose kinase; Nov435, Novozym 435; OAC, olivetolic acid cyclase; OLS, olivetol synthase; PanK, pantothenate kinase; PNP, purine nucleoside phosphorylase; POX, pyruvate kinase; PPM, phosphopentomutase; PTA, phosphotransferacetylase; SP, sucrose phosphorylase; ThiM, hydroxyethylthiazole kinase; UP, uridine phosphorylase. successfully employed in such syntheses [165]. However, few examples exist for the simultaneous usage of several enzymes and chemical conversions in one pot for drug synthesis. Here, the production of (_S_,_S_)-sertraline is shown, which uses two enzymes and a chemical reaction, in separately performed steps [142]. Sertraline is the API in Zoloft(r) (Pfizer), used for the treatment of depression and anxiety [166; 167]. Enzymatic steps were integrated to receive a stereoselective keto-precursor of serntraline [142]. For this purpose, the racemic mixture of this keto-precursor was selectively reduced by a KRED and subsequently re-oxidized by either a chemical step or a by a lacase to get the enantiopure keto-educt. Suitable KRED and reaction conditions were selected by the screening of enzyme variants and temperature, pH, and racemic substrate concentrations. Finally, the enantiopure alcohol-precursor was gained with a >99% _ee_ and 29% conversion after 7 h. The final animation was performed with a chemical step, since a biocatalytic animation of the racemic reduct by IREDs or TAs were unsuccessful. (_S_,_S_)-sertraline was obtained with a >99% _ee_ and an overall yield of 16% [142]. Figure 15: (**A**) Multi-enzymatic reactions for the synthesis of 2′3′-cyclic GMP-AMP [153], and (**B**) MK-1454 [154]. AcK, acetate kinase; ADK, adenosine kinase; AK, adenylate kinase; cGAS, cyclic GMP-AMP synthase; GK, guanylate kinase; PPK2, polyphosphate kinase 2. Several approaches for the challenging synthesis of drugs by multi-enzymatic reactions were developed, including the rational and bioretrysynthetic building of the reaction route, creation of enantioselective products by specialized enzymes, and performance improvement by adjusted enzyme ratios or equilibrium shifts by ISPR. Partly, product titers were higher using the enzyme cascades than in cell-based productions. These examples demonstrate the broad versatility and applicability of enzyme cascades for API synthesis and as more research is put to this topic, more examples and even industrial applications will be the outcome. However, the development of new processes not only includes the development of the synthesis step, but also an integrated view of upstream and downstream. It is usually not the synthesis step that incurs the highest costs, but the downstream process. Nevertheless, the higher the titer and space-time yield and the fewer the by-products in the synthesis step, the easier the downstream processing. Therefore, the development and optimization of the most efficient biocatalysis possible will continue to be of concern in the future.