Stephan C. Hammer

New generation of biocatalysts for organic synthesis.

The use of enzymes as catalysts for the preparation of novel compounds has received steadily increasing attention over the past few years. High demands are placed on the identification of new biocatalysts for organic synthesis. The catalysis of more ambitious reactions reflects the high expectations of this field of research. Enzymes play an increasingly important role as biocatalysts in the synthesis of key intermediates for the pharmaceutical and chemical industry, and new enzymatic technologies and processes have been established. Enzymes are an important part of the spectrum of catalysts available for synthetic chemistry. The advantages and applications of the most recent and attractive biocatalysts--reductases, transaminases, ammonia lyases, epoxide hydrolases, and dehalogenases--will be discussed herein and exemplified by the syntheses of interesting compounds.

Squalene hopene cyclases are protonases for stereoselective Brønsted acid catalysis

For many important reactions catalyzed in chemical laboratories, the corresponding enzymes are missing, representing a restriction in biocatalysis. Although nature provides highly developed machineries appropriate to catalyze such reactions, their potential is often ignored. This also applies to Brønsted acid catalysis, a powerful method to promote a myriad of chemical transformations. Here, we report on the unique protonation machinery of a squalene hopene cyclase (SHC). Active site engineering of this highly evolvable enzyme yielded a platform for enzymatic Brønsted acid catalysis in water. This is illustrated by activation of different functional groups (alkenes, epoxides and carbonyls), enabling the highly stereoselective syntheses of various cyclohexanoids while uncoupling SHC from polycyclization chemistry. This work highlights the potential of systematic investigation on natures catalytic machineries to generate unique catalysts.

Imine Reductase‐Catalyzed Intermolecular Reductive Amination of Aldehydes and Ketones

Imine reductases (IREDs) have emerged as promising biocatalysts for the synthesis of chiral amines. In this study, the asymmetric imine reductase‐catalyzed intermolecular reductive amination with NADPH as the hydrogen source was investigated. A highly chemo‐ and stereoselective imine reductase was applied for the reductive amination by using a panel of carbonyls with different amine nucleophiles. Primary and secondary amine products were generated in moderate to high yields with high enantiomeric excess values. The formation of the imine intermediate was studied between carbonyl substrates and methylamine in aqueous solution in the pH range of 4.0 to 9.0 by 1H NMR spectroscopy. We further measured the kinetics of the reductive amination of benzaldehyde with methylamine. This imine reductase‐catalyzed approach constitutes a powerful and direct method for the synthesis of valuable amines under mild reaction conditions.

Anti-Markovnikov alkene oxidation by metal-oxo-mediated enzyme catalysis.

Teaching an enzyme to switch sites There has been a recent flurry of activity in modifying enzymes to conduct unnatural chemical reactions more cleanly or selectively than synthetic chemical catalysts. Hammer et al. now report application of a cytochrome P450 variant to an oxidation that has largely eluded efficient catalysis. They used directed evolution to mutate the enzyme so that it placed oxygen at the less substituted carbon of the C=C double bond in styrenes, forming aldehyde products. They thereby attained opposite site selectivity to that of the widely used palladium-catalyzed Wacker-Tsuji oxidation. Science, this issue p. 215 Directed evolution modifies cytochrome P450 to catalyze an unnatural reaction that has bedeviled chemical catalysis. Catalytic anti-Markovnikov oxidation of alkene feedstocks could simplify synthetic routes to many important molecules and solve a long-standing challenge in chemistry. Here we report the engineering of a cytochrome P450 enzyme by directed evolution to catalyze metal-oxo–mediated anti-Markovnikov oxidation of styrenes with high efficiency. The enzyme uses dioxygen as the terminal oxidant and achieves selectivity for anti-Markovnikov oxidation over the kinetically favored alkene epoxidation by trapping high-energy intermediates and catalyzing an oxo transfer, including an enantioselective 1,2-hydride migration. The anti-Markovnikov oxygenase can be combined with other catalysts in synthetic metabolic pathways to access a variety of challenging anti-Markovnikov functionalization reactions.

Entropy is Key to the Formation of Pentacyclic Terpenoids by Enzyme-Catalyzed Polycyclization†

Polycyclizations constitute a cornerstone of chemistry and biology. Multicyclic scaffolds are generated by terpene cyclase enzymes in nature through a carbocationic polycyclization cascade of a prefolded polyisoprene backbone, for which electrostatic stabilization of transient carbocationic species is believed to drive catalysis. Computational studies and site-directed mutagenesis were used to assess the contribution of entropy to the polycyclization cascade catalyzed by the triterpene cyclase from A. acidocaldarius. Our results show that entropy contributes significantly to the rate enhancement through the release of water molecules through specific channels. A single rational point mutation that results in the disruption of one of these water channels decreased the entropic contribution to catalysis by 60 kcal mol(-1) . This work demonstrates that entropy is the key to enzyme-catalyzed polycyclizations, which are highly relevant in biology since 90 % of all natural products contain a cyclic subunit.

Selectivity in the Cyclization of Citronellal Introduced by Squalene Hopene Cyclase Variants

The squalene hopene cyclase from Alicyclobacillus acidocaldarius (AacSHC) is a highly efficient enzyme catalyst for stereoselective Brønsted acid catalysis. We engineered AacSHC to catalyze the selective Prins cyclization of citronellal. Four active site variants were identified for the diastereoselective cyclization of (S)‐citronellal to stereoisomers (−)‐iso‐isopulegol, (+)‐isopulegol and (−)‐neo‐isopulegol, respectively. The replacement of active site residues resulted in two triple variants that catalyzed the transformation of (R)‐citronellal to give the isomers (+)‐neo‐isopulegol and (−)‐isopulegol with up to >99 % de, respectively. The newly designed library of functionally diverse active site geometries exhibits high selective control during citronellal cyclization, leading exclusively to a single diastereomer of the desired isopulegol. Whereas the cyclization of citronellal with chemical catalysts was observed to produce the isopulegol isomer with the lowest energy, the reaction with AacSHC variants proceeded with higher product selectivity. The results of this study show that variants of AacSHC are excellent catalysts for the highly selective formation of isopulegol stereoisomers.

Schlummerndes Synthesepotenzial in Enzymen: Wie können wir es wecken?

Enzymes are highly attractive catalysts for organic synthesis. Traditionally, new enzymatic function has been identified by screening large libraries of mutants and microorganisms. Recently, the field has developed to use small, functionally rich libraries combined with a chemical-based enzyme engineering approach. Key advances derive from bioinformatics along with a better understanding of the mechanisms of natural protein evolution, all this in combination with chemical intuition.