Enzymes are renowned for their catalytic e ciency and selectivity, but relatively few carbon-carbon bond forming enzymes have found their way into the biocatalysis toolbox. While engineering can overcome the challenges associated with C-C bond formation for some enzyme systems, the broader synthetic potential of biocatalysis is hindered by the lack of high-quality C-C bond forming transformations. Here we show that the enzyme UstD performs a highly selective decarboxylative aldol addition with diverse aldehyde substrates to make non-standard, γ-hydroxy amino acids. We increased the activity of UstD through three rounds of classic directed evolution and an additional round of computationally-guided engineering. The enzyme that emerged, UstD2.0, is e cient in a whole-cell biocatalysis format, which circumvents the need for enzyme puri cation, thereby facilitating its use in traditional organic settings. This new, highly stereoselective enzyme represents a unique expansion of the biosynthetic toolbox. The products are highly desirable, functionally rich bioactive γ-hydroxy amino acids that we demonstrate can be prepared stereoselectively on gram-scale. The X-ray crystal structure of UstD2.0 at 2.25 Å reveals the active site and the molecular basis for the remarkably promiscuity of this catalyst. Taking inspiration from the versatile reactivity of enamines in organic synthesis, we hypothesize that the enamine intermediate of UstD can be engineered to react with electrophiles other than aldehydes. The advent of structural information enabled by engineering of UstD2.0 provides a foundation for probing the unique mechanism of UstD and will guide efforts to expand the reactivity of this unique enzyme. Main TextMajor advances have been made in the practical use of enzymes for enantioselective functional group manipulations 9 . For example, enantioselective reduction of ketones or enantiospeci c hydrolysis of racemic esters are now routine in process chemistry. There have also been impressive strides made in enzymatic C-H activation 10 . However, the development of enzymes to form C-C bonds on preparative scale lags far behind traditional synthetic organic methodology 11 .To ll this gap, we were drawn to a recently described pyridoxal phosphate (PLP) dependent enzyme involved in the biosynthesis of Ustiloxin B, an inhibitor of microtubilin polymerization (Fig. 1A) 12 . This enzyme, UstD, decarboxylates the side chain of l-aspartate (1), forming a putative nucleophilic enamine intermediate (Fig. 1B). This enamine then attacks an aliphatic aldehyde appended to a cyclic tetrapeptide, resulting in the formation of a γ-hydroxy amino acid side chain. The loss of CO 2 renders this enantioselective C-C bond forming reaction irreversible. However, the native substrate for UstD is a complex, cyclic peptide, and it was unknown if this enzyme would react promiscuously with alternative substrates; if so, the enzyme would directly produce γ-hydroxy amino acids (Fig. 1B). Such non-standard amino acids (nsAAs) are found in bioactive natur...

Multi-enzyme biocatalytic cascades are emerging as practical routes for the synthesis of complex bioactive molecules. However, the relative sparsity of water-stable carbon electrophiles limits the synthetic complexity of molecules made from such cascades. Here, we develop a chemoenzymatic platform that leverages styrene oxide isomerase (SOI) to convert readily accessible aryl epoxides into α-aryl aldehydes through Meinwald rearrangement. These unstable aldehyde intermediates are then intercepted with a C−C bond forming enzyme, ObiH, that catalyzes a transaldolase reaction with L-threonine to yield synthetically challenging β-hydroxy-α-amino acids. Co-expression of both enzymes in E. coli yields a whole-cell biocatalyst capable of synthesizing a variety of stereopure non-standard amino acids (nsAA) and can be produced on a gram scale. We used isotopically labeled substrates to probe the mechanism of SOI, which we show to catalyze a concerted isomerization featuring a stereospecific 1,2-hydride shift. The viability of in situ generated α-aryl aldehydes was further established by intercepting them with a recently engineered decarboxylative aldolase to yield γ-hydroxy nsAAs. Together, these data establish a versatile method of producing α-aryl aldehydes in simple, wholecell conditions and show that these intermediates are useful synthons in C−C bond forming cascades.

The formation of carbon-carbon bonds lies at the heart of organic chemistry, but relatively few C-C bond forming enzymes have found their way into the biocatalysis toolbox. We report that the enzyme UstD performs a highly selective decarboxylative aldol addition with diverse aldehyde substrates to make non-standard, γ-hydroxy amino acids. We increased the activity of UstD through three rounds of classic directed evolution and an additional round of computationally-guided engineering. The enzyme that emerged, UstD^2.0, is very efficient in a whole-cell biocatalysis format and readily crystallizes. The X-ray crystal structure of UstD^2.0at 2.25 Å reveals the active site and empowers future studies. The utility of UstD^2.0 was demonstrated via the stereoselective gram-scale syntheses of non-standard amino acids.

Enzymes are renowned for their catalytic efficiency and selectivity, but relatively few carbon-carbon bond forming enzymes have found their way into the biocatalysis toolbox. While engineering can overcome the challenges associated with C-C bond formation for some enzyme systems, the broader synthetic potential of biocatalysis is hindered by the lack of high-quality C-C bond forming transformations. Here we show that the enzyme UstD performs a highly selective decarboxylative aldol addition with diverse aldehyde substrates to make non-standard, γ-hydroxy amino acids. We increased the activity of UstD through three rounds of classic directed evolution and an additional round of computationally-guided engineering. The enzyme that emerged, UstD2.0, is efficient in a whole-cell biocatalysis format, which circumvents the need for enzyme purification, thereby facilitating its use in traditional organic settings. This new, highly stereoselective enzyme represents a unique expansion of the biosynthetic toolbox. The products are highly desirable, functionally rich bioactive γ-hydroxy amino acids that we demonstrate can be prepared stereoselectively on gram-scale. The X-ray crystal structure of UstD2.0 at 2.25 Å reveals the active site and the molecular basis for the remarkably promiscuity of this catalyst. Taking inspiration from the versatile reactivity of enamines in organic synthesis, we hypothesize that the enamine intermediate of UstD can be engineered to react with electrophiles other than aldehydes. The advent of structural information enabled by engineering of UstD2.0 provides a foundation for probing the unique mechanism of UstD and will guide efforts to expand the reactivity of this unique enzyme.

The formation of carbon-carbon bonds lies at the heart of organic chemistry, but relatively few C-C bond forming enzymes have found their way into the biocatalysis toolbox. We report that the enzyme UstD performs a highly selective decarboxylative aldol addition with diverse aldehyde substrates to make non-standard, γ-hydroxy amino acids. We increased the activity of UstD through three rounds of classic directed evolution and an additional round of computationally-guided engineering. The enzyme that emerged, UstD^2.0, is very efficient in a whole-cell biocatalysis format and readily crystallizes. The X-ray crystal structure of UstD^2.0at 2.25 Å reveals the active site and empowers future studies. The utility of UstD^2.0 was demonstrated via the stereoselective gram-scale syntheses of non-standard amino acids.