Incorporation of beta-selenolo[3,2-b]pyrrolyl-alanine into proteins for phase determination in protein X-ray crystallography

Non-Canonical Amino Acids in Analyses of Protease Structure and Function

All known organisms encode 20 canonical amino acids by base triplets in the genetic code. The cellular translational machinery produces proteins consisting mainly of these amino acids. Several hundred natural amino acids serve important functions in metabolism, as scaffold molecules, and in signal transduction. New side chains are generated mainly by post-translational modifications, while others have altered backbones, such as the β- or γ-amino acids, or they undergo stereochemical inversion, e.g., in the case of D-amino acids. In addition, the number of non-canonical amino acids has further increased by chemical syntheses. Since many of these non-canonical amino acids confer resistance to proteolytic degradation, they are potential protease inhibitors and tools for specificity profiling studies in substrate optimization and enzyme inhibition. Other applications include in vitro and in vivo studies of enzyme kinetics, molecular interactions and bioimaging, to name a few. Amino acids with bio-orthogonal labels are particularly attractive, enabling various cross-link and click reactions for structure-functional studies. Here, we cover the latest developments in protease research with non-canonical amino acids, which opens up a great potential, e.g., for novel prodrugs activated by proteases or for other pharmaceutical compounds, some of which have already reached the clinical trial stage.

Tryptophan Synthase: Biocatalyst Extraordinaire

Tryptophan synthase (TrpS) has emerged as a paragon of noncanonical amino acid (ncAA) synthesis and is an ideal biocatalyst for synthetic and biological applications. TrpS catalyzes an irreversible, C-C bond-forming reaction between indole and serine to make l-tryptophan; native TrpS complexes possess fairly broad specificity for indole analogues, but are difficult to engineer to extend substrate scope or to confer other useful properties due to allosteric constraints and their heterodimeric structure. Directed evolution freed the catalytically relevant TrpS β-subunit (TrpB) from allosteric regulation by its TrpA partner and has enabled dramatic expansion of the enzyme's substrate scope. This review examines the long and storied career of TrpS from the perspective of its application in ncAA synthesis and biocatalytic cascades.

An Engineered Escherichia coli Strain with Synthetic Metabolism for in-Cell Production of Translationally Active Methionine Derivatives

In the last decades, it has become clear that the canonical amino acid repertoire codified by the universal genetic code is not up to the needs of emerging biotechnologies. For this reason, extensive genetic code re-engineering is essential to expand the scope of ribosomal protein translation, leading to reprogrammed microbial cells equipped with an alternative biochemical alphabet to be exploited as potential factories for biotechnological purposes. The prerequisite for this to happen is a continuous intracellular supply of noncanonical amino acids through synthetic metabolism from simple and cheap precursors. We have engineered an Escherichia coli bacterial system that fulfills these requirements through reconfiguration of the methionine biosynthetic pathway and the introduction of an exogenous direct trans-sulfuration pathway. Our metabolic scheme operates in vivo, rescuing intermediates from core cell metabolism and combining them with small bio-orthogonal compounds. Our reprogrammed E. coli strain is capable of the in-cell production of l-azidohomoalanine, which is directly incorporated into proteins in response to methionine codons. We thereby constructed a prototype suitable for economic, versatile, green sustainable chemistry, pushing towards enzyme chemistry and biotechnology-based production.

Synthesis and characterization of an unnatural boron and nitrogen-containing tryptophan analogue and its incorporation into proteins

A boron and nitrogen containing unnatural analogue of tryptophan is synthesized and incorporated into proteins.

A boron and nitrogen containing unnatural analogue of tryptophan is synthesized through the functionalization of BN-indole. The spectroscopic properties of BN-tryptophan are reported with respect to the natural tryptophan, and the incorporation of BN-tryptophan into proteins expressed in E. coli using selective pressure incorporation is described. This work shows that a cellular system can recognize the unnatural, BN-containing tryptophan. More importantly, it presents the first example of an azaborine containing amino acid being incorporated into proteins.

Evolution of translation machinery in recoded bacteria enables multi-site incorporation of nonstandard amino acids

Expansion of the genetic code with nonstandard amino acids (nsAAs) has enabled biosynthesis of proteins with diverse new chemistries. However, this technology has been largely restricted to proteins containing a single or few nsAA instances. Here we describe an in vivo evolution approach in a genomically recoded Escherichia coli strain for the selection of orthogonal translation systems capable of multi-site nsAA incorporation. We evolved chromosomal aminoacyl-tRNA synthetases (aaRSs) with up to 25-fold increased protein production for p-acetyl-L-phenylalanine and p-azido-L-phenylalanine (pAzF). We also evolved aaRSs with tunable specificities for 14 nsAAs, including an enzyme that efficiently charges pAzF while excluding 237 other nsAAs. These variants enabled production of elastin-like-polypeptides with 30 nsAA residues at high yields (∼50 mg/L) and high accuracy of incorporation (>95%). This approach to aaRS evolution should accelerate and expand our ability to produce functionalized proteins and sequence-defined polymers with diverse chemistries.

Tailoring the substrate specificity of yeast phenylalanyl-tRNA synthetase toward a phenylalanine analog using multiple-site-specific incorporation

A yeast phenylalanyl-tRNA synthetase variant with T415G mutation (yPheRS (T415G)) was rationally designed to recognize various phenylalanine (Phe) analogs allowing site-specific incorporation into an amber site of a protein in E. coli. However, the relaxed substrate specificity of yPheRS (T415G) led to a significant tryptophan (Trp) misincorporation restricting the utility of yPheRS for biosynthesis of proteins containing a Phe analog. In order to obtain yPheRS variants with high substrate-specificity toward a Phe analog, we developed a general high-throughput screening method. This method uses fluorescence reduction of green fluorescence protein (GFP) upon efficient introduction of a Phe analog into multiple sites of GFP by breaking the degeneracy of the Phe codons. Combined use of positive and negative screenings of a yPheRS saturation library led to a yPheRS variant (yPheRS_naph) very selective toward 2-l-naphthylalanine (2Nal), a model Phe analog. The yPheRS_naph exhibited 6-fold higher relative activity toward 2Nal (vs Trp) in ATP-PPi exchange assays and led to high-fidelity incorporation of 2Nal into an amber site of murine dihydrofolate reductase in both minimal and rich media. These results successfully demonstrate that the high-throughput screening method developed can be used to evolve yPheRS to be very selective toward a Phe analog.

Biological applications of expanded genetic codes

Substantial efforts in the past decade have resulted in the systematic expansion of genetic codes, allowing for the direct ribosomal incorporation of ∼100 unnatural amino acids into bacteria, yeast, mammalian cells, and animals. Here, we illustrate the versatility of expanded genetic codes in biology and bioengineering, focusing on the application of expanded genetic codes to problems in protein, cell, synthetic, and experimental evolutionary biology. As the expanded genetic code field continues to develop, its place as a foundational technology in the whole of biological sciences will solidify.

Anomalous diffraction in crystallographic phase evaluation

X-ray diffraction patterns from crystals of biological macromolecules contain sufficient information to define atomic structures, but atomic positions are inextricable without having electron-density images. Diffraction measurements provide amplitudes, but the computation of electron density also requires phases for the diffracted waves. The resonance phenomenon known as anomalous scattering offers a powerful solution to this phase problem. Exploiting scattering resonances from diverse elements, the methods of MAD (multiwavelength anomalous diffraction) and SAD (single-wavelength anomalous diffraction) now predominate for de novo determinations of atomic-level biological structures. This review describes the physical underpinnings of anomalous diffraction methods, the evolution of these methods to their current maturity, the elements, procedures and instrumentation used for effective implementation, and the realm of applications.

Coupling bioorthogonal chemistries with artificial metabolism: intracellular biosynthesis of azidohomoalanine and its incorporation into recombinant proteins

In this paper, we present a novel, “single experiment” methodology based on genetic engineering of metabolic pathways for direct intracellular production of non-canonical amino acids from simple precursors, coupled with expanded genetic code. In particular, we engineered the intracellular biosynthesis of l-azidohomoalanine from O-acetyl-l-homoserine and NaN₃, and achieved its direct incorporation into recombinant target proteins by AUG codon reassignment in a methionine-auxotroph E. coli strain. In our system, the host’s methionine biosynthetic pathway was first diverted towards the production of the desired non-canonical amino acid by exploiting the broad reaction specificity of recombinant pyridoxal phosphate-dependent O-acetylhomoserine sulfhydrylase from Corynebacterium glutamicum. Then, the expression of the target protein barstar, accompanied with efficient l-azidohomoalanine incorporation in place of l-methionine, was accomplished. This work stands as proof-of-principle and paves the way for additional work towards intracellular production and site-specific incorporation of biotechnologically relevant non-canonical amino acids directly from common fermentable sources.

Manipulation of enzyme properties by noncanonical amino acid incorporation

Since wild-type enzymes do not always have the properties needed for various applications, enzymes are often engineered to obtain desirable properties through protein engineering techniques. In the past decade, complementary to the widely used rational protein design and directed evolution techniques, noncanonical amino acid incorporation (NCAAI) has become a new and effective protein engineering technique. Recently, NCAAI has been used to improve intrinsic functions of proteins, such as enzymes and fluorescent proteins, beyond the capacities obtained with natural amino acids. Herein, recent progress on improving enzyme properties through NCAAI in vivo is reviewed and the challenges of current approaches and future directions are also discussed. To date, both NCAAI methods-residue- and site-specific incorporation-have been primarily used to improve the catalytic turnover number and substrate binding affinity of enzymes. Numerous strategies used to minimize structural perturbation and stability loss of a target enzyme upon NCAAI are also explored. Considering the generality of NCAAI incorporation, we expect its application could be expanded to improve other enzyme properties, such as substrate specificity and solvent resistance in the near future.

Discovery of Escherichia coli methionyl-tRNA synthetase mutants for efficient labeling of proteins with azidonorleucine in vivo

Incorporation of noncanonical amino acids into cellular proteins often requires engineering new aminoacyl-tRNA synthetase activity into the cell. A screening strategy that relies on cell-surface display of reactive amino acid side-chains was used to identify a diverse set of methionyl-tRNA synthetase (MetRS) mutants that allow efficient incorporation of the methionine (Met) analog azidonorleucine (Anl). We demonstrate that the extent of cell-surface labeling in vivo is a good indicator of the rate of Anl activation by the MetRS variant harbored by the cell. By screening at low Anl concentrations in Met-supplemented media, MetRS variants with improved activities toward Anl and better discrimination against Met were identified.

Unnatural amino acid incorporation into virus-like particles

Virus-like particles composed of hepatitis B virus (HBV) or bacteriophage Qbeta capsid proteins have been labeled with azide- or alkyne-containing unnatural amino acids by expression in a methionine auxotrophic strain of E. coli. The substitution does not affect the ability of the particles to self-assemble into icosahedral structures indistinguishable from native forms. The azide and alkyne groups were addressed by Cu(I)-catalyzed [3 + 2] cycloaddition: HBV particles were decomposed by the formation of more than 120 triazole linkages per capsid in a location-dependent manner, whereas Qbeta suffered no such instability. The marriage of these well-known techniques of sense-codon reassignment and bioorthogonal chemical coupling provides the capability to construct polyvalent particles displaying a wide variety of functional groups with near-perfect control of spacing.

Metal-catalyzed 1,2-shift of diverse migrating groups in allenyl systems as a new paradigm toward densely functionalized heterocycles

A general, mild, and efficient 1,2-migration/cycloisomerization methodology toward multisubstituted 3-thio-, seleno-, halo-, aryl-, and alkyl-furans and pyrroles, as well as fused heterocycles, valuable building blocks for synthetic chemistry, has been developed. Moreover, regiodivergent conditions have been identified for C-4 bromo- and thio-substituted allenones and alkynones for the assembly of regioisomeric 2-hetero substituted furans selectively. It was demonstrated that, depending on reaction conditions, ambident substrates can be selectively transformed into furan products, as well as undergo selective 6-exo-dig or Nazarov cyclizations. Our mechanistic investigations have revealed that the transformation proceeds via allenylcarbonyl or allenylimine intermediates followed by 1,2-group migration to the allenyl sp carbon during cycloisomerization. It was found that 1,2-migration of chalcogens and halogens predominantly proceeds via formation of irenium intermediates. Analogous intermediate can also be proposed for 1,2-aryl shift. Furthermore, it was shown that the cycloisomerization cascade can be catalyzed by Brønsted acids, albeit less efficiently, and commonly observed reactivity of Lewis acid catalysts cannot be attributed to the eventual formation of proton. Undoubtedly, thermally induced or Lewis acid-catalyzed transformations proceed via intramolecular Michael addition or activation of the enone moiety pathways, whereas certain carbophilic metals trigger carbenoid/oxonium type pathway. However, a facile cycloisomerization in the presence of cationic complexes, as well as observed migratory aptitude in the cycloisomerization of unsymmetrically disubstituted aryl- and alkylallenes, strongly supports electrophilic nature for this transformation. Full mechanistic details, as well as the scope of this transformation, are discussed.

In vivo engineering of proteins with nitrogen-containing tryptophan analogs

Recently, it has become possible to reprogram the protein synthesis machinery such that numerous noncanonical amino acids can be translated into target sequences yielding tailor-made proteins. The canonical amino acid tryptophan (Trp) encoded by a single nucleotide triplet (UGG) is a particularly interesting target for protein engineering and design. Trp-residues can be substituted with a variety of analogs and surrogates generated biosynthetically or by organic chemistry. Among them, nitrogen-containing tryptophan analogs occupy a central position, as they have distinct chemical properties in comparison with aliphatic amines and imines. They resemble purine bases of DNA and share their capacity for pH-sensitive intramolecular charge transfer. These special properties of the analogs can be directly transmitted into related protein structures via in vivo ribosome-mediated translation. Proteins expressed in this way are further endowed with unique properties like new spectral, altered redox and titration features or might serve as useful biomaterials. We present and discuss current works and future developments in protein engineering with nitrogen-containing tryptophan analogs and related compounds as well as their relevance for academic and applicative research.

Aminotryptophan-containing barstar: structure--function tradeoff in protein design and engineering with an expanded genetic code

The indole ring of the canonical amino acid tryptophan (Trp) possesses distinguished features, such as sterical bulk, hydrophobicity and the nitrogen atom which is capable of acting as a hydrogen bond donor. The introduction of an amino group into the indole moiety of Trp yields the structural analogs 4-aminotryptophan ((4-NH(2))Trp) and 5-aminotryptophan ((5-NH(2))Trp). Their hydrophobicity and spectral properties are substantially different when compared to those of Trp. They resemble the purine bases of DNA and share their capacity for pH-sensitive intramolecular charge transfer. The Trp --> aminotryptophan substitution in proteins during ribosomal translation is expected to result in related protein variants that acquire these features. These expectations have been fulfilled by incorporating (4-NH(2))Trp and (5-NH(2))Trp into barstar, an intracellular inhibitor of the ribonuclease barnase from Bacillus amyloliquefaciens. The crystal structure of (4-NH(2))Trp-barstar is similar to that of the parent protein, whereas its spectral and thermodynamic behavior is found to be remarkably different. The T(m) value of (4-NH(2))Trp- and (5-NH(2))Trp-barstar is lowered by about 20 degrees Celsius, and they exhibit a strongly reduced unfolding cooperativity and substantial loss of free energy in folding. Furthermore, folding kinetic study of (4-NH(2))Trp-barstar revealed that the denatured state is even preferred over native one. The combination of structural and thermodynamic analyses clearly shows how structures of substituted barstar display a typical structure-function tradeoff: the acquirement of unique pH-sensitive charge transfer as a novel function is achieved at the expense of protein stability. These findings provide a new insight into the evolution of the amino acid repertoire of the universal genetic code and highlight possible problems regarding protein engineering and design by using an expanded genetic code.

Isosteric replacement of sulfur with other chalcogens in peptides and proteins

The review addresses the functional and structural properties of the two series of chalcogen analogues of amino acids in peptides and proteins, the methionine and the serine/cysteine series, and discusses the synthesis of the related selenium/tellurium analogues as well as their use in peptide synthesis and protein expression. Advances in synthetic methodologies and recombinant technologies and their combined applications in native and expressed protein ligation allows the isomorphous character of selenium- and tellurium-containing amino acids to be exploited for production of heavy metal mutants of proteins and thus to facilitate the phasing problem in x-ray crystallography. In addition, selenocysteine has been recognized as an ideal tool for the production of selenoenzymes with new catalytic activities. Moreover, the fully isomorphous character of disulfide replacement with diselenide is well suited to increase the robustness of cystine frameworks in cystine-rich peptides and proteins and for the de novo design of even non-native cystine frameworks by exploiting the highly negative redox potential of selenols.

Reassignment of sense codons in vivo

The genetic code maps one or more of the 61 sense codons onto a set of 20 canonical amino acids. Reassignment of sense codons to non-canonical amino acids in model organisms such as Escherichia coli has been achieved through manipulation of the cellular protein synthesis machinery. Specifically, control of amino acid pools, coupled with engineering of the aminoacyl-tRNA synthetase activity of the host, has enabled assignment of sense codons to a wide variety of non-canonical amino acids under conditions routinely used for expression of recombinant proteins. Codon reassignment is leading to important advances in protein engineering and bioorganic chemistry. Here we summarize some of those advances, and provide detailed protocols for codon reassignment.

Synthesis of single- and multiple-stranded cystine-rich peptides

The large abundance of bioactive single- and multiple-stranded cystine-rich peptides in nature has fostered the development of orthogonal thiol-protection schemes and of efficient chemistries for regioselective disulfide formation in synthetic replica for decades. In parallel to these entirely synthetic strategies, an increased knowledge of oxidative refolding mechanisms of proteins has been accumulated, and the collective experience with air oxidation of cysteine-rich peptides into their native disulfide frameworks have largely confirmed Anfinsen's principle of the self-assembly of polypeptide chains. In fact, a continuously growing number of cysteine-rich bioactive peptides from the most diverse sources and with differing cysteine patterns were found to retain the critical sequence-encoded structural information for correct oxidative folding into the native structures as dominant isomers, although in the biosynthetic pathways the mature peptide forms are mostly generated by posttranslational processing of folded precursors. Such self-assembly processes can be optimized by opportune manipulation of the experimental conditions or by induction of productive intermediates. But there are also numerous cases where folding and disulfide formation are thermodynamically not coupled and where the application of a defined succession of regioselective cysteine pairings still represents the method of choice to install the desired native or non-native cystine frameworks. Among our contributions to the state of the art in the synthesis of cystine-rich peptides, we have mainly addressed the induction of correct oxidative refolding of single-stranded cysteine-rich peptides into their native structures by the use of selenocysteine and suitable strategies for disulfide-mediated assembly of monomers into defined oligomers as mimics of homo- and heterotrimeric collagens as a synthetic approach for the development of new biomaterials.

Designing novel spectral classes of proteins with a tryptophan-expanded genetic code

Fluorescence methods are now well-established and powerful tools to study biological macromolecules. The canonical amino acid tryptophan (Trp), encoded by a single UGG triplet, is the main reporter of intrinsic fluorescence properties of most natural proteins and peptides and is thus an attractive target for tailoring their spectral properties. Recent advances in research have provided substantial evidence that the natural protein translational machinery can be genetically reprogrammed to introduce a large number of non-coded (i.e. noncanonical) Trp analogues and surrogates into various proteins. Especially attractive targets for such an engineering approach are fluorescent proteins in which the chromophore is formed post-translationally from an amino acid sequence, like the green fluorescent protein from Aequorea victoria. With the currently available translationally active fluoro-, hydroxy-, amino-, halogen-, and chalcogen-containing Trp analogues and surrogates, the traditional methods for protein engineering and design can be supplemented or even fully replaced by these novel approaches. Future research will provide a further increase in the number of Trp-like amino acids that are available for redesign (by engineering of the genetic code) of native Trp residues and enable novel strategies to generate proteins with tailored spectral properties.

Expanding the Genetic Code

Although chemists can synthesize virtually any small organic molecule, our ability to rationally manipulate the structures of proteins is quite limited, despite their involvement in virtually every life process. For most proteins, modifications are largely restricted to substitutions among the common 20 amino acids. Herein we describe recent advances that make it possible to add new building blocks to the genetic codes of both prokaryotic and eukaryotic organisms. Over 30 novel amino acids have been genetically encoded in response to unique triplet and quadruplet codons including fluorescent, photoreactive, and redox-active amino acids, glycosylated amino acids, and amino acids with keto, azido, acetylenic, and heavy-atom-containing side chains. By removing the limitations imposed by the existing 20 amino acid code, it should be possible to generate proteins and perhaps entire organisms with new or enhanced properties.

Prolegomena to Future Experimental Efforts on Genetic Code Engineering by Expanding Its Amino Acid Repertoire

Protein synthesis and its relation to the genetic code was for a long time a central issue in biology. Rapid experimental progress throughout the past decade, crowned with the recently elucidated ribosomal structures, provided an almost complete description of this process. In addition important experiments provided solid evidence that the natural protein translation machinery can be reprogrammed to encode genetically a vast number of non-coded (i.e. noncanonical) amino acids. Indeed, in the set of 20 canonical amino acids as prescribed by the universal genetic code, many desirable functionalities, such as halogeno, keto, cyano, azido, nitroso, nitro, and silyl groups, as well as C=C or C[triple bond]C bonds, are absent. The ability to encode genetically such chemical diversity will enable us to reprogram living cells, such as bacteria, to express tailor-made proteins exhibiting functional diversity. Accordingly, genetic code engineering has developed into an exciting emerging research field at the interface of biology, chemistry, and physics.

Non-canonical amino acids in protein engineering

Methods for engineering proteins that contain non-canonical amino acids have advanced rapidly in the past few years. Novel amino acids can be introduced into recombinant proteins in either a residue-specific or site-specific fashion. The methods are complementary: residue-specific incorporation allows engineering of the overall physical and chemical behavior of proteins and protein-like macromolecules, whereas site-specific methods allow mechanistic questions to be probed in atomistic detail. Challenges remain in the engineering of the translational apparatus and in the design of schemes that can be used to encode both canonical and non-canonical amino acids.

Selenomethionine and selenocysteine double labeling strategy for crystallographic phasing

A protocol for the quantitative incorporation of both selenomethionine and selenocysteine into recombinant proteins overexpressed in Escherichia coli is described. This methodology is based on the use of a suitable cysteine auxotrophic strain and a minimal medium supplemented with selenium-labeled methionine and cysteine. The proteins chosen for these studies are the cathelin-like motif of protegrin-3 and a nucleoside-diphosphate kinase. Analysis of the purified proteins by electrospray mass spectrometry and X-ray crystallography revealed that both cysteine and methionine residues were isomorphously replaced by selenocysteine and selenomethionine. Moreover, selenocysteines allowed the formation of unstrained and stable diselenide bridges in place of the canonical disulfide bonds. In addition, we showed that NDP kinase contains a selenocysteine adduct on Cys122. This novel selenium double-labeling method is proposed as a general approach to increase the efficiency of the MAD technique used for phase determination in protein crystallography.

Probing the role of tryptophans in Aequorea victoria green fluorescent proteins with an expanded genetic code

The expanded genetic code in combination with site-directed mutagenesis was used to probe spectroscopic and structural roles of tryptophan (Trp) residues in Aequorea victoria green fluorescent proteins (avGFPs). Nine different halogen-, chalcogen-, and methyl-containing Trp isosteric analogues and surrogates were incorporated into avGFPs containing indole moieties in, and outside of, the chromophore, by the use of the selective pressure incorporation method. Such isosteric replacements introduced minimal local geometry changes in indole moieties, often to the level of single atomic exchange ('atomic mutation') and do not affect three-dimensional structures of avGFPs but induce changes in spectral properties. Our approach offers a new platform to re-evaluate issues like resonance transfer, mechanisms of chromophore formation and maturation, as well as the importance of local geometry and weak sulphur-aromatic interactions for avGFP spectral properties and structural stability. The library of novel tailor-made avGFP mutants and variants generated in this work has demonstrated not only the potentials of the expanded genetic code to study spectroscopic functions, but also a new approach to generate tailor-made proteins with interesting and useful spectral properties.

Synthesis and incorporation of [6,7]-selenatryptophan into dihydrofolate reductase

Until recently, the only selenium containing amino acid which could be used to completely substitute for a wild type amino acid was selenomethionine (SeMet). In the last decade the preparation of SeMet containing proteins has proved to be valuable tools in the determination of three-dimensional structure by multiwavelength anomalous diffraction (MAD) techniques. The potential utility of a selenium containing tryptophan analog, beta-seleno[3,2-b]pyrrolyl-L-alanine ([4,5]selenatryptophan), has recently been demonstrated in the literature. This finding shows promise for the bioincorporation of its positional isomer, beta-selenolo[2,3-b]pyrrolyl-L-alanine ([6,7]selenatryptophan), thereby adding to the essential arsenal of selenium-containing amino acids for use in the characterization of proteins. The synthesis of [6,7]selenatryptophan by enzymatic biotransformation with tryptophan synthase from selenolo[2,3-b]pyrrole was carried out as well as its characterization by NMR spectroscopy and thin layer chromatography. Selenatryptophyl dihydrofolate reductase ([6,7]SeTrp-DHFR) was then synthesized in vivo, purified, and found to exhibit no perturbations to enzymatic activity.