Structure and chemistry of 4-methylideneimidazole-5-one containing enzymes

Exploring the Kinetics and Thermodynamics of a Novel Histidine Ammonia-Lyase from Geobacillus kaustophilus

Histidine ammonia-lyase (HAL) plays a pivotal role in the non-oxidative deamination of L-histidine to produce trans-urocanic, a crucial process in amino acid metabolism. This study examines the cloning, purification, and biochemical characterization of a novel HAL from Geobacillus kaustophilus (GkHAL) and eight active site mutants to assess their effects on substrate binding, catalysis, thermostability, and secondary structure. The GkHAL enzyme was successfully overexpressed and purified to homogeneity. Its primary sequence displayed 40.7% to 43.7% similarity with other known HALs and shared the same oligomeric structure in solution. Kinetic assays showed that GkHAL has optimal activity at 85 °C and pH 8.5, with high thermal stability even after preincubation at high temperatures. Mutations at Y52, H82, N194, and E411 resulted in a complete loss of catalytic activity, underscoring their essential role in enzyme function, while mutations at residues Q274, R280, and F325 did not abolish activity but did reduce catalytic efficiency. Notably, mutants R280K and F325Y displayed novel activity with L-histidinamide, expanding the substrate specificity of HAL enzymes. Circular dichroism (CD) analysis showed minor secondary structure changes in the mutants but no significant effect on global GkHAL folding. These findings suggest that GkHAL could be a promising candidate for potential biotechnological applications.

Structure and Substrate Specificity of S-Methyl Thiourocanate Hydratase

Nicotinamide adenine dinucleotide (NAD+) is a common cofactor in enzyme-catalyzed reactions that involve hydride transfers. In contrast, urocanase and urocanase-like enzymes use NAD+ for covalent electrophilic catalysis. Deciphering avenues by which this unusual catalytic strategy has diversified by evolution may point to approaches for the design of novel enzymes. In this report, we describe the S-methyl thiourocanate hydratase (S-Me-TUC) from Variovorax sp. RA8 as a novel member of this small family of NAD+-dependent hydratases. This enzyme catalyzes the 1,4-addition of water to S-methyl thiourocanate as the second step in the catabolism of S-methyl ergothioneine. The crystal structure of this enzyme in complex with the cofactor and a product analogue identifies critical sequence motifs that explain the narrow and nonoverlapping substrate scopes of S-methyl thiourocanate-, urocanate-, thiourocanate-, and Nτ-methyl urocanate-specific hydratases. The discovery of a S-methyl ergothioneine catabolic pathway also suggests that S-methylation or alkylation may be a significant activity in the biology of ergothioneine.

On the evolution of coenzyme biosynthesis

Covering: up to 2022The report provides a broad approach to deciphering the evolution of coenzyme biosynthetic pathways. Here, these various pathways are analyzed with respect to the coenzymes required for this purpose. Coenzymes whose biosynthesis relies on a large number of coenzyme-mediated reactions probably appeared on the scene at a later stage of biological evolution, whereas the biosyntheses of pyridoxal phosphate (PLP) and nicotinamide (NAD+) require little additional coenzymatic support and are therefore most likely very ancient biosynthetic pathways.

Unlocking New Reactivities in Enzymes by Iminium Catalysis

The application of biocatalysis in conquering challenging synthesis requires the constant input of new enzymes. Developing novel biocatalysts by absorbing catalysis modes from synthetic chemistry has yielded fruitful new-to-nature enzymes. Organocatalysis was originally bio-inspired and has become the third pillar of asymmetric catalysis. Transferring organocatalytic reactions back to enzyme platforms is a promising approach for biocatalyst creation. Herein, we summarize recent developments in the design of novel biocatalysts that adopt iminium catalysis, a fundamental branch in organocatalysis. By repurposing existing enzymes or constructing artificial enzymes, various biocatalysts for iminium catalysis have been created and optimized via protein engineering to promote valuable abiological transformations. Recent advances in iminium biocatalysis illustrate the power of combining chemomimetic biocatalyst design and directed evolution to generate useful new-to-nature enzymes.

Bacterial Degradation of Nτ-Methylhistidine

The first three enzymatic steps by which organisms degrade histidine are universally conserved. A histidine ammonia-lyase (EC 4.3.1.3) catalyzes 1,2-elimination of the α-amino group from l-histidine; a urocanate hydratase (EC 4.2.1.49) converts urocanate to 4-imidazolone-5-propionate, and this intermediate is hydrolyzed to N-formimino-l-glutamate by an imidazolonepropionase (EC 3.5.2.7). Surprisingly, despite broad distribution in many species from all kingdoms of life, this pathway has rarely served as a template for the evolution of other metabolic processes. The only other known pathway with a similar logic is that of ergothioneine degradation. In this report, we describe a new addition to this exclusive collection. We show that the firmicute Bacillus terra and other soil-dwelling bacteria contain enzymes for the degradation of Nτ-methylhistidine to l-glutamate and N-methylformamide. Our results indicate that in some environments, Nτ-methylhistidine can accumulate to concentrations that make its efficient degradation a competitive skill. In addition, this process describes the first biogenic source of N-methylformamide.

Transcriptome of the coralline alga Calliarthron tuberculosum (Corallinales, Rhodophyta) reveals convergent evolution of a partial lignin biosynthesis pathway

The discovery of lignins in the coralline red alga Calliarthron tuberculosum raised new questions about the deep evolution of lignin biosynthesis. Here we present the transcriptome of C. tuberculosum supported with newly generated genomic data to identify gene candidates from the monolignol biosynthetic pathway using a combination of sequence similarity-based methods. We identified candidates in the monolignol biosynthesis pathway for the genes 4CL, CCR, CAD, CCoAOMT, and CSE but did not identify candidates for PAL, CYP450 (F5H, C3H, C4H), HCT, and COMT. In gene tree analysis, we present evidence that these gene candidates evolved independently from their land plant counterparts, suggesting convergent evolution of a complex multistep lignin biosynthetic pathway in this red algal lineage. Additionally, we provide tools to extract metabolic pathways and genes from the newly generated transcriptomic and genomic datasets. Using these methods, we extracted genes related to sucrose metabolism and calcification. Ultimately, this transcriptome will provide a foundation for further genetic and experimental studies of calcifying red algae.

Biomedical applications of microbial phenylalanine ammonia lyase: Current status and future prospects

Phenylalanine ammonia lyase (PAL) has recently emerged as an important therapeutic enzyme with several biomedical applications. The enzyme catabolizes l-phenylalanine to trans-cinnamate and ammonia. PAL is widely distributed in higher plants, some algae, ferns, and microorganisms, but absent in animals. Although microbial PAL has been extensively exploited in the past for producing industrially important metabolites, its high substrate specificity and catalytic efficacy lately spurred interest in its biomedical applications. PEG-PAL drug named Palynziq™, isolated from Anabaena variabilis has been recently approved for the treatment of adult phenylketonuria (PKU) patients. Further, it has exhibited high potency in regressing tumors and treating tyrosine related metabolic abnormalities like tyrosinemia. Several therapeutically valuable metabolites have been biosynthesized via its catalytic action including dietary supplements, antimicrobial peptides, aspartame, amino-acids, and their derivatives. This review focuses on all the prospective biomedical applications of PAL. It also provides an overview of the structure, production parameters, and various strategies to improve the therapeutic potential of this enzyme. Engineered PAL with improved pharmacodynamic and pharmacokinetic properties will further establish this enzyme as a highly efficient biological drug.

Directed evolution of Anabaena variabilis phenylalanine ammonia-lyase (PAL) identifies mutants with enhanced activities

There is broad interest in engineering phenylalanine ammonia-lyase (PAL) for its biocatalytic applications in industry and medicine. While site-specific mutagenesis has been employed to improve PAL stability or substrate specificity, combinatorial techniques are poorly explored. Here, we report development of a directed evolution technique to engineer PAL enzymes. Central to this approach is a high-throughput enrichment that couples E. coli growth to PAL activity. Starting with the PAL used in the formulation of pegvaliase for PKU therapy, we report previously unidentified mutations that increase turnover frequency almost twofold after only a single round of engineering.

Exploring the therapeutic potential of modern and ancestral phenylalanine/tyrosine ammonia-lyases as supplementary treatment of hereditary tyrosinemia

Phenylalanine/tyrosine ammonia-lyases (PAL/TALs) have been approved by the FDA for treatment of phenylketonuria and may harbour potential for complementary treatment of hereditary tyrosinemia Type I. Herein, we explore ancestral sequence reconstruction as an enzyme engineering tool to enhance the therapeutic potential of PAL/TALs. We reconstructed putative ancestors from fungi and compared their catalytic activity and stability to two modern fungal PAL/TALs. Surprisingly, most putative ancestors could be expressed as functional tetramers in Escherichia coli and thus retained their ability to oligomerize. All ancestral enzymes displayed increased thermostability compared to both modern enzymes, however, the increase in thermostability was accompanied by a loss in catalytic turnover. One reconstructed ancestral enzyme in particular could be interesting for further drug development, as its ratio of specific activities is more favourable towards tyrosine and it is more thermostable than both modern enzymes. Moreover, long-term stability assessment showed that this variant retained substantially more activity after prolonged incubation at 25 °C and 37 °C, as well as an increased resistance to incubation at 60 °C. Both of these factors are indicative of an extended shelf-life of biopharmaceuticals. We believe that ancestral sequence reconstruction has potential for enhancing the properties of enzyme therapeutics, especially with respect to stability. This work further illustrates that resurrection of putative ancestral oligomeric proteins is feasible and provides insight into the extent of conservation of a functional oligomerization surface area from ancestor to modern enzyme.

Investigation into isomerization reaction of phenylalanine aminomutase from Pantoea agglomerans

Phenylalanine aminomutase (PaPAM) from Pantoea agglomerans is a member of the MIO (4-methylene-imidazol-5-one) family of enzymes, which isomerizes α-phenylalanine to β-phenylalanine, and could be used to synthesize unnatural β-arylalanine. However, the mechanism of isomerization reaction is not clear. To investigate the mechanism, the gene (pam), which encodes PaPAM, was first expressed in E.coli, and recombinant PaPAM was prepared using affinity chromatography. Then, ¹⁵N-(2S)-α-phenylalanine, (2S)-(3-²H₂)-α-phenylalanine and (2S,3S)-[2,3-²H₂]-α-phenylalanine were used as substrates to analyze the mechanism of isomerization reaction. The results of MS and NMR showed that the isomerization reaction was performed through the intramolecular exchange of NH₂ with pro-3R hydrogen of α-phenylalanine. The PaPAM shuttles the α-NH₂ of α-phenylalanine to β site to replace the pro-3R hydrogen. Simultaneously, the pro-3R hydrogen is shifted to α site to produce β-phenylalanine. Furthermore, a key residue, Phe at position 455 in the active site, was determined to control the exchange way using molecular docking and sequence alignment of MIO family enzymes. The results indicated that the key 455 Phe residue is involved in changing the binding orientation of the carboxyl group of the intermediate trans-cinnamic acid to control the NH₂-H pair exchange.

Dehydroamino acids: chemical multi-tools for late-stage diversification

α,β-Dehydroamino acids (dhAAs) are noncanonical amino acids that are found in a wide array of natural products and can be easily installed into peptides and proteins. dhAAs exhibit remarkable synthetic flexibility, readily undergoing a number of reactions, such as polar and single-electron additions, transition metal catalyzed cross-couplings, and cycloadditions. Because of the relatively mild conditions required for many of these reactions, dhAAs are increasingly being used as orthogonal chemical handles for late-stage modification of biomolecules. Still, only a fraction of the chemical reactivity of dhAAs has been exploited in such biorthogonal applications. Herein, we provide an overview of the broad spectrum of chemical reactivity of dhAAs, with special emphasis on recent efforts to adapt such transformations for biomolecules such as natural products, peptides, and proteins. We also discuss examples of enzymes from natural product biosynthetic pathways that have been found to catalyze many similar reactions; these enzymes provide mild, regio- and stereoselective, biocatalytic alternatives for future development. We anticipate that the continued investigation of the innate reactivity of dhAAs will furnish a diverse portfolio dhAA-based chemistries for use in chemical biology and drug discovery.

Directed Evolution of a Designer Enzyme Featuring an Unnatural Catalytic Amino Acid

The impressive rate accelerations that enzymes display in nature often result from boosting the inherent catalytic activities of side chains by their precise positioning inside a protein binding pocket. Such fine-tuning is also possible for catalytic unnatural amino acids. Specifically, the directed evolution of a recently described designer enzyme, which utilizes an aniline side chain to promote a model hydrazone formation reaction, is reported. Consecutive rounds of directed evolution identified several mutations in the promiscuous binding pocket, in which the unnatural amino acid is embedded in the starting catalyst. When combined, these mutations boost the turnover frequency (kcat ) of the designer enzyme by almost 100-fold. This results from strengthening the catalytic contribution of the unnatural amino acid, as the engineered designer enzymes outperform variants, in which the aniline side chain is replaced with a catalytically inactive tyrosine residue, by more than 200-fold.

A designer enzyme for hydrazone and oxime formation featuring an unnatural catalytic aniline residue

Creating designer enzymes with the ability to catalyse abiological transformations is a formidable challenge. Efforts toward this goal typically consider only canonical amino acids in the initial design process. However, incorporating unnatural amino acids that feature uniquely reactive side chains could significantly expand the catalytic repertoire of designer enzymes. To explore the potential of such artificial building blocks for enzyme design, here we selected p-aminophenylalanine as a potentially novel catalytic residue. We demonstrate that the catalytic activity of the aniline side chain for hydrazone and oxime formation reactions is increased by embedding p-aminophenylalanine into the hydrophobic pore of the multidrug transcriptional regulator from Lactococcus lactis. Both the recruitment of reactants by the promiscuous binding pocket and a judiciously placed aniline that functions as a catalytic residue contribute to the success of the identified artificial enzyme. We anticipate that our design strategy will prove rewarding to significantly expand the catalytic repertoire of designer enzymes in the future.

Functional characterization of phenylalanine ammonia-lyase- and cinnamate 4-hydroxylase-encoding genes from Lycoris radiata, a galanthamine-producing plant

Galanthamine (GAL), the well-known Amaryllidaceae alkaloid, is a clinically used drug for the treatment of Alzheimer's disease. L-Phenylalanine (Phe) and trans-cinnamic acid (CA) were enzymatically transformed into the catechol portion of GAL. Herein, a Phe ammonia-lyase-encoding gene LrPAL3 and a cinnamate 4-hydroxylase-encoding gene LrC4H were cloned from Lycoris radiata, a GAL-producing plant. LrPAL3 was overexpressed in Escherichia coli and purified to homogeneity. LrPAL3 catalyzes the forward deamination conversion of L-Phe into trans-CA. The 3-chloro- and 4-fluoro-L-Phe were deaminated to generate the corresponding 3-chloro- and 4-fluoro-trans-CA by LrPAL3. LrPAL3-catalyzed reverse hydroamination was confirmed by the conversion of trans-CA into L-Phe with exceptional regio- and stereo-selectivity. LrC4H was overexpressed in E. coli with tCamCPR, a cytochrome P450 reductase-encoding gene. LrC4H catalyzes the regioselective para-hydroxylation on trans-CA to form p-coumaric acid. The transcriptional levels of both LrPAL3 and LrC4H were positively associated with the GAL contents within the leaves and flowers of L. radiata, which suggested that their expression and function are co-regulated and involved in the biosynthesis of GAL. The present investigations on the biosynthetic genes of GAL will promote the development of synthetic biology platforms for this kind of important drug via metabolic engineering.

Evolutionary convergence in the biosyntheses of the imidazole moieties of histidine and purines

Background

The imidazole group is an ubiquitous chemical motif present in several key types of biomolecules. It is a structural moiety of purines, and plays a central role in biological catalysis as part of the side-chain of histidine, the amino acid most frequently found in the catalytic site of enzymes. Histidine biosynthesis starts with both ATP and the pentose phosphoribosyl pyrophosphate (PRPP), which is also the precursor for the de novo synthesis of purines. These two anabolic pathways are also connected by the imidazole intermediate 5-aminoimidazole-4-carboxamide ribotide (AICAR), which is synthesized in both routes but used only in purine biosynthesis. Rather surprisingly, the imidazole moieties of histidine and purines are synthesized by different, non-homologous enzymes. As discussed here, this phenomenon can be understood as a case of functional molecular convergence.

Results

In this work, we analyze these polyphyletic processes and argue that the independent origin of the corresponding enzymes is best explained by the differences in the function of each of the molecules to which the imidazole moiety is attached. Since the imidazole present in histidine is a catalytic moiety, its chemical arrangement allows it to act as an acid or a base. On the contrary, the de novo biosynthesis of purines starts with an activated ribose and all the successive intermediates are ribotides, with the key β-glycosidic bondage joining the ribose and the imidazole moiety. This prevents purine ribonucleotides to exhibit any imidazole-dependent catalytic activity, and may have been the critical trait for the evolution of two separate imidazole-synthesizing-enzymes. We also suggest that, in evolutionary terms, the biosynthesis of purines predated that of histidine.

Conclusions

As reviewed here, other biosynthetic routes for imidazole molecules are also found in extant metabolism, including the autocatalytic cyclization that occurs during the formation of creatinine from creatine phosphate, as well as the internal cyclization of the Ala-Ser-Gly motif of some members of the ammonia-lyase and aminomutase families, that lead to the MIO cofactor. The diversity of imidazole-synthesizing pathways highlights the biological significance of this key chemical group, whose biosyntheses evolved independently several times.

Expanding the substrate scope of phenylalanine ammonia-lyase from Petroselinum crispum towards styrylalanines

The substrate scope of phenylalanine ammonia-lyase from Petroselinum crispum (PcPAL) towards the l-enantiomers of racemic styrylalanines rac-1a–d were extended by reshaping the aromatic binding pocket of the active site of PcPAL by point mutations of F137.

Ironing out their differences: dissecting the structural determinants of a phenylalanine aminomutase and ammonia lyase

Deciphering the structural features that functionally separate ammonia lyases from aminomutases is of interest because it may allow for the engineering of more efficient aminomutases for the synthesis of unnatural amino acids (e.g., β-amino acids). However, this has proved to be a major challenge that involves understanding the factors that influence their activity and regioselectivity differences. Herein, we report evidence of a structural determinant that dictates the activity differences between a phenylalanine ammonia lyase (PAL) and aminomutase (PAM). An inner loop region that closes the active sites of both PAM and PAL was mutated within PAM (PAM residues 77-97) in a stepwise approach to study the effects when the equivalent residue(s) found in the PAL loop were introduced into the PAM loop. Almost all of the single loop mutations triggered a lyase phenotype in PAM. Experimental and computational evidence suggest that the induced lyase features result from inner loop mobility enhancements, which are possibly caused by a 310-helix cluster, flanking α-helices, and hydrophobic interactions. These findings pinpoint the inner loop as a structural determinant of the lyase and mutase activities of PAM.

Asymmetric Preparation of prim-, sec-, and tert-Amines Employing Selected Biocatalysts

This account focuses on the application of ω-transaminases, lyases, and oxidases for the preparation of amines considering mainly work from our own lab. Examples are given to access α-chiral primary amines from the corresponding ketones as well as terminal amines from primary alcohols via a two-step biocascade. 2,6-Disubstituted piperidines, as examples for secondary amines, are prepared by biocatalytical regioselective asymmetric monoamination of designated diketones followed by spontaneous ring closure and a subsequent diastereoselective reduction step. Optically pure tert-amines such as berbines and N-methyl benzylisoquinolines are obtained by kinetic resolution via an enantioselective aerobic oxidative C-C bond formation.

A new member of the 4-methylideneimidazole-5-one-containing aminomutase family from the enediyne kedarcidin biosynthetic pathway

4-Methylideneimidazole-5-one (MIO)-containing aminomutases catalyze the conversion of L-α-amino acids to β-amino acids with either an (R) or an (S) configuration. L-phenylalanine and L-tyrosine are the only two natural substrates identified to date. The enediyne chromophore of the chromoprotein antitumor antibiotic kedarcidin (KED) harbors an (R)-2-aza-3-chloro-β-tyrosine moiety reminiscent of the (S)-3-chloro-5-hydroxy-β-tyrosine moiety of the C-1027 enediyne chromophore, the biosynthesis of which uncovered the first known MIO-containing aminomutase, SgcC4. Comparative analysis of the KED and C-1027 biosynthetic gene clusters inspired the proposal for (R)-2-aza-3-chloro-β-tyrosine biosynthesis starting from 2-aza-L-tyrosine, featuring KedY4 as a putative MIO-containing aminomutase. Here we report the biochemical characterization of KedY4, confirming its proposed role in KED biosynthesis. KedY4 is an MIO-containing aminomutase that stereospecifically catalyzes the conversion of 2-aza-L-tyrosine to (R)-2-aza-β-tyrosine, exhibiting no detectable activity toward 2-aza-L-phenylalanine or L-tyrosine as an alternative substrate. In contrast, SgcC4, which stereospecifically catalyzes the conversion of L-tyrosine to (S)-β-tyrosine in C-1027 biosynthesis, exhibits minimal activity with 2-aza-L-tyrosine as an alternative substrate but generating (S)-2-aza-β-tyrosine, a product with the opposite stereochemistry of KedY4. This report of KedY4 broadens the scope of known substrates for the MIO-containing aminomutase family, and comparative studies of KedY4 and SgcC4 provide an outstanding opportunity to examine how MIO-containing aminomutases control substrate specificity and product enantioselectivity.

4-methylideneimidazole-5-one-containing aminomutases in enediyne biosynthesis

Many natural products contain unusual aromatic β-amino acids or moieties derived therefrom. The biosynthesis of these β-amino acids was first elucidated during a biosynthetic study of the enediyne antitumor antibiotic C-1027, when an enzyme, SgcC4, was discovered to convert L-tyrosine to (S)-β-tyrosine. SgcC4 is similar in sequence and structure to 4-methylideneimidazole-5-one (MIO)-containing ammonia lyases. Whereas the ammonia lyases use the electrophilic power of the MIO group to catalyze the release of ammonia from aromatic amino acids to generate α,β-unsaturated carboxylic acids as final products, SgcC4 retains the α,β-unsaturated carboxylic acid and amine as intermediates and reappends the amino group to the β-carbon, affording a β-amino acid as the final product. The study of SgcC4 led to the subsequent discovery of other MIO-containing aminomutases with altered substrate specificity and product stereochemistry, including MdpC4 from the biosynthetic pathway of the enediyne antitumor antibiotic maduropeptin. This chapter describes protocols for the enzymatic and structural characterization of these MIO-containing aminomutases as exemplified by SgcC4 and MdpC4. These protocols are applicable to the study of other aminomutases.

Aminomutases: mechanistic diversity, biotechnological applications and future perspectives

Aminomutases carry out the chemically challenging exchange of a hydrogen atom and an amine substituent present on neighboring carbon atoms. In recent years, aminomutases have been intensively investigated for their biophysical, structural and mechanistic characteristics. The reactions catalyzed by these enzymes have considerable potential for biotechnological applications. Here, we present an overview of this diverse group of enzymes, with a focus on enzymatic mechanisms and recent developments in their use in applied biocatalysis.

Mutational analysis of phenylalanine ammonia lyase to improve reactions rates for various substrates

Phenylalanine ammonia lyases (PAL) catalyze the reversible, non-reductive amination of trans-cinnamic acid to l-phenylalanine in the presence of high ammonia concentrations. Since neither cofactor recycling nor other additives are needed and by this asymmetric synthesis theoretical yields of 100% can be reached, it is an interesting reaction for industrial processes. In this study we demonstrate the superior properties of p-nitro-cinnamic acid (p-n-CA) in the amination reaction using the PAL from Petroselinum crispum (pcPAL). By focused-directed evolution, three mutants were identified showing increased reaction rates and decreased substrate inhibition. Together, the F137V mutant with p-n-CA showed a 15-fold increased reaction rate compared with the pcPAL WT with the natural cinnamic acid. The high reaction rates were also proven in preparative scale experiments. Activities towards other p-substituted cinnamic acids showing different electronic effects of the substituent were analyzed. Focused-directed evolution around the carboxylic acid- and amine-binding site always decreased PAL activity, due to a sensitive H-bond network.

Engineering of an enantioselective tyrosine aminomutase by mutation of a single active site residue in phenylalanine aminomutase

By replacing a single active-site residue Cys107 with Ser in phenylalanine aminomutase (PAM), the enzyme gained tyrosine aminomutase (TAM) activity while retaining PAM activity and high enantioselectivity. This engineered enantioselective TAM also catalyzed formation of β-tyrosine from p-coumaric acid and may prove to be useful for the synthesis of enantiopure β-tyrosine and its derivatives.

Probing the active site of MIO-dependent aminomutases, key catalysts in the biosynthesis of beta-amino acids incorporated in secondary metabolites

The tyrosine aminomutase SgTAM produces (S)-ss-tyrosine from L-tyrosine in the biosynthesis of the enediyne antitumor antibiotic C-1027. This conversion is promoted by the methylideneimidazole-5-one (MIO) prosthetic group. MIO was first identified in the homologous family of ammonia lyases, which deaminate aromatic amino acids to form alpha,ss-unsaturated carboxylates. Studies of substrate specificity have been described for lyases but there have been limited reports in altering the substrate specificity of aminomutases. Furthermore, it remains unclear as to what structural properties are responsible for catalyzing the presumed readdition of the amino group into the alpha,ss-unsaturated intermediates to form ss-amino acids. Attempts to elucidate specificity and mechanistic determinants of SgTAM have also proved to be difficult as it is recalcitrant to perturbations to the active site via mutagenesis. An X-ray cocrystal structure of the SgTAM mutant of the catalytic base with L-tyrosine verified important substrate binding residues as well as the enzymatic base. Further mutagenesis revealed that removal of these crucial interactions renders the enzyme inactive. Proposed structural determinants for mutase activity probed via mutagenesis, time-point assays and X-ray crystallography revealed a complicated role for these residues in maintaining key quaternary structure properties that aid in catalysis.