Evolution of enzymatic activities in the enolase superfamily: identification of the general acid catalyst in the active site of D-glucarate dehydratase from Escherichia coli

The role of complex interactions between the intestinal flora and host in regulating intestinal homeostasis and inflammatory bowel disease

Pharmacological treatment of inflammatory bowel disease (IBD) is inefficient and difficult to discontinue appropriately, and enterobacterial interactions are expected to provide a new target for the treatment of IBD. We collected recent studies on the enterobacterial interactions among the host, enterobacteria, and their metabolite products and discuss potential therapeutic options. Intestinal flora interactions in IBD are affected in the reduced bacterial diversity, impact the immune system and are influenced by multiple factors such as host genetics and diet. Enterobacterial metabolites such as SCFAs, bile acids, and tryptophan also play important roles in enterobacterial interactions, especially in the progression of IBD. Therapeutically, a wide range of sources of probiotics and prebiotics exhibit potential therapeutic benefit in IBD through enterobacterial interactions, and some have gained wide recognition as adjuvant drugs. Different dietary patterns and foods, especially functional foods, are novel therapeutic modalities that distinguish pro-and prebiotics from traditional medications. Combined studies with food science may significantly improve the therapeutic experience of patients with IBD. In this review, we provide a brief overview of the role of enterobacteria and their metabolites in enterobacterial interactions, discuss the advantages and disadvantages of the potential therapeutic options derived from such metabolites, and postulate directions for further research.

The Birth of Genomic Enzymology: Discovery of the Mechanistically Diverse Enolase Superfamily

Enzymes are categorized into superfamilies by sequence, structural, and mechanistic similarities. The evolutionary implications can be profound. Until the mid-1990s, the approach was fragmented largely due to limited sequence and structural data. However, in 1996, Babbitt et al. published a paper in Biochemistry that demonstrated the potential power of mechanistically diverse superfamilies to identify common ancestry, predict function, and, in some cases, predict specificity. This Perspective describes the findings of the original work and reviews the current understanding of structure and mechanism in the founding family members. The outcomes of the genomic enzymology approach have reached far beyond the functional assignment of members of the enolase superfamily, inspiring the study of superfamilies and the adoption of sequence similarity networks and genome context and yielding fundamental insights into enzyme evolution.

Microbiome Analysis of Mucosal Ileoanal Pouch in Ulcerative Colitis Patients Revealed Impairment of the Pouches Immunometabolites

The pathogenesis of ulcerative colitis (UC) is unknown, although genetic loci and altered gut microbiota have been implicated. Up to a third of patients with moderate to severe UC require proctocolectomy with ileal pouch ano-anastomosis (IPAA). We aimed to explore the mucosal microbiota of UC patients who underwent IPAA.

METHODS

For microbiome analysis, mucosal specimens were collected from 34 IPAA individuals. Endoscopic and histological examinations of IPAA were normal in 21 cases, while pouchitis was in 13 patients. 19 specimens from the healthy control (10 from colonic and 9 from ileum) were also analyzed. Data were analyzed using an ensemble of software packages: QIIME2, coda-lasso, clr-lasso, PICRUSt2, and ALDEx2.

RESULTS

IPAA specimens had significantly lower bacterial diversity as compared to normal. The microbial composition of the normal pouch was also decreased also when compared to pouchitis. Faecalibacterium prausnitzii, Gemmiger formicilis, Blautia obeum, Ruminococcus torques, Dorea formicigenerans, and an unknown species from Roseburia were the most uncommon in pouch/pouchitis, while an unknown species from Enterobacteriaceae was over-represented. Propionibacterium acnes and Enterobacteriaceae were the species most abundant in the pouchitis and in the normal pouch, respectively. Predicted metabolic pathways among the IPAA bacterial communities revealed an important role of immunometabolites such as SCFA, butyrate, and amino acids.

CONCLUSIONS

Our findings showed specific bacterial signature hallmarks of dysbiosis and could represent bacterial biomarkers in IPAA patients useful to develop novel treatments in the future by modulating the gut microbiota through the administration of probiotic immunometabolites-producing bacterial strains and the addition of specific prebiotics and the faecal microbiota transplantation.

Nothing lasts forever: understanding microbial biodegradation of polyfluorinated compounds and perfluorinated alkyl substances

Poly‐ and perfluorinated chemicals, including perfluorinated alkyl substances (PFAS), are pervasive in today’s society, with a negative impact on human and ecosystem health continually emerging. These chemicals are now subject to strict government regulations, leading to costly environmental remediation efforts. Commercial polyfluorinated compounds have been called ‘forever chemicals’ due to their strong resistance to biological and chemical degradation. Environmental cleanup by bioremediation is not considered practical currently. Implementation of bioremediation will require uncovering and understanding the rare microbial successes in degrading these compounds. This review discusses the underlying reasons why microbial degradation of heavily fluorinated compounds is rare. Fluorinated and chlorinated compounds are very different with respect to chemistry and microbial physiology. Moreover, the end product of biodegradation, fluoride, is much more toxic than chloride. It is imperative to understand these limitations, and elucidate physiological mechanisms of defluorination, in order to better discover, study, and engineer bacteria that can efficiently degrade polyfluorinated compounds.

Whole exome sequencing analyses reveal gene-microbiota interactions in the context of IBD

OBJECTIVE

Both the gut microbiome and host genetics are known to play significant roles in the pathogenesis of IBD. However, the interaction between these two factors and its implications in the aetiology of IBD remain underexplored. Here, we report on the influence of host genetics on the gut microbiome in IBD.

DESIGN

To evaluate the impact of host genetics on the gut microbiota of patients with IBD, we combined whole exome sequencing of the host genome and whole genome shotgun sequencing of 1464 faecal samples from 525 patients with IBD and 939 population-based controls. We followed a four-step analysis: (1) exome-wide microbial quantitative trait loci (mbQTL) analyses, (2) a targeted approach focusing on IBD-associated genomic regions and protein truncating variants (PTVs, minor allele frequency (MAF) >5%), (3) gene-based burden tests on PTVs with MAF <5% and exome copy number variations (CNVs) with site frequency <1%, (4) joint analysis of both cohorts to identify the interactions between disease and host genetics.

RESULTS

We identified 12 mbQTLs, including variants in the IBD-associated genes IL17REL, MYRF, SEC16A and WDR78. For example, the decrease of the pathway acetyl-coenzyme A biosynthesis, which is involved in short chain fatty acids production, was associated with variants in the gene MYRF (false discovery rate <0.05). Changes in functional pathways involved in the metabolic potential were also observed in participants carrying rare PTVs or CNVs in CYP2D6, GPR151 and CD160 genes. These genes are known for their function in the immune system. Moreover, interaction analyses confirmed previously known IBD disease-specific mbQTLs in TNFSF15.

CONCLUSION

This study highlights that both common and rare genetic variants affecting the immune system are key factors in shaping the gut microbiota in the context of IBD and pinpoints towards potential mechanisms for disease treatment.

Enzymology of Alternative Carbohydrate Catabolic Pathways

The Embden–Meyerhof–Parnas (EMP) and Entner–Doudoroff (ED) pathways are considered the most abundant catabolic pathways found in microorganisms, and ED enzymes have been shown to also be widespread in cyanobacteria, algae and plants. In a large number of organisms, especially common strains used in molecular biology, these pathways account for the catabolism of glucose. The existence of pathways for other carbohydrates that are relevant to biomass utilization has been recognized as new strains have been characterized among thermophilic bacteria and Archaea that are able to transform simple polysaccharides from biomass to more complex and potentially valuable precursors for industrial microbiology. Many of the variants of the ED pathway have the key dehydratase enzyme involved in the oxidation of sugar derived from different families such as the enolase, IlvD/EDD and xylose-isomerase-like superfamilies. There are the variations in structure of proteins that have the same specificity and generally greater-than-expected substrate promiscuity. Typical biomass lignocellulose has an abundance of xylan, and four different pathways have been described, which include the Weimberg and Dahms pathways initially oxidizing xylose to xylono-gamma-lactone/xylonic acid, as well as the major xylose isomerase pathway. The recent realization that xylan constitutes a large proportion of biomass has generated interest in exploiting the compound for value-added precursors, but few chassis microorganisms can grow on xylose. Arabinose is part of lignocellulose biomass and can be metabolized with similar pathways to xylose, as well as an oxidative pathway. Like enzymes in many non-phosphorylative carbohydrate pathways, enzymes involved in L-arabinose pathways from bacteria and Archaea show metabolic and substrate promiscuity. A similar multiplicity of pathways was observed for other biomass-derived sugars such as L-rhamnose and L-fucose, but D-mannose appears to be distinct in that a non-phosphorylative version of the ED pathway has not been reported. Many bacteria and Archaea are able to grow on mannose but, as with other minor sugars, much of the information has been derived from whole cell studies with additional enzyme proteins being incorporated, and so far, only one synthetic pathway has been described. There appears to be a need for further discovery studies to clarify the general ability of many microorganisms to grow on the rarer sugars, as well as evaluation of the many gene copies displayed by marine bacteria.

A continuous assay for l-talarate/galactarate dehydratase using circular dichroism

l-Talarate/galactarate dehydratase (TGD) is a member of the enolase superfamily of enzymes and catalyzes the dehydration of either meso-galactarate or l-talarate to form 5-keto-4-deoxy-d-glucarate (5-KDG). To facilitate study of this enzyme and other galactarate dehydratases, a continuous circular dichroism-based assay has been developed. Using recombinant enzyme from Salmonella typhimurium (StTGD), the rates of StTGD-catalyzed conversion of m-galactarate to 5-KDG were determined by following the change in ellipticity at 323 nm. The apparent molar ellipticity ([θ]₃₂₃) for the 5-KDG formed was determined to be 202 ± 2 deg cm² dmol^-1, which was used to convert observed rates (Δθ/Δt) into concentration-dependent rates (Δc/Δt). The kinetic parameters K_m, k_cat, and k_cat/K_m were 0.38 ± 0.05 mM, 4.8 ± 0.1 s^-1, and 1.3 (±0.2) × 10⁴ M^-1s^-1, respectively. These values are in excellent agreement with those published previously [Yew, W.S. et al. (2007) Biochemistry46, 9564-9577] using a coupled assay system. To demonstrate the utility of the assay, the inhibition constant (K_i = 10.7 ± 0.4 mM) was determined for the competitive inhibitor tartronate. The continuous CD-based assay offers a practical and efficient alternative method to the coupled assay that requires access to 5-KDG aldolase, and to the labor-intensive, fixed-time assays.

Characterization and mutagenesis of two novel iron-sulphur cluster pentonate dehydratases

We describe here the identification and characterization of two novel enzymes belonging to the IlvD/EDD protein family, the D-xylonate dehydratase from Caulobacter crescentus, Cc XyDHT, (EC 4.2.1.82), and the L-arabonate dehydratase from Rhizobium leguminosarum bv. trifolii, Rl ArDHT (EC 4.2.1.25), that produce the corresponding 2-keto-3-deoxy-sugar acids. There is only a very limited amount of characterization data available on pentonate dehydratases, even though the enzymes from these oxidative pathways have potential applications with plant biomass pentose sugars. The two bacterial enzymes share 41 % amino acid sequence identity and were expressed and purified from Escherichia coli as homotetrameric proteins. Both dehydratases were shown to accept pentonate and hexonate sugar acids as their substrates and require Mg(2+) for their activity. Cc XyDHT displayed the highest activity on D-xylonate and D-gluconate, while Rl ArDHT functioned best on D-fuconate, L-arabonate and D-galactonate. The configuration of the OH groups at C2 and C3 position of the sugar acid were shown to be critical, and the C4 configuration also contributed substantially to the substrate recognition. The two enzymes were also shown to contain an iron-sulphur [Fe-S] cluster. Our phylogenetic analysis and mutagenesis studies demonstrated that the three conserved cysteine residues in the aldonic acid dehydratase group of IlvD/EDD family members, those of C60, C128 and C201 in Cc XyDHT, and of C59, C127 and C200 in Rl ArDHT, are needed for coordination of the [Fe-S] cluster. The iron-sulphur cluster was shown to be crucial for the catalytic activity (kcat) but not for the substrate binding (Km) of the two pentonate dehydratases.

Catabolism of Hexuronides, Hexuronates, Aldonates, and Aldarates

Following elucidation of the regulation of the lactose operon in Escherichia coli, studies on the metabolism of many sugars were initiated in the early 1960s. The catabolic pathways of D-gluconate and of the two hexuronates, D-glucuronate and D-galacturonate, were investigated. The post genomic era has renewed interest in the study of these sugar acids and allowed the complete characterization of the D-gluconate pathway and the discovery of the catabolic pathways for L-idonate, D-glucarate, galactarate, and ketogluconates. Among the various sugar acids that are utilized as sole carbon and energy sources to support growth of E. coli, galacturonate, glucuronate, and gluconate were shown to play an important role in the colonization of the mammalian large intestine. In the case of sugar acid degradation, the regulators often mediate negative control and are inactivated by interaction with a specific inducer, which is either the substrate or an intermediate of the catabolism. These regulators coordinate the synthesis of all the proteins involved in the same pathway and, in some cases, exert crosspathway control between related catabolic pathways. This is particularly well illustrated in the case of hexuronide and hexuronate catabolism. The structural genes encoding the different steps of hexuronate catabolism were identified by analysis of numerous mutants affected for growth with galacturonate or glucuronate. E. coli is able to use the diacid sugars D-glucarate and galactarate (an achiral compound) as sole carbon source for growth. Pyruvate and 2-phosphoglycerate are the final products of the D-glucarate/galactarate catabolism.

Enzymatic and structural characterization of rTSγ provides insights into the function of rTSβ

In humans, the gene encoding a reverse thymidylate synthase (rTS) is transcribed in the reverse direction of the gene encoding thymidylate synthase (TS) that is involved in DNA biosynthesis. Three isoforms are found: α, β, and γ, with the transcript of the α-isoform overlapping with that of TS. rTSβ has been of interest since the discovery of its overexpression in methotrexate and 5-fluorouracil resistant cell lines. Despite more than 20 years of study, none of the rTS isoforms have been biochemically or structurally characterized. In this study, we identified rTSγ as an l-fuconate dehydratase and determined its high-resolution crystal structure. Our data provide an explanation for the observed difference in enzymatic activities between rTSβ and rTSγ, enabling more informed proposals for the possible function of rTSβ in chemotherapeutic resistance.

A link between gut community metabolism and pathogenesis: molecular hydrogen-stimulated glucarate catabolism aids Salmonella virulence

Glucarate, an oxidized product of glucose, is a major serum organic acid in humans. Still, its role as a carbon source for a pathogen colonizing hosts has not been studied. We detected high-level expression of a potential glucarate permease encoding gene gudT when Salmonella enterica serovar Typhimurium are exposed to hydrogen gas (H₂), a gaseous by-product of gut commensal metabolism. A gudT strain of Salmonella is deficient in glucarate-dependent growth, however, it can still use other monosaccharides, such as glucose or galactose. Complementation of the gudT mutant with a plasmid harbouring gudT restored glucarate-dependent growth to wild-type (WT) levels. The gudT mutant exhibits attenuated virulence: the mean time of death for mice inoculated with WT strain was 2 days earlier than for mice inoculated with the gudT strain. At 4 days postinoculation, liver and spleen homogenates from mice inoculated with a gudT strain contained significantly fewer viable Salmonella than homogenates from animals inoculated with the parent. The parent strain grew well H₂-dependently in a minimal medium with amino acids and glucarate provided as the sole carbon sources, whereas the gudT strain achieved approximately 30% of the parent strain's yield. Glucarate-mediated growth of a mutant strain unable to produce H₂ was stimulated by H₂ addition, presumably owing to the positive transcriptional response to H₂. Gut microbiota-produced molecular hydrogen apparently signals Salmonella to catabolize an alternative carbon source available in the host. Our results link a gut microbiome-produced diffusible metabolite to augmenting bacterial pathogenesis.

Predicting enzyme-substrate specificity with QM/MM methods: a case study of the stereospecificity of (D)-glucarate dehydratase

The stereospecificity of d-glucarate dehydratase (GlucD) is explored by QM/MM calculations. Both the substrate binding and the chemical steps of GlucD contribute to substrate specificity. Although the identification of transition states remains computationally intensive, we suggest that QM/MM computations on ground states or intermediates can capture aspects of specificity that cannot be obtained using docking or molecular mechanics methods.

Divergent evolution in enolase superfamily: strategies for assigning functions

Nature's strategies for evolving catalytic functions can be deciphered from the information contained in the rapidly expanding protein sequence databases. However, the functions of many proteins in the protein sequence and structure databases are either uncertain (too divergent to assign function based on homology) or unknown (no homologs), thereby limiting the utility of the databases. The mechanistically diverse enolase superfamily is a paradigm for understanding the structural bases for evolution of enzymatic function. We describe strategies for assigning functions to members of the enolase superfamily that should be applicable to other superfamilies.

Transient knockdown and overexpression reveal a developmental role for the zebrafish enosf1b gene

BACKGROUND

Despite detailed in vivo knowledge of glycolytic enolases and many bacterial non-enolase members of the superfamily, little is known about the in vivo function of vertebrate non-enolase enolase superfamily members (ENOSF1s). Results of previous studies suggest involvement of the β splice form of ENOSF1 in breast and colon cancers. This study used the zebrafish (Danio rerio) as a vertebrate model of ENOSF1β function.

RESULTS

Whole mount in situ hybridization (WISH) showed that zebrafish ENOSF1β (enosf1b) is zygotic and expressed ubiquitously through the first 24 hours post fertilization (hpf). After 24 hpf, enosf1b expression is restricted to the notochord. Embryos injected with enosf1b-EGFP mRNA grew slower than EGFP mRNA-injected embryos but caught up to the EGFP-injected embryos by 48 hpf. Embryos injected with ATG or exon 10 enosf1b mRNA-targeting morpholinos had kinked notochords, shortened anterior-posterior axes, and circulatory edema. WISH for ntl or pax2a expression showed that embryos injected with either morpholino have deformed notochord and pronephros. TUNEL staining revealed increased apoptosis in the peri-notochord region.

CONCLUSIONS

This study is the first report of ENOSF1 function in a vertebrate and shows that ENOSF1 is required for embryonic development. Increased apoptosis following enosf1b knockdown suggests a potential survival advantage for increased ENOSF1β expression in human cancers.

Revisiting gap locations in amino acid sequence alignments and a proposal for a method to improve them by introducing solvent accessibility

In comparative modeling, the quality of amino acid sequence alignment still constitutes a major bottleneck in the generation of high quality models of protein three-dimensional (3D) structures. Substantial efforts have been made to improve alignment quality by revising the substitution matrix, introducing multiple sequences, replacing dynamic programming with hidden Markov models, and incorporating 3D structure information. Improvements in the gap penalty have not been a major focus, however, following the development of the affine gap penalty and of the secondary structure dependent gap penalty. We revisited the correlation between protein 3D structure and gap location in a large protein 3D structure data set, and found that the frequency of gap locations approximated to an exponential function of the solvent accessibility of the inserted residues. The nonlinearity of the gap frequency as a function of accessibility corresponded well to the relationship between residue mutation pattern and residue accessibility. By introducing this relationship into the gap penalty calculation for pairwise alignment between template and target amino acid sequences, we were able to obtain a sequence alignment much closer to the structural alignment. The quality of the alignments was substantially improved on a pair of sequences with identity in the “twilight zone” between 20 and 40%. The relocation of gaps by our new method made a significant improvement in comparative modeling, exemplified here by the Bacillus subtilis yitF protein. The method was implemented in a computer program, ALAdeGAP (ALignment with Accessibility dependent GAp Penalty), which is available at http://cib.cf.ocha.ac.jp/target_protein/. Proteins 2011; © 2011 Wiley-Liss, Inc.

Computation-facilitated assignment of the function in the enolase superfamily: a regiochemically distinct galactarate dehydratase from Oceanobacillus iheyensis

The structure of an uncharacterized member of the enolase superfamily from Oceanobacillus iheyensis (GI 23100298, IMG locus tag Ob2843, PDB entry 2OQY ) was determined by the New York SGX Research Center for Structural Genomics (NYSGXRC). The structure contained two Mg(2+) ions located 10.4 A from one another, with one located in the canonical position in the (beta/alpha)(7)beta-barrel domain (although the ligand at the end of the fifth beta-strand is His, unprecedented in structurally characterized members of the superfamily); the second is located in a novel site within the capping domain. In silico docking of a library of mono- and diacid sugars to the active site predicted a diacid sugar as a likely substrate. Activity screening of a physical library of acid sugars identified galactarate as the substrate (k(cat) = 6.8 s(-1), K(M) = 620 microM, k(cat)/K(M) = 1.1 x 10(4) M(-1) s(-1)), allowing functional assignment of Ob2843 as galactarate dehydratase (GalrD-II). The structure of a complex of the catalytically impaired Y90F mutant with Mg(2+) and galactarate allowed identification of a Tyr 164-Arg 162 dyad as the base that initiates the reaction by abstraction of the alpha-proton and Tyr 90 as the acid that facilitates departure of the beta-OH leaving group. The enzyme product is 2-keto-d-threo-4,5-dihydroxyadipate, the enantiomer of the product obtained in the GalrD reaction catalyzed by a previously characterized bifunctional l-talarate/galactarate dehydratase (TalrD/GalrD). On the basis of the different active site structures and different regiochemistries, we recognize that these functions represent an example of apparent, not actual, convergent evolution of function. The structure of GalrD-II and its active site architecture allow identification of the seventh functionally and structurally characterized subgroup in the enolase superfamily. This study provides an additional example in which an integrated sequence- and structure-based strategy employing computational approaches is a viable approach for directing functional assignment of unknown enzymes discovered in genome projects.

Evolution of enzymatic activities in the enolase superfamily: L-rhamnonate dehydratase

The l-rhamnonate dehydratase (RhamD) function was assigned to a previously uncharacterized family in the mechanistically diverse enolase superfamily that is encoded by the genome of Escherichia coli K-12. We screened a library of acid sugars to discover that the enzyme displays a promiscuous substrate specificity: l-rhamnonate (6-deoxy- l-mannonate) has the "best" kinetic constants, with l-mannonate, l-lyxonate, and d-gulonate dehydrated less efficiently. Crystal structures of the RhamDs from both E. coli K-12 and Salmonella typhimurium LT2 (95% sequence identity) were obtained in the presence of Mg (2+); the structure of the RhamD from S. typhimurium was also obtained in the presence of 3-deoxy- l-rhamnonate (obtained by reduction of the product with NaBH 4). Like other members of the enolase superfamily, RhamD contains an N-terminal alpha + beta capping domain and a C-terminal (beta/alpha) 7beta-barrel (modified TIM-barrel) catalytic domain with the active site located at the interface between the two domains. In contrast to other members, the specificity-determining "20s loop" in the capping domain is extended in length and the "50s loop" is truncated. The ligands for the Mg (2+) are Asp 226, Glu 252 and Glu 280 located at the ends of the third, fourth and fifth beta-strands, respectively. The active site of RhamD contains a His 329-Asp 302 dyad at the ends of the seventh and sixth beta-strands, respectively, with His 329 positioned to function as the general base responsible for abstraction of the C2 proton of l-rhamnonate to form a Mg (2+)-stabilized enediolate intermediate. However, the active site does not contain other acid/base catalysts that have been implicated in the reactions catalyzed by other members of the MR subgroup of the enolase superfamily. Based on the structure of the liganded complex, His 329 also is expected to function as the general acid that both facilitates departure of the 3-OH group in a syn-dehydration reaction and delivers a proton to carbon-3 to replace the 3-OH group with retention of configuration.

Divergent evolution in the enolase superfamily: the interplay of mechanism and specificity

The members of the mechanistically diverse enolase superfamily catalyze different overall reactions. Each shares a partial reaction in which an active site base abstracts the alpha-proton of the carboxylate substrate to generate an enolate anion intermediate that is stabilized by coordination to the essential Mg(2+) ion; the intermediates are then directed to different products in the different active sites. In this minireview, our current understanding of structure/function relationships in the divergent members of the superfamily is reviewed, and the use of this knowledge for our future studies is proposed.

Potential use of sugar binding proteins in reactors for regeneration of CO2 fixation acceptor D-Ribulose-1,5-bisphosphate

Sugar binding proteins and binders of intermediate sugar metabolites derived from microbes are increasingly being used as reagents in new and expanding areas of biotechnology. The fixation of carbon dioxide at emission source has recently emerged as a technology with potentially significant implications for environmental biotechnology. Carbon dioxide is fixed onto a five carbon sugar D-ribulose-1,5-bisphosphate. We present a review of enzymatic and non-enzymatic binding proteins, for 3-phosphoglycerate (3PGA), 3-phosphoglyceraldehyde (3PGAL), dihydroxyacetone phosphate (DHAP), xylulose-5-phosphate (X5P) and ribulose-1,5-bisphosphate (RuBP) which could be potentially used in reactors regenerating RuBP from 3PGA. A series of reactors combined in a linear fashion has been previously shown to convert 3-PGA, (the product of fixed CO₂ on RuBP as starting material) into RuBP (Bhattacharya et al., 2004; Bhattacharya, 2001). This was the basis for designing reactors harboring enzyme complexes/mixtures instead of linear combination of single-enzyme reactors for conversion of 3PGA into RuBP. Specific sugars in such enzyme-complex harboring reactors requires removal at key steps and fed to different reactors necessitating reversible sugar binders. In this review we present an account of existing microbial sugar binding proteins and their potential utility in these operations.

Superfamily active site templates

We show that three-dimensional signatures consisting of only a few functionally important residues can be diagnostic of membership in superfamilies of enzymes. Using the enolase superfamily as a model system, we demonstrate that such a signature, or template, can identify superfamily members in structural databases with high sensitivity and specificity. This is remarkable because superfamilies can be highly diverse, with members catalyzing many different overall reactions; the unifying principle can be a conserved partial reaction or chemical capability. Our definition of a superfamily thus hinges on the disposition of residues involved in a conserved function, rather than on fold similarity alone. A clear advantage of basing structure searches on such active site templates rather than on fold similarity is the specificity with which superfamilies with distinct functional characteristics can be identified within a large set of proteins with the same fold, such as the (beta/alpha)8 barrels. Preliminary results are presented for an additional group of enzymes with a different fold, the haloacid dehalogenase superfamily, suggesting that this approach may be generally useful for assigning reading frames of unknown function to specific superfamilies and thereby allowing inference of some of their functional properties.

Triggering loops and enzyme function: identification of loops that trigger and modulate movements

Enzyme function often involves a conformational change. There is a general agreement that loops play a vital role in correctly positioning the catalytically important residues. Nevertheless, predicting the functional loops and most importantly their role in enzyme function remains a difficult task. A major reason for this difficulty is that loops that undergo conformational change are frequently not well conserved in their primary sequence. beta1,4-Galactosyltransferase is one such enzyme. There, the amino acid sequence of a long loop that undergoes a large conformational change upon substrate binding is not well conserved. Our molecular dynamics simulations show that the large conformational change in the long loop is brought about by a second, interacting loop. Interestingly, while the structural change of the second loop is much smaller than that of the long loop, its sequence (particularly glycine residues) is highly conserved. We further examine the generality of the proposition that there are loops that trigger movements but nevertheless show little or no structural changes in crystals. We focus on two other enzymes, enolase and lipase. We chose these enzymes, since they too undergo conformational change upon ligand binding, however, they have different folds and different functions. Through multiple sets of simulations we show that the conformational change of the functional loop(s) is brought about through communication of flexibility by triggering loops that have several glycine residues. We further propose that similar to the conservation of common favorable fold types and structural motifs, evolution has also conserved common "skillful" mechanisms. Mechanisms may be conserved across different folds, sequences and functions, with adaptation to specific enzymatic roles.

The structure of 3-methylaspartase from Clostridium tetanomorphum functions via the common enolase chemical step

Methylaspartate ammonia-lyase (3-methylaspartase, MAL; EC ) catalyzes the reversible anti elimination of ammonia from L-threo-(2S,3S)-3-methylaspartic acid to give mesaconic acid. This reaction lies on the main catabolic pathway for glutamate in Clostridium tetanomorphum. MAL requires monovalent and divalent cation cofactors for full catalytic activity. The enzyme has attracted interest because of its potential use as a biocatalyst. The structure of C. tetanomorphum MAL has been solved to 1.9-A resolution by the single-wavelength anomalous diffraction method. A divalent metal ion complex of the protein has also been determined. MAL is a homodimer with each monomer consisting of two domains. One is an alpha/beta-barrel, and the other smaller domain is mainly beta-strands. The smaller domain partially occludes the C terminus of the barrel and forms a large cleft. The structure identifies MAL as belonging to the enolase superfamily of enzymes. The metal ion site is located in a large cleft between the domains. Potential active site residues have been identified based on a combination of their proximity to a metal ion site, molecular modeling, and sequence homology. In common with all members of the enolase superfamily, the carboxylic acid of the substrate is co-ordinated by the metal ions, and a proton adjacent to a carboxylic acid group of the substrate is abstracted by a base. In MAL, it appears that Lys(331) removes the alpha-proton of methylaspartic acid. This motif is the defining mechanistic characteristic of the enolase superfamily of which all have a common fold. The degree of structural conservation is remarkable given only four residues are absolutely conserved.