Construction and expression of human aldolase A and B expression plasmids in Escherichia coli host

E. coli expression plasmids for human aldolases A and B (EC 4.1.2.13) have been constructed from the pIN-III expression vector and their cDNAs, and expressed in E. coli strain JM83. Enzymatically active forms of human aldolase have been generated in the cells when transfected with either pHAA47, a human aldolase A expression plasmid, or pHAB 141, a human aldolase B expression plasmid. These enzymes are indistinguishable from authentic enzymes with respect to molecular size, amino acid sequences at the NH2- and COOH-terminal regions, the Km for substrate, fructose 1,6-bisphosphate and the activity ratio of fructose 1,6-bisphosphate/fructose 1-phosphate (FDP/F1P), although net electric charge and the Km for FDP of synthetic aldolase B differed from those for a previously reported human liver aldolase B. In addition, both the expressed aldolases A and B complement the temperature-sensitive phenotype of the aldolase mutant of E. coli h8. These data argue that the expressed aldolases are structurally and functionally similar to the authentic human aldolases, and would provide a system for analysis of the structure-function relationship of human aldolases A and B.     

Purification and properties of the native form of rabbit liver aldolase. Evidence for proteolytic modification after tissue extraction.

Aldolase was purified from rabbit liver by affinity-elution chromatography. By taking precautions to avoid rupture of lysosomes during the isolation procedure, a stable form of liver aldolase was obtained. The stable form of the enzyme had a specific activity with respect to fructose 1,6-bisphosphate cleavage of 20-28 mumol/min per mg of protein and a fructose 1,6-bisphosphate cleavage of 20-28mumol/min per mg of protein and a frutose 1,6-bisphosphate/fructose 1-phosphate activity ratio of 4. It was distinguishable from rabbit muscle aldolase, as previously isolated, on the basis of its electrophoretic mobility and N-terminal analysis. Muscle and liver aldolases were immunologically distinct. The stable liver aldolase was degraded with a lysosomal extract to a form with catalytic properties resembling those reported for aldolase B4. It is postulated that liver aldolase prepared by previously described methods has been modified by proteolysis and does not constitute the native form of the enzyme.     

Expression, purification, and characterization of natural mutants of human aldolase B. Role of quaternary structure in catalysis

Fructaldolases (EC 4.1.2.13) are ancient enzymes of glycolysis that catalyze the reversible cleavage of phosphofructose esters into cognate triose (phosphates). Three vertebrate isozymes of Class I aldolase have arisen by gene duplication and display distinct activity profiles with fructose 1,6-bisphosphate and with fructose 1-phosphate. We describe the biochemical and biophysical characterization of seven natural human aldolase B variants, identified in patients suffering from hereditary fructose intolerance and expressed as recombinant proteins in E. coli, from which they were purified to homogeneity. The mutant aldolases were all missense variants and could be classified into two principal groups: catalytic mutants, with retained tetrameric structure but altered kinetic properties (W147R, R303W, and A337V), and structural mutants, in which the homotetramers readily dissociate into subunits with greatly impaired enzymatic activity (A149P, A174D, L256P, and N334K). Investigation of these two classes of mutant enzyme suggests that the integrity of the quaternary structure of aldolase B is critical for maintaining its full catalytic function.     

Classification of fructose-1,6-bisphosphate aldolases based on 18O retention in the cleavage reaction.

Oxygen (18) was used as a mechanistic probe in the investigation of several different sources of fructose 1,6-bisphosphate aldolases (EC 4.1.2.13) which, due to differences in some physical and chemical properties, could not be clearly put in either Class I or Class II. Aldolases may be identified as belonging to a particular class on the basis of the amount of 180 retained in the dihydroxyacetone phosphate produced in the cleavage of [2-Oxygen (18)] fructose 1,6-biphosphate. The mechanism of Class I aldolases involves an obligatory exchange of the C-2 oxygen atom of fructose 1,6-bisphosphate, leading to the absence of 180 in the product. For Class II aldolases, the C-2 oxygen atom is retained in the aldol cleavage reaction. Aldolases from spinach and L. casei base intermediate. Aldosase from C. perfringens was found to be Class II, suggesting a metal-chelate intermediate. Results with Euglena aldolase confirmed that this organism contained both types of aldolases with approximately 78% Class II. The data show that despite a wide variety of physical and chemical properties, there are important mechanistic similarities within each class of enzyme and significant differences between the two classes. The determination of 180 retention in the product of the cleavage reaction using [2-180] fructose 1,6-biphosphate is an accurate means of classifying these enzymes since it is a measure of a property which is directly related to the mechanisms of the reactions.     

Exploring CP12 binding proteins revealed aldolase as a new partner for the phosphoribulokinase/glyceraldehyde 3-phosphate dehydrogenase/CP12 complex--purification and kinetic characterization of this enzyme from Chlamydomonas reinhardtii

Possible binding proteins of CP12 in a green alga, Chlamydomonas reinhardtii, were investigated. We covalently immobilized CP12 on a resin and then used it to trap CP12 partners. Thus, we found an association between CP12 and phosphoribulokinase (EC 2.7.1.19), glyceraldehyde 3-phosphate dehydrogenase (EC 1.2.1.13) and aldolase. Immunoprecipitation with purified CP12 antibodies supported these data. The dissociation constant between CP12 and fructose 1,6-bisphosphate (EC 4.1.2.13) aldolase was measured by surface plasmon resonance and is equal to 0.48 +/- 0.05 mum and thus corroborated an interaction between CP12 and aldolase. However, the association is even stronger between aldolase and the phosphoribulokinase/glyceraldehyde 3-phosphate dehydrogenase/CP12 complex and the dissociation constant between them is equal to 55+/-5 nm. Moreover, owing to the fact that aldolase has been poorly studied in C. reinhardtii, we purified it and analyzed its kinetic properties. The enzyme displayed Michaelis-Menten kinetics with fructose 1,6-bisphosphate and sedoheptulose 1,7-bisphosphate, with a catalytic constant equal to 35 +/- 1 s(-1) and 4 +/- 0.1 s(-1), respectively. The K(m) value for fructose 1,6-bisphosphate was equal to 0.16 +/- 0.02 mm and 0.046 +/- 0.005 mm for sedoheptulose 1,7-bisphosphate. The catalytic efficiency of aldolase was thus 219 +/- 31 s(-1).mm(-1) with fructose 1,6-bisphosphate and 87 +/- 9 s(-1).mm(-1) with sedoheptulose 1,7-bisphosphate. In the presence of the complex, this parameter for fructose 1,6-bisphosphate increased to 310 +/- 23 s(-1).mm(-1), whereas no change was observed with sedoheptulose 1,7-bisphosphate. The condensation reaction of aldolase to form fructose 1,6-bisphosphate was also investigated but no effect of CP12 or the complex on this reaction was observed.     

Exploring substrate binding and discrimination in fructose1, 6-bisphosphate and tagatose 1,6-bisphosphate aldolases.

Fructose 1,6-bisphosphate aldolase catalyses the reversible condensation of glycerone-P and glyceraldehyde 3-phosphate into fructose 1,6-bisphosphate. A recent structure of the Escherichia coli Class II fructose 1,6-bisphosphate aldolase [Hall, D.R., Leonard, G.A., Reed, C.D., Watt, C.I., Berry, A. & Hunter, W.N. (1999) J. Mol. Biol. 287, 383-394] in the presence of the transition state analogue phosphoglycolohydroxamate delineated the roles of individual amino acids in binding glycerone-P and in the initial proton abstraction steps of the mechanism. The X-ray structure has now been used, together with sequence alignments, site-directed mutagenesis and steady-state enzyme kinetics to extend these studies to map important residues in the binding of glyceraldehyde 3-phosphate. From these studies three residues (Asn35, Ser61 and Lys325) have been identified as important in catalysis. We show that mutation of Ser61 to alanine increases the Km value for fructose 1, 6-bisphosphate 16-fold and product inhibition studies indicate that this effect is manifested most strongly in the glyceraldehyde 3-phosphate binding pocket of the active site, demonstrating that Ser61 is involved in binding glyceraldehyde 3-phosphate. In contrast a S61T mutant had no effect on catalysis emphasizing the importance of an hydroxyl group for this role. Mutation of Asn35 (N35A) resulted in an enzyme with only 1.5% of the activity of the wild-type enzyme and different partial reactions indicate that this residue effects the binding of both triose substrates. Finally, mutation of Lys325 has a greater effect on catalysis than on binding, however, given the magnitude of the effects it is likely that it plays an indirect role in maintaining other critical residues in a catalytically competent conformation. Interestingly, despite its proximity to the active site and high sequence conservation, replacement of a fourth residue, Gln59 (Q59A) had no significant effect on the function of the enzyme. In a separate study to characterize the molecular basis of aldolase specificity, the agaY-encoded tagatose 1,6-bisphosphate aldolase of E. coli was cloned, expressed and kinetically characterized. Our studies showed that the two aldolases are highly discriminating between the diastereoisomers fructose bisphosphate and tagatose bisphosphate, each enzyme preferring its cognate substrate by a factor of 300-1500-fold. This produces an overall discrimination factor of almost 5 x 105 between the two enzymes. Using the X-ray structure of the fructose 1,6-bisphosphate aldolase and multiple sequence alignments, several residues were identified, which are highly conserved and are in the vicinity of the active site. These residues might potentially be important in substrate recognition. As a consequence, nine mutations were made in attempts to switch the specificity of the fructose 1,6-bisphosphate aldolase to that of the tagatose 1,6-bisphosphate aldolase and the effect on substrate discrimination was evaluated. Surprisingly, despite making multiple changes in the active site, many of which abolished fructose 1, 6-bisphosphate aldolase activity, no switch in specificity was observed. This highlights the complexity of enzyme catalysis in this family of enzymes, and points to the need for further structural studies before we fully understand the subtleties of the shaping of the active site for complementarity to the cognate substrate.     

The proton exchange of the pro-S hydrogen atom at C-1 in dihydroxyacetone phosphate and D-fructose 1,6-bisphosphate catalysed by class-I and class-II aldolases.

The efficacy of class-I and class-II aldolases in catalysing the C-1 proton exchange in fructose 1,6-bisphosphate and dihydroxyacetone phosphate was investigated. The rate of this reaction was at least two orders of magnitude slower in class-II than in the class-I aldolases. It is suggested that this difference reflects the formation of different intermediates in the reactions catalysed by the two classes of aldolase.     

Microbial aldolases as C–C bonding enzymes—unknown treasures and new developments

Aldolases are a specific group of lyases that catalyze the reversible stereoselective addition of a donor compound (nucleophile) onto an acceptor compound (electrophile). Whereas most aldolases are specific for their donor compound in the aldolization reaction, they often tolerate a wide range of aldehydes as acceptor compounds. C–C bonding by aldolases creates stereocenters in the resulting aldol products. This makes aldolases interesting tools for asymmetric syntheses of rare sugars or sugar-derived compounds as iminocyclitols, statins, epothilones, and sialic acids. Besides the well-known fructose 1,6-bisphosphate aldolase, other aldolases of microbial origin have attracted the interest of synthetic bio-organic chemists in recent years. These are either other dihydroxyacetone phosphate aldolases or aldolases depending on pyruvate/phosphoenolpyruvate, glycine, or acetaldehyde as donor substrate. Recently, an aldolase that accepts dihydroxyacetone or hydroxyacetone as a donor was described. A further enlargement of the arsenal of available chemoenzymatic tools can be achieved through screening for novel aldolase activities and directed evolution of existing aldolases to alter their substrate- or stereospecifities. We give an update of work on aldolases, with an emphasis on microbial aldolases.     

Molecular cloning, expression, purification, and characterization of fructose 1,6-bisphosphate aldolase from Mycobacterium tuberculosis--a novel Class II A tetramer

The Class II fructose 1,6-bisphosphate aldolase (fda, Rv0363c) from the pathogen Mycobacterium tuberculosis H37RV was subcloned in the Escherichia coli vector pT7-7 and purified to near homogeneity. The specific activity (35 U/mg) is approximately 9 times higher than previously reported for the enzyme partially purified from the pathogen. Attempts to express the enzyme with an N-terminal fusion tag yielded inactive, mostly insoluble protein. The native recombinant enzyme is zinc-dependent and has a catalytic efficiency for fructose 1,6-bisphosphate cleavage higher than most Class II aldolases characterized to date. The aldolase has a Km of 20 microM, a kcat of 21 s(-1), and a pH optimum of 7.8. The molecular mass of the enzyme subunits as determined by mass spectrometry is in agreement with the mass calculated on the basis of its gene sequence minus the terminal methionine, 36,413 Da. The enzyme is a homotetramer and retains only two zinc ions per tetramer when transferred to a metal-free buffer, as determined by ICP-MS and by a colorimetric assay using 4-(2-pyridylazo)-resorcinol (PAR) as a chelator. The E. coli expression system reported in this study will facilitate the further characterization of this enzyme and the screening for potential inhibitors.     

Cooperative effect of fructose bisphosphate and glyceraldehyde-3-phosphate dehydrogenase on aldolase action

The combination of binding and kinetic approaches is suggested to study (i) the mechanism of substrate-modulated dynamic enzyme associations; (ii) the specificity of enzyme interactions. The effect of complex formation between aldolase and glyceraldehyde-3-phosphate dehydrogenase (D-glyceraldehyde-3-phosphate:NAD+ oxidoreductase (phosphorylating), EC 1.2.1.12) on aldolase catalysis was investigated under pseudo-first-order conditions. No change in kcat but a significant increase in KM of fructose 1,6-bisphosphate for aldolase was found when both enzymes were obtained from muscle. In contrast, kcat rather than KM changed if dehydrogenase was isolated from yeast. Next, the conversion of fructose 1-phosphate was not affected by interactions between enzyme couples isolated from muscle. The influence of fructose phosphates on the enzyme-complex formation was studied by means of covalently attached fluorescent probe. We found that the interaction ws not perturbed by the presence of fructose 1-phosphate; however, fructose 1,6-bisphosphate altered the dissociation constant of the enzyme complex. A molecular model for fructose 1,6-bisphosphate-modulated enzyme interaction has been evaluated which suggests that high levels of fructose bisphosphate would drive the formation of the 'channelling' complex between aldolase and glyceraldehyde-3-phosphate dehydrogenase.     

Synthesis and cleavage of octulose bisphosphates with liver and muscle aldolases

Rabbit muscle aldolase was used to synthesize d-glycero-d-altro-octulose 1,8-bisphosphate and d-glycero-d-ido-octulose 1,8-bisphosphate. The products, isolated by ion-exchange chromatography, were characterized with the cysteine-sulfuric acid reaction and shown to be 90–95% pure by analysis for organic phosphorus and for dihydroxyacetone phosphate formed on cleavage with aldolase. The kinetic constants for synthesis and cleavage of these octulose bisphosphates with muscle and liver aldolases were determined. In the direction of cleavage both octulose bisphosphates were excellent substrates for liver aldolase, comparable to fructose 1,6-bisphosphate with respect to both V and K_m. With muscle aldolase the rate of cleavage was 1–5% of that with fructose bisphosphate and comparable to that with fructose 1-phosphate. In the direction of synthesis, ribose 5-phosphate was a better substrate than arabinose 5-phosphate for both the liver and muscle enzymes, although for both pentose phosphates the values of K_m fell in the range between 5 and 25 mm. It is concluded that reactions catalyzed by aldolase might account for the reported presence of these eight-carbon sugar phosphate esters in liver and in red cells.     

Two Distinct Aldolases of Class II Type in the Cyanoplasts and in the Cytosol of the Alga Cyanophora paradoxa

Two aldolases from the alga Cyanophora paradoxa (Glaucocystophyta) can be separated by chromatography on diethylaminoethyl-Fractogel. The two aldolases are inhibited by 1 mM ethylene-diaminetetraacetate (EDTA) and, therefore, are class II aldolases. When cells of C. paradoxa were fractionated, one aldolase was associated with the cytosol fraction and the other was associated with the cyanoplast fraction. The Km(fructose-1,6-bisphosphate) was 600 [mu]M for the cytosolic aldolase and 340 [mu]M for the cyanoplast aldolase. The activity of the cytosolic aldolase was increased up to 4-fold by 100 mM K+ and slightly inhibited by Li+ and Cs+, whereas the cyanoplast aldolase was not affected by these ions. Inactivation by 1 mM EDTA could be partly restored by the addition of Co2+ or Mn2+ and to a lesser extent by Zn2+ or Mg2+. The molecular masses of the native cytosolic and cyanoplast aldolases are about 90 and 85 kD, respectively, as estimated by velocity centrifugation in sucrose gradients. Implications for the evolution of class I and II aldolases in chloroplasts of higher plants and algae will be discussed.     

Carboxyl terminus region modulates catalytic activity of recombinant maize aldolase

Site-directed mutagenesis was utilized to study the functional role of the COOH-terminal region in recombinant maize aldolase. A single mutation was created in each of the last nine amino acids of the COOH terminus and characterized kinetically. Point mutations in the COOH-terminal region were found to influence both the rate of fructose 1,6-bisphosphate and fructose 1-phosphate cleavage. Catalytic efficiency, kcat/Km, was not affected by the mutations within experimental error consistent with this region of the COOH terminus modulating product release. Concentrations of the carbanion-enamine enzyme intermediate complex produced upon substrate cleavage increased with the severity of the point mutation. A condensation assay was developed to directly measure fructose 1,6-bisphosphate synthesized by aldolases in the presence of high triose phosphate concentrations. The maximal rate of aldol condensation of triose phosphates, D-glyceralehyde-3-P and dihydroxyacetone-P, was affected by the point mutations to the same extent as the maximal rate of substrate cleavage. Interpretation of the data is consistent with point mutations in the COOH terminus predominantly affecting the proton exchange with the dihydroxyacetone-P enzymatic complex at the carbanion-enamine step and that this step is probably rate-limiting in the catalytic mechanism of recombinant maize aldolase. The role of the COOH-terminal region in aldolases is thus consistent with a sequence dependent modulation of catalytic activity.     

Stereochemistry of nonnatural aldol reactions catalyzed by DHAP aldolases

A coupled enzymatic assay was developed for quantitative determination of the stereoisomeric products formed in aldol reactions catalyzed by dihydroxyacetone phosphate (DHAP)-dependent aldolases. Three of the four stereoisomers could be determined directly; the fourth one was calculated. This procedure is based on the reversibility of the aldol reaction and requires no derivatization or work-up of the product samples, only removal or inactivation of the biocatalyst. In comparison with other methods the enzymatic assay is highly accurate and fast. Determination of isomer formation with 10 different acceptor substrates applying this procedure gave unprecedented insight in the stereochemistry of fructose-1,6-bisphosphate aldolase from Staphylococcus carnosus and l-rhamnulose-1-phosphate aldolase from E. coli.     

Characterization of chloroplastic fructose 1,6-bisphosphate aldolases as lysine-methylated proteins in plants

In pea (Pisum sativum), the protein-lysine methyltransferase (PsLSMT) catalyzes the trimethylation of Lys-14 in the large subunit (LS) of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), the enzyme catalyzing the CO(2) fixation step during photosynthesis. Homologs of PsLSMT, herein referred to as LSMT-like enzymes, are found in all plant genomes, but methylation of LS Rubisco is not universal in the plant kingdom, suggesting a species-specific protein substrate specificity of the methyltransferase. In this study, we report the biochemical characterization of the LSMT-like enzyme from Arabidopsis thaliana (AtLSMT-L), with a focus on its substrate specificity. We show that, in Arabidopsis, LS Rubisco is not naturally methylated and that the physiological substrates of AtLSMT-L are chloroplastic fructose 1,6-bisphosphate aldolase isoforms. These enzymes, which are involved in the assimilation of CO(2) through the Calvin cycle and in chloroplastic glycolysis, are trimethylated at a conserved lysyl residue located close to the C terminus. Both AtLSMT-L and PsLSMT are able to methylate aldolases with similar kinetic parameters and product specificity. Thus, the divergent substrate specificity of LSMT-like enzymes from pea and Arabidopsis concerns only Rubisco. AtLSMT-L is able to interact with unmethylated Rubisco, but the complex is catalytically unproductive. Trimethylation does not modify the kinetic properties and tetrameric organization of aldolases in vitro. The identification of aldolases as methyl proteins in Arabidopsis and other species like pea suggests a role of protein lysine methylation in carbon metabolism in chloroplasts.     

Saccharomyces cerevisiae aldolase mutants.

Six mutants lacking the glycolytic enzyme fructose 1,6-bisphosphate aldolase have been isolated in the yeast Saccharomyces cerevisiae by inositol starvation. The mutants grown on gluconeogenic substrates, such as glycerol or alcohol, and show growth inhibition by glucose and related sugars. The mutations are recessive, segregate as one gene in crosses, and fall in a single complementation group. All of the mutants synthesize an antigen cross-reacting to the antibody raised against yeast aldolase. The aldolase activity in various mutant alleles measured as fructose 1,6-bisphosphate cleavage is between 1 to 2% and as condensation of triose phosphates to fructose 1,6-bisphosphate is 2 to 5% that of the wild-type. The mutants accumulate fructose 1,6-bisphosphate from glucose during glycolysis and dihydroxyacetone phosphate during gluconeogenesis. This suggests that the aldolase activity is absent in vivo.     

Studies on chimeric fusion proteins of human aldolase isozymes A and B.

Several kinds of fusion proteins between human aldolases A and B were prepared by recombinant DNA technology and their enzymic properties were examined. AB chimeras, which have aldolase A at the N-terminal region and aldolase B at the C-terminal region, were scarcely obtained, while BA chimeras were abundant (Kitajima et al., (1990), J. Biol. Chem., 265, 17493-17498). All the BAB chimeras, aldolase A fragments inserted in aldolase B, showed activity assignable to aldolase B type, which imply an essential role of Tyr residue at the C-terminus of aldolase A in the binding of fructose-1,6-bisphosphate (Fru-1,6-P2). BAB chimeras also showed reactivity to effectors such as fructose-2,6-bisphosphate (Fru-2,6-P2) and pyridoxal 5-phosphate (PLP), in a similar manner to aldolase B. BAB108 has a similarity to the BA108 chimera, but acts differently from other BAB chimeras, suggesting that its structure around active site looks like that of aldolase A.     

Structure of a Class I Tagatose-1,6-bisphosphate Aldolase: INVESTIGATION INTO AN APPARENT LOSS OF STEREOSPECIFICITY*

Tagatose-1,6-bisphosphate aldolase from Streptococcus pyogenes is a class I aldolase that exhibits a remarkable lack of chiral discrimination with respect to the configuration of hydroxyl groups at both C3 and C4 positions. The enzyme catalyzes the reversible cleavage of four diastereoisomers (fructose 1,6-bisphosphate (FBP), psicose 1,6-bisphosphate, sorbose 1,6-bisphosphate, and tagatose 1,6-bisphosphate) to dihydroxyacetone phosphate (DHAP) and d-glyceraldehyde 3-phosphate with high catalytic efficiency. To investigate its enzymatic mechanism, high resolution crystal structures were determined of both native enzyme and native enzyme in complex with dihydroxyacetone-P. The electron density map revealed a (α/β)₈ fold in each dimeric subunit. Flash-cooled crystals of native enzyme soaked with dihydroxyacetone phosphate trapped a covalent intermediate with carbanionic character at Lys²⁰⁵, different from the enamine mesomer bound in stereospecific class I FBP aldolase. Structural analysis indicates extensive active site conservation with respect to class I FBP aldolases, including conserved conformational responses to DHAP binding and conserved stereospecific proton transfer at the DHAP C3 carbon mediated by a proximal water molecule. Exchange reactions with tritiated water and tritium-labeled DHAP at C3 hydrogen were carried out in both solution and crystalline state to assess stereochemical control at C3. The kinetic studies show labeling at both pro-R and pro-S C3 positions of DHAP yet detritiation only at the C3 pro-S-labeled position. Detritiation of the C3 pro-R label was not detected and is consistent with preferential cis-trans isomerism about the C2–C3 bond in the carbanion as the mechanism responsible for C3 epimerization in tagatose-1,6-bisphosphate aldolase.     

Fructose 1,6-bisphosphate aldolase of Drosophila melanogaster: comparative sequence analyses around the substrate-binding lysyl residue

The amino acid sequence of a 103 residue segment encompassing the substrate-binding active site lysyl residue of fructose 1,6-bisphosphate aldolase from Drosophila melanogaster is determined. The sequence is identical to more than 70% with the structure of rabbit muscle aldolase and with the known partial sequences of the sturgeon muscle, trout muscle, and ox liver enzymes. The homology of the insect enzyme with the vertebrate aldolases strongly implies a similar tertiary structure folding.     

Structure of tagatose-1,6-bisphosphate aldolase. Insight into chiral discrimination, mechanism, and specificity of class II aldolases

Tagatose-1,6-bisphosphate aldolase (TBPA) is a tetrameric class II aldolase that catalyzes the reversible condensation of dihydroxyacetone phosphate with glyceraldehyde 3-phosphate to produce tagatose 1,6-bisphosphate. The high resolution (1.45 A) crystal structure of the Escherichia coli enzyme, encoded by the agaY gene, complexed with phosphoglycolohydroxamate (PGH) has been determined. Two subunits comprise the asymmetric unit, and a crystallographic 2-fold axis generates the functional tetramer. A complex network of hydrogen bonds position side chains in the active site that is occupied by two cations. An unusual Na+ binding site is created using a pi interaction with Tyr183 in addition to five oxygen ligands. The catalytic Zn2+ is five-coordinate using three histidine nitrogens and two PGH oxygens. Comparisons of TBPA with the related fructose-1,6-bisphosphate aldolase (FBPA) identifies common features with implications for the mechanism. Because the major product of the condensation catalyzed by the enzymes differs in the chirality at a single position, models of FBPA and TBPA with their cognate bisphosphate products provide insight into chiral discrimination by these aldolases. The TBPA active site is more open on one side than FBPA, and this contributes to a less specific enzyme. The availability of more space and a wider range of aldehyde partners used by TBPA together with the highly specific nature of FBPA suggest that TBPA might be a preferred enzyme to modify for use in biotransformation chemistry.