Crystal structure of a histidine-tagged serine hydrolase involved in the carbazole degradation (CarC enzyme)

2-Hydroxy-6-oxo-6-(2(')-aminophenyl)-hexa-2,4-dienoate hydrolases (CarC enzymes) from two carbazole-degrading bacteria were purified using recombinant Escherichia coli strains with the histidine (His)-tagged purification system. The His-tagged CarC (ht-CarC) enzymes from Pseudomonas resinovorans strain CA10 (ht-CarC(CA10)) and Janthinobacterium sp. strain J3 (ht-CarC(J3)) exhibited hydrolase activity toward 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoate as the purified native CarC(CA10) did. ht-CarC(J3) was crystallized in the space group I422 with cell dimensions of a=b=130.3A, c=84.5A in the hexagonal setting, and the crystal structure of ht-CarC(J3) was determined at 1.86A resolution. The final refined model of ht-CarC(J3) yields an R-factor of 21.6%, although the electron-density corresponding to Ile146 to Asn155 was ambiguous in the final model. We compared the known structures of BphD from Rhodococcus sp. strain RHA1 and CumD from Pseudomonas fluorescens strain IP01. The backbone conformation of ht-CarC(J3) was better superimposed with CumD than with BphD(RHA1). The side-chain directions of Arg185 and Trp262 residues in the substrate binding pockets of these enzymes were different among these proteins, suggesting that these residues may take a conformational change during the catalytic cycles.     

Pseudomonas putida Mutants Defective in the Metabolism of the Products of meta Fission of Catechol and Its Methyl Analogues

A selection procedure is described which was used to isolate mutants of Pseudomonas putida strain U in the following enzymes of the meta-fission pathway of phenol and cresols: hydrolase, tautomerase, and decarboxylase. Strains deficient in the hydrolase are unable to use either o- or m-cresol as a sole carbon source and were shown to accumulate 2-hydroxy-6-keto-2,4-heptadienoate when incubated in the presence of o- or m-cresol. When 2-hydroxymuconic semialdehyde (plus nicotinamide adenine dinucleotide, oxidized form) was metabolized by phenol-induced extracts of tautomerase-deficient strains, the enol tautomer of 4-oxalocrotonate accumulated and was then converted slowly to the keto tautomer by a nonenzymatic reaction. Phenol-induced extracts of decarboxylase-deficient strains accumulated the keto tautomer of 4-oxalocrotonate from 2-hydroxymuconic semialdehyde (plus nicotinamide adenine dinucleotide, oxidized form). Strains with an inactive decarboxylase are unable to completely metabolize either phenol or p-cresol. Tautomerase-defective strains are unable to grow with p-cresol as the sole carbon source and grow only very slowly on phenol.     

Structural Determinants Allowing Transferase Activity in SENSITIVE TO FREEZING 2, Classified as a Family I Glycosyl Hydrolase

SENSITIVE TO FREEZING 2 (SFR2) is classified as a family I glycosyl hydrolase but has recently been shown to have galactosyltransferase activity in Arabidopsis thaliana. Natural occurrences of apparent glycosyl hydrolases acting as transferases are interesting from a biocatalysis standpoint, and knowledge about the interconversion can assist in engineering SFR2 in crop plants to resist freezing. To understand how SFR2 evolved into a transferase, the relationship between its structure and function are investigated by activity assay, molecular modeling, and site-directed mutagenesis. SFR2 has no detectable hydrolase activity, although its catalytic site is highly conserved with that of family 1 glycosyl hydrolases. Three regions disparate from glycosyl hydrolases are identified as required for transferase activity as follows: a loop insertion, the C-terminal peptide, and a hydrophobic patch adjacent to the catalytic site. Rationales for the effects of these regions on the SFR2 mechanism are discussed.     

Microbial metabolism of chlorosalicylates: accelerated evolution by natural genetic exchange

Methylsalicylate-grown cells of Pseudomonas sp. WR 401 cometabolized 3-, 4- and 5-substituted halosalicylates to the corresponding halocatechols. Further degradation was unproductive due to the presence of high levels of catechol 2,3-dioxygenase. This strain acquired the ability to utilize 3-chlorobenzoate following acquisition of genes from Pseudomonas sp. B 13 which are necessary for the assimilation of chlorocatechols. This derivative (WR 4011) was unable to use 4- or 5-chlorosalicylates. Derivatives able to use these compounds were obtained by plating WR 4011 on 5-chlorosalicylate minimal medium; one such derivative was designated WR 4016. The acquisition of this property was accompanied by concomitant loss of the methylsalicylate phenotype. During growth on 4- or 5-chlorosalicylate the typical enzymes of chlorocatechol assimilation were detected in cell free extracts, whereas catechol 2,3-dioxygenase activity was not induced. Repeated subcultivation of WR 4016 in the presence of 3-chlorosalicylate produced variants (WR 4016-1) which grew on all three isomers.Abbreviations CS chlorosalicylate - MS methylsalicylate - 3CB 3-chlorobenzoate - nal^r nalidixin-resistant - str^r streptomycin-resistant - C230 catechol-2,3-dioxygenase - C120 catechol-1,2-dioxygenase - HMSH 2-hydroxymuconic semialdehyde hydrolase or 2-hydroxy-6-oxo-hexa-2,4-dienoic acid-hydrolase - HMSD 2-hydroxymuconic semialdehyde dehydrogenase - Dienlacton hydrolase 4-carboxymethylenebut-2-en-4-olide hydrolase     

Novel organization of catechol <Emphasis Type="Italic">meta</Emphasis> pathway genes in the nitrobenzene degrader <Emphasis Type="Italic">Comamonas</Emphasis> sp. JS765 and its evolutionary implication

The catechol meta cleavage pathway is one of the central metabolic pathways for the degradation of aromatic compounds. A novel organization of the pathway genes, different from that of classical soil microorganisms, has been observed in Sphingomonas sp HV3 and Pseudomonas sp. DJ77. In a Comamonas sp. JS765, cdoE encoding catechol 2,3-dioxygenase shares a common ancestry only with tdnC of a Pseudomonas putida strain, while codG encoding 2-hydroxymuconic semialdehyde dehydrogenase shows a higher degree of similarity to those genes in classical bacteria. Located between cdoE and cdoG are several putative genes, whose functions are unknown. These genes are not found in meta pathway operons of other microorganisms with the exception of cdoX2, which is similar to cmpX in strain HV3. Therefore, the gene cluster in JS765 reveals a third type of gene organization of the meta pathway.     

The meta cleavage operon of TOL degradative plasmid pWWO comprises 13 genes

Summary The meta-cleavage operon of TOL plasmid pWWO of Pseudomonas putida encodes a set of enzymes which transform benzoate/toluates to Krebs cycle intermediates via extradiol (meta-) cleavage of (methyl)catechol. The genetic organization of the operon was characterized by cloning of the meta-cleavage genes into an expression vector and identification of their products in Escherichia coli maxicells. This analysis showed that the meta-cleavage operon contains 13 genes whose order and products (in kilodaltons) are The xyIXYZ genes encode three subunits of toluate 1,2-dioxygenase. The xylL, xyIE, xyIG, xylF, xylJ, xylK, xylI and xylH genes encode 1,2-dihydroxy-3,5-cyclohexadiene-1-carboxylate dehydrogenase, catechol 2,3-dioxygenase, 2-hydroxymuconic semialdehyde dehydrogenase, 2-hydroxymuconic semialdehyde hydrolase, 2-oxopent-4-enoate hydratase, 4-hydroxy-2-oxovalerate aldolase, 4-oxalocrotonate decarboxylase and 4-oxalocrotonate tautomerase, respectively. The functions of xyIT and xylQ are not known at present. The comparison of the coding capacity and the sizes of the products of the meta-cleavage operon genes indicated that most of the DNA between xyIX and xyIH consists of coding sequences.     

Evolutionary relationships between catabolic pathways for aromatics: Conservation of gene order and nucleotide sequences of catechol oxidation genes of pWW0 and NAH7 plasmids

Summary TOL plasmid pWW0 and plasmid NAH7 encode catabolic enzymes required for oxidative degradation of toluene and naphthalene, respectively. The gene order of the catabolic operon of NAH7 for salicylate oxidation was determined to be: promoter-nahG (the structural gene for salicylate hydroxylase)-nahH (catechol 2,3-dioxygenase)-nahI (hydroxymuconic semialdehyde dehydrogenase)-nahN (hydroxymuconic semialdehyde hydrolase)-nahL (2-oxopent-4-enoate hydratase). This order is identical to that of the isofunctional genes of TOL plasmid pWW0. The complete nucleotide sequence of nahH was determined and compared with that of xylE, the isofunctional gene of TOL plasmid pWW0. There were 20% and 16% differences in their nucleotide and amino acid sequences, respectively. The homology between the NAH7 and TOL pWW0 plasmids ends upstream of the Shine-Dalgarno sequences of nahH and xylE, but the homology continues downstream of these genes. This observation suggested that genes for the catechol oxidative enzymes of NAH7 and TOL pWW0 were derived from a common ancestral sequence which was transferred as a discrete segment of DNA between plasmids.     

Molecular characterisation of a Rhodococcus ohp operon

The ohp operon of Rhodococcus strain V49 consists of five genes, ohpR, ohpA, ohpB, ohpC and ohpD which encode putative regulator and transport proteins and confirmed monooxygenase, hydroxymuconic semialdehyde hydrolase and catechol 2,3-dioxygenase enzymes, respectively. These enzymes catalyse the conversion of 3-(2- hydroxyphenyl)propionic acid to the corresponding linear product via a meta-cleavage pathway. Confirmation that the ohp gene cluster formed an operon was provided by gene disruption during which expression of Bacillus levansucrase was confirmed in Rhodococcus. Following biochemical assays of cell-free extracts from recombinant Escherichia coli expressing ohpB (monooxygenase), ohpC (hydroxymuconic-semialdehyde hydrolase) and ohpD (catechol 2,3-dioxygenase), the ortho-hydroxyphenylpropionic acid catabolic pathway in Rhodococcus strain V49 (ATCC 19070) has been predicted.     

Unique regulation of the active site of the serine esterase S-formylglutathione hydrolase

S-Formylglutathione hydrolases (SFGHs) are highly conserved thioesterases present in prokaryotes and eukaryotes, and form part of the formaldehyde detoxification pathway, as well as functioning as xenobiotic-hydrolysing carboxyesterases. As defined by their sensitivity to covalent modification, SFGHs behave as cysteine hydrolases, being inactivated by thiol alkylating agents, while being insensitive to inhibition by organophosphates such as paraoxon. As such, the enzyme has been classified as an esterase D in animals, plants and microbes. While SFGHs do contain a conserved cysteine residue that has been implicated in catalysis, sequence analysis also reveals the classic catalytic triad of a serine hydrolase. Using a combination of selective protein modification and X-ray crystallography, AtSFGH from Arabidopsis thaliana has been shown to be a serine hydrolase rather than a cysteine hydrolase. Uniquely, the conserved reactive cysteine (Cys59) previously implicated in catalysis lies in close proximity to the serine hydrolase triad, serving a gate-keeping function in comprehensively regulating access to the active site. Thus, any covalent modification of Cys59 inhibited all hydrolase activities of the enzyme. When isolated from Escherichia coli, a major proportion of recombinant AtSFGH was recovered with the Cys59 forming a mixed disulfide with glutathione. Reversible disulfide formation with glutathione could be demonstrated to regulate hydrolase activity in vitro. The importance of Cys59 in regulating AtSFGH in planta was demonstrated in transient expression assays in Arabidopsis protoplasts. As determined by fluorescence microscopy, the Cys59Ser mutant enzyme was shown to rapidly hydrolyse 4-methylumbelliferyl acetate in paraoxon-treated cells, while the native enzyme was found to be inactive. Our results clarify the classification of AtSFGHs as hydrolases and suggest that the regulatory and conserved cysteine provides an unusual redox-sensitive regulation to an enzyme functioning in both primary and xenobiotic metabolism in prokaryotes and eukaryotes.     

Transposon mutagenesis analysis of meta-cleavage pathway operon genes of the TOL plasmid of Pseudomonas putida mt-2.

Hybrid plasmids containing the regulated meta-cleavage pathway operon of TOL plasmid pWWO were mutagenized with transposon Tn1000 or Tn5. The resulting insertion mutant plasmids were examined for their ability to express eight of the catabolic enzymes in Escherichia coli. The physical locations of the insertions in each of 28 Tn1000 and 5 Tn5 derivative plasmids were determined by restriction endonuclease cleavage analysis. This information permitted the construction of a precise physical and genetic map of the meta-cleavage pathway operon. The gene order xylD (toluate dioxygenase), L (dihydroxycyclohexidiene carboxylate dehydrogenase), E (catechol 2,3-dioxygenase), G (hydroxymuconic semialdehyde dehydrogenase), F (hydroxymuconic semialdehyde hydrolase), J (2-oxopent-4-enoate hydratase), I (4-oxalocrotonate decarboxylase), and H (4-oxalocrotonate tautomerase) was established, and gene sizes were estimated. Tn1000 insertions within catabolic genes exerted polar effects on distal structural genes of the operon, but not on an adjacent regulatory gene xylS.     

Functional insight for beta-glucuronidase in Escherichia coli and Staphylococcus sp. RLH1

Glycosyl hydrolases hydrolyze the glycosidic bond either in carbohydrates or between carbohydrate and non-carbohydrate moiety. The beta-glucuronidase (beta D-glucuronoside glucuronosohydrolase; EC 3.2.1.31) enzyme belongs to the family-2 glycosyl hydrolase. The E. coli borne beta-glucuronidase gene (uidA) was devised as a gene fusion marker in plant genetic transformation experiments. Recent plant transformation vectors contain a novel beta-glucuronidase (gusA) derived from Staphylococcus sp. RLH1 for E. coli uidA. It is known to have a ten fold higher sensitivity compared to E. coli beta-glucuronidase. The functional superiority of Staphylococcus (gusA) over E. coli (uidA) activity is not fully known. The comparison of secondary structural elements among them revealed an increased percentage of random coils in Staphylococcus beta-glucuronidase. The 3D model of gusA shows catalytic site residues 396Glu, 508Glu and 471Tyr of gusA in loop regions. Accessible surface area (ASA) calculations on the 3D model showed increased ASA for active site residues in Staphylococcus beta-glucuronidase. Increased random coil, the presence of catalytic residues in loops, greater solvent accessibility of active residues and increased charged residues in gusA of Staphylococcus might facilitate interaction with the solvent. This hypothesizes the enhanced catalytic activity of beta-glucuronidase in Staphylococcus sp. RLH1 compared to that in E. coli.     

Novel organization of catechol meta-pathway genes in Sphingomonas sp. HV3 pSKY4 plasmid

Sphingomonas sp. strain HV3 (formerly Pseudomonas sp. HV3), which degrades aromatics and chloroaromatics, harbors a mega-plasmid, pSKY4. A sequenced 4 kb fragment of the plasmid reveals a novel gene organization for catechol meta-pathway genes. The putative meta operon starts with the cmpF gene encoding a 2-hydroxymuconic semialdehyde hydrolase. The gene has a 6 bp overlap with the previously characterized ring-cleavage gene, catechol 2,3-dioxygenase, cmpE. Downstream of cmpE is a 429 bp open reading frame of unknown function. Gene cmpC, encoding a 2-hydroxymuconic semialdehyde dehydrogenase, starts 44 bp further downstream. It has the highest homology to 2-hydroxymuconic semialdehyde dehydrogenases of dmp and xyl pathways and to XylC from the marine oligotroph Cycloclasticus oligotrophus. The gene organization is different from other known meta pathways. This is the first report of organization of plasmid-encoded meta-pathway genes in the genus Sphingomonas.     

Degradation of 4-Chlorophenol via the meta Cleavage Pathway by Comamonas testosteroni JH5

Comamonas testosteroni JH5 used 4-chlorophenol (4-CP) as its sole source of energy and carbon up to a concentration of 1.8 mM, accompanied by the stoichiometric release of chloride. The degradation of 4-CP mixed with the isomeric 2-CP by resting cells led to the accumulation of 3-chlorocatechol (3-CC), which inactivated the catechol 2,3-dioxygenase. As a result, further 4-CP breakdown was inhibited and 4-CC accumulated as a metabolite. In the crude extract of 4-CP-grown cells, catechol 1,2-dioxygenase and muconate cycloisomerase activities were not detected, whereas the activities of catechol 2,3-dioxygenase, 2-hydroxymuconic semialdehyde dehydrogenase, 2-hydroxymuconic semialdehyde hydrolase, and 2-oxopent-4-enoate hydratase were detected. These enzymes of the meta cleavage pathway showed activity with 4-CC and with 5-chloro-2-hydroxymuconic semialdehyde. The activities of the dioxygenase and semialdehyde dehydrogenase were constitutive. Two key metabolites of the meta cleavage pathway, the meta cleavage product (5-chloro-2-hydroxymuconic semialdehyde) and 5-chloro-2-hydroxymuconic acid, were detected. Thus, our previous postulation that C. testosteroni JH5 uses the meta cleavage pathway for the complete mineralization of 4-CP was confirmed.     

Metabolism of 2,2'-dihydroxybiphenyl by Pseudomonas sp. strain HBP1: production and consumption of 2,2',3-trihydroxybiphenyl.

Cells of Pseudomonas sp. strain HBP1 grown on 2-hydroxy- or 2,2'-dihydroxybiphenyl contain NADH-dependent monooxygenase activity that hydroxylates 2,2'-dihydroxybiphenyl. The product of this reaction was identified as 2,2',3-trihydroxybiphenyl by 1H nuclear magnetic resonance and mass spectrometry. Furthermore, the monooxygenase activity also hydroxylates 2,2',3-trihydroxybiphenyl at the C-3' position, yielding 2,2',3,3'-tetrahydroxybiphenyl as a product. An estradiol ring cleavage dioxygenase activity that acts on both 2,2',3-tri- and 2,2',3,3'-tetrahydroxybiphenyl was partially purified. Both substrates yielded yellow meta-cleavage compounds that were identified as 2-hydroxy-6-(2-hydroxyphenyl)-6-oxo-2,4-hexadienoic acid and 2-hydroxy-6-(2,3-dihydroxyphenyl)-6-oxo-2,4-hexadienoic acid, respectively, by gas chromatography-mass spectrometry analysis of their respective trimethylsilyl derivatives. The meta-cleavage products were not stable in aqueous incubation mixtures but gave rise to their cyclization products, 3-(chroman-4-on-2-yl)pyruvate and 3-(8-hydroxychroman-4-on-2-yl)pyruvate, respectively. In contrast to the meta-cleavage compounds, which were turned over to salicylic acid and 2,3-dihydroxybenzoic acid, the cyclization products are not substrates to the meta-cleavage product hydrolase activity. NADH-dependent salicylate monooxygenase activity catalyzed the conversions of salicylic acid and 2,3-dihydroxybenzoic acid to catechol and pyrogallol, respectively. The partially purified estradiol ring cleavage dioxygenase activity that acted on the hydroxybiphenyls also produced 2-hydroxymuconic semialdehyde and 2-hydroxymuconic acid from catechol and pyrogallol, respectively.     

Crystal structures of two Bacillus carboxylesterases with different enantioselectivities

Naproxen esterase (NP) from Bacillus subtilis Thai I-8 is a carboxylesterase that catalyzes the enantioselective hydrolysis of naproxenmethylester to produce S-naproxen (E > 200). It is a homolog of CesA (98% sequence identity) and CesB (64% identity), both produced by B. subtilis strain 168. CesB can be used for the enantioselective hydrolysis of 1,2-O-isopropylideneglycerol (solketal) esters (E > 200 for IPG-caprylate). Crystal structures of NP and CesB, determined to a resolution of 1.75 Å and 2.04 Å, respectively, showed that both proteins have a canonical α/β hydrolase fold with an extra N-terminal helix stabilizing the cap subdomain. The active site in both enzymes is located in a deep hydrophobic groove and includes the catalytic triad residues Ser130, His274, and Glu245. A product analog, presumably 2-(2-hydroxyethoxy)acetic acid, was bound in the NP active site. The enzymes have different enantioselectivities, which previously were shown to result from only a few amino acid substitutions in the cap domain. Modeling of a substrate in the active site of NP allowed explaining the different enantioselectivities. In addition, Ala156 may be a determinant of enantioselectivity as well, since its side chain appears to interfere with the binding of certain R-enantiomers in the active site of NP. However, the exchange route for substrate and product between the active site and the solvent is not obvious from the structures. Flexibility of the cap domain might facilitate such exchange. Interestingly, both carboxylesterases show higher structural similarity to meta-cleavage compound (MCP) hydrolases than to other α/β hydrolase fold esterases.     

Structure and mechanism of HpcH: a metal ion dependent class II aldolase from the homoprotocatechuate degradation pathway of Escherichia coli

Microorganisms are adept at degrading chemically resistant aromatic compounds. One of the longest and most well characterized aromatic catabolic pathways is the 4-hydroxyphenylacetic acid degradation pathway of Escherichia coli. The final step involves the conversion of 4-hydroxy-2-oxo-heptane-1,7-dioate into pyruvate and succinic semialdehyde. This reaction is catalyzed by 4-hydroxy-2-oxo-heptane-1,7-dioate aldolase (HpcH), a member of the divalent metal ion dependent class II aldolase enzymes that have great biosynthetic potential. We have solved the crystal structure of HpcH in the apo form, and with magnesium and the substrate analogue oxamate bound, to 1.6 A and 2.0 A, respectively. Comparison with similar structures of the homologous 2-dehydro-3-deoxygalactarate aldolase, coupled with site-directed mutagenesis data, implicate histidine 45 and arginine 70 as key catalytic residues.     

Protein structural change upon ligand binding correlates with enzymatic reaction mechanism

Overall structural changes of enzymes in response to ligand binding were investigated by database analysis of 62 non-redundant enzymes whose ligand-unbound and ligand-bound forms were available in the Protein Data Bank. The results of analysis indicate that transferases often undergo large rigid-body domain motions upon ligand binding, while other enzymes, most typically, hydrolases, change their structures to a small extent. It was also found that the solvent accessibility of the substrate molecule was low in transferases but high in hydrolases. These differences are explained by the enzymatic reaction mechanisms. The transferase reaction requires the catalytic groups to be insulated from the water environment, and thus transferases bury the ligand molecule inside the protein by closing the cleft. On the other hand, the hydrolase reaction involves the surrounding water molecules and occurs at the protein surface, requiring only a small structural change.     

Co-metabolism of methyl- and chloro-substituted catechols by an Achromobacter sp. possessing a new meta-cleaving oxygenase

Co-metabolism of 3-methylcatechol, 4-chlorocatechol and 3,5-dichlorocatechol by an Achromobacter sp. was shown to result in the accumulation of 2-hydroxy-3-methylmuconic semialdehyde, 4-chloro-2-hydroxymuconic semialdehyde and 3,5-dichloro-2-hydroxymuconic semialdehyde respectively. Formation of these products indicated that cleavage of the aromatic nucleus of the substituted catechols was accomplished by a new meta-cleaving enzyme, catechol 1,6-oxygenase. This enzyme was equally active on both chloro- and methyl-substituted catechols.     

The metabolism of aromatic ring fission products by Bacillus stearothermophilus strain IC3

Bacillus stearothermophilus IC3 degraded the meta cleavage product of catechol, 2-hydroxymuconic semialdehyde, to pyruvate and acetaldehyde via the 4-oxalocrotonate pathway. The pathway was identical to those previously delineated in several mesophilic organisms. However, all the enzymes showed activity at 55 degrees C and other properties (substrate specificities and effects of metal ions) also differed from those displayed by the mesophilic enzymes. All enzymes of this meta cleavage pathway, except the 2-hydroxy-6-oxohepta-2,4-dienoate hydrolase and 4-hydroxy-2-oxovalerate aldolase activities, were induced by growth on phenol.     

Stereospecific enzymes in the degradation of aromatic compounds by pseudomonas putida

Two reactions in the catabolism of catechol by meta-fission, namely, hydration of 2-oxopent-4-enoate (vinylpyruvate) and aldol fission of the product, are catalyzed by stereospecific enzymes. The absolute configuration of this hydration product was shown to be l(S)-4-hydroxy-2-oxopentanoate. Vinylpyruvate hydratase, purified almost to homogeneity, had a molecular weight of about 287,000 and was dissociated in sodium dodecyl sulfate, without prior treatment with mercaptoethanol, into a species with an approximate molecular weight of 28,000. The hydratase was highly specific for its substrates; thus, although 2-oxo-cis-hex-4-enoate was also hydrated, structurally similar compounds such as the trans isomer, vinylacetic and crotonic acids, and the ring-fission products of catechol and methylcatechols were not attacked. Vinylpyruvate hydratase was activated by Mn(2+) ions. On the basis of these observations, a mechanism is proposed which closely resembles that for 4-hydroxy-2-oxopentanoate aldolase. A possible evolutionary connection between functionally related, divalent cation-activated hydro-lyases and aldolases is discussed. It was also demonstrated that l-(S)-4-hydroxy-2-oxohexanoate is the biologically active enantiomer of this hydroxy acid.