Coenzyme B12-dependent diol dehydratase: regulation of apoenzyme synthesis in Klebsiella pneumoniae (Aerobacter aerogenes) ATCC 8724

Reactivating chaperones for coenzyme B12-dependent diol and glycerol dehydratases and ethanolamine ammonia-lyase

Adenosylcobalamin (AdoCbl) or coenzyme B₁₂-dependent enzymes tend to undergo mechanism-based inactivation during catalysis or inactivation in the absence of substrate. Such inactivation may be inevitable because they use a highly reactive radical for catalysis, and side reactions of radical intermediates result in the damage of the coenzyme. How do living organisms address such inactivation when enzymes are inactivated by undesirable side reactions? We discovered reactivating factors for radical B₁₂ eliminases. They function as releasing factors for damaged cofactor(s) from enzymes and thus mediate their exchange for intact AdoCbl. Since multiple turnovers and chaperone functions were demonstrated, they were renamed "reactivases" or "reactivating chaperones." They play an essential role in coenzyme recycling as part of the activity-maintaining systems for B₁₂ enzymes. In this chapter, we describe our investigations on reactivating chaperones, including their discovery, gene cloning, preparation, characterization, activity assays, and mechanistic studies, that have been conducted using a wide range of biochemical and structural methods that we have developed.

Coenzyme B12-dependent eliminases: Diol and glycerol dehydratases and ethanolamine ammonia-lyase

Adenosylcobalamin (AdoCbl) or coenzyme B₁₂-dependent enzymes catalyze intramolecular group-transfer reactions and ribonucleotide reduction in a wide variety of organisms from bacteria to animals. They use a super-reactive primary-carbon radical formed by the homolysis of the coenzyme's Co-C bond for catalysis and thus belong to the larger class of "radical enzymes." For understanding the general mechanisms of radical enzymes, it is of great importance to establish the general mechanism of AdoCbl-dependent catalysis using enzymes that catalyze the simplest reactions-such as diol dehydratase, glycerol dehydratase and ethanolamine ammonia-lyase. These enzymes are often called "eliminases." We have studied AdoCbl and eliminases for more than a half century. Progress has always been driven by the development of new experimental methodologies. In this chapter, we describe our investigations on these enzymes, including their metabolic roles, gene cloning, preparation, characterization, activity assays, and mechanistic studies, that have been conducted using a wide range of biochemical and structural methodologies we have developed.

Key enzymes catalyzing glycerol to 1,3-propanediol

Biodiesel can replace petroleum diesel as it is produced from animal fats and vegetable oils, and it produces about 10 % (w/w) glycerol, which is a promising new industrial microbial carbon, as a major by-product. One of the most potential applications of glycerol is its biotransformation to high value chemicals such as 1,3-propanediol (1,3-PD), dihydroxyacetone (DHA), succinic acid, etc., through microbial fermentation. Glycerol dehydratase, 1,3-propanediol dehydrogenase (1,3-propanediol-oxydoreductase), and glycerol dehydrogenase, which were encoded, respectively, by dhaB, dhaT, and dhaD and with DHA kinase are encompassed by the dha regulon, are the three key enzymes in glycerol bioconversion into 1,3-PD and DHA, and these are discussed in this review article. The summary of the main research direction of these three key enzyme and methods of glycerol bioconversion into 1,3-PD and DHA indicates their potential application in future enzymatic research and industrial production, especially in biodiesel industry.

Diol Dehydratase-Reactivase Is Essential for Recycling of Coenzyme B12 in Diol Dehydratase

Holoenzymes of adenosylcobalamin-dependent diol and glycerol dehydratases undergo mechanism-based inactivation by glycerol and O2 inactivation in the absence of substrate, which accompanies irreversible cleavage of the coenzyme Co-C bond. The inactivated holodiol dehydratase and the inactive enzyme·cyanocobalamin complex were (re)activated by incubation with NADH, ATP, and Mg(2+) (or Mn(2+)) in crude extracts of Klebsiella oxytoca, suggesting the presence of a reactivating system in the extract. The reducing system with NADH could be replaced by FMNH2. When inactivated holoenzyme or the enzyme·cyanocobalamin complex, a model of inactivated holoenzyme, was incubated with purified recombinant diol dehydratase-reactivase (DD-R) and an ATP:cob(I)alamin adenosyltransferase in the presence of FMNH2, ATP, and Mg(2+), diol dehydratase activity was restored. Among the three adenosyltransferases (PduO, EutT, and CobA) of this bacterium, PduO and CobA were much more efficient for the reactivation than EutT, although PduO showed the lowest adenosyltransfease activity toward free cob(I)alamin. These results suggest that (1) diol dehydratase activity is maintained through coenzyme recycling by a reactivating system for diol dehydratase composed of DD-R, PduO adenosyltransferase, and a reducing system, (2) the releasing factor DD-R is essential for the recycling of adenosycobalamin, a tightly bound, prosthetic group-type coenzyme, and (3) PduO is a specific adenosylating enzyme for the DD reactivation, whereas CobA and EutT exert their effects through free synthesized coenzyme. Although FMNH2 was mainly used as a reductant in this study, a natural reducing system might consist of PduS cobalamin reductase and NADH.

Diol dehydratase-reactivating factor is a reactivase--evidence for multiple turnovers and subunit swapping with diol dehydratase

Adenosylcobalamin-dependent diol dehydratase (DD) undergoes suicide inactivation by glycerol, one of its physiological substrates, resulting in the irreversible cleavage of the coenzyme Co-C bond. The damaged cofactor remains tightly bound to the active site. The DD-reactivating factor reactivates the inactivated holoenzyme in the presence of ATP and Mg(2+) by mediating the exchange of the tightly bound damaged cofactor for free intact coenzyme. In this study, we demonstrated that this reactivating factor mediates the cobalamin exchange not stoichiometrically but catalytically in the presence of ATP and Mg(2+). Therefore, we concluded that the reactivating factor is a sort of enzyme. It can be designated DD reactivase. The reactivase showed broad specificity for nucleoside triphosphates in the activation of the enzyme·cyanocobalamin complex. This result is consistent with the lack of specific interaction with the adenine ring of ADP in the crystal structure of the reactivase. The specificities of the reactivase for divalent metal ions were also not strict. DD formed 1:1 and 1:2 complexes with the reactivase in the presence of ADP and Mg(2+). Upon complex formation, one β subunit was released from the (αβ)₂ tetramer of the reactivase. This result, together with the similarity in amino acid sequences and folds between the DD β subunit and the reactivase β subunit, suggests that subunit displacement or swapping takes place upon formation of the enzyme·reactivase complex. This would result in the dissociation of the damaged cofactor from the inactivated holoenzyme, as suggested by the crystal structures of the reactivase and DD.

Characterization of glycerol dehydratase expressed by fusing its alpha- and beta-subunits

The gdh and gdhr genes, encoding B(12)-dependent glycerol dehydratase (GDH) and glycerol dehydratase reactivase (GDHR), respectively, in Klebsiella pneumoniae, were cloned and expressed in E. coli. Part of the beta-subunit was lost during GDH purification when co-expressing alpha, beta and gamma subunit. This was overcome by fusing the beta-subunit to alpha- or gamma-subunit with/without the insertion of a linker peptide between the fusion moieties. The kinetic properties of the fusion enzymes were characterized and compared with wild type enzyme. The results demonstrated that the fusion protein GDHALB/C, constructed by linking the N-terminal of beta-subunit to the C-terminal of alpha subunit through a (Gly(4)Ser)(4) linker peptide, had the greatest catalytic activity. Similar to the wild-type enzyme, GDHALB/C underwent mechanism-based inactivation by glycerol during catalysis and could be reactivated by GDHR.

Molecular basis for specificities of reactivating factors for adenosylcobalamin-dependent diol and glycerol dehydratases

Adenosylcobalamin-dependent diol and glycerol dehydratases are isofunctional enzymes and undergo mechanism-based inactivation by a physiological substrate glycerol during catalysis. Inactivated holoenzymes are reactivated by their own reactivating factors that mediate the ATP-dependent exchange of an enzyme-bound, damaged cofactor for free adenosylcobalamin through intermediary formation of apoenzyme. The reactivation takes place in two steps: (a) ADP-dependent cobalamin release and (b) ATP-dependent dissociation of the resulting apoenzyme-reactivating factor complexes. The in vitro experiments with purified proteins indicated that diol dehydratase-reactivating factor (DDR) cross-reactivates the inactivated glycerol dehydratase, whereas glycerol dehydratase-reactivating factor (GDR) did not cross-reactivate the inactivated diol dehydratase. We investigated the molecular basis of their specificities in vitro by using purified preparations of cognate and noncognate enzymes and reactivating factors. DDR mediated the exchange of glycerol dehydratase-bound cyanocobalamin for free adeninylpentylcobalamin, whereas GDR cannot mediate the exchange of diol dehydratase-bound cyanocobalamin for free adeninylpentylcobalamin. As judged by denaturing PAGE, the glycerol dehydratase-DDR complex was cross-formed, although the diol dehydratase-GDR complex was not formed. There were no specificities of reactivating factors in the ATP-dependent dissociation of enzyme-reactivating factor complexes. Thus, it is very likely that the specificities of reactivating factors are determined by the capability of reactivating factors to form complexes with apoenzymes. A modeling study based on the crystal structures of enzymes and reactivating factors also suggested why DDR cross-forms a complex with glycerol dehydratase, and why GDR does not cross-form a complex with diol dehydratase.

Release of a damaged cofactor from a coenzyme B12-dependent enzyme: X-ray structures of diol dehydratase-reactivating factor

The crystal structures of ADP bound and nucleotide-free forms of molecular chaperone-like diol dehydratase-reactivating factor (DDR) were determined at 2.0 and 3.0 A, respectively. DDR exists as a dimer of heterodimer (alphabeta)2. The alpha subunit has four domains: ATPase domain, swiveling domain, linker domain, and insert domain. The beta subunit, composed of a single domain, has a similar fold to the beta subunit of diol dehydratase (DD). The binding of an ADP molecule to the nucleotide binding site of DDR causes a marked conformational change of the ATPase domain of the alpha subunit, which would weaken the interactions between the DDR alpha and beta subunits and make the displacement of the DDR beta subunit by DD through the beta subunit possible. The binding of the DD beta subunit to the DDR alpha subunit induces steric repulsion between the DDR alpha and DD alpha subunits that would lead to the release of a damaged cofactor from inactivated holoDD.

Identification of a reactivating factor for adenosylcobalamin-dependent ethanolamine ammonia lyase

The holoenzyme of adenosylcobalamin-dependent ethanolamine ammonia lyase undergoes suicidal inactivation during catalysis as well as inactivation in the absence of substrate. The inactivation involves the irreversible cleavage of the Co-C bond of the coenzyme. We found that the inactivated holoenzyme undergoes rapid and continuous reactivation in the presence of ATP, Mg2+, and free adenosylcobalamin in permeabilized cells (in situ), homogenate, and cell extracts of Escherichia coli. The reactivation was observed in the permeabilized E. coli cells carrying a plasmid containing the E. coli eut operon as well. From coexpression experiments, it was demonstrated that the eutA gene, adjacent to the 5' end of ethanolamine ammonia lyase genes (eutBC), is essential for reactivation. It encodes a polypeptide consisting of 467 amino acid residues with predicted molecular weight of 49,599. No evidence was obtained that shows the presence of the auxiliary protein(s) potentiating the reactivation or associating with EutA. It was demonstrated with purified recombinant EutA that both the suicidally inactivated and O2-inactivated holoethanolamine ammonia lyase underwent rapid reactivation in vitro by EutA in the presence of adenosylcobalamin, ATP, and Mg2+. The inactive enzyme-cyanocobalamin complex was also activated in situ and in vitro by EutA under the same conditions. Thus, it was concluded that EutA is the only component of the reactivating factor for ethanolamine ammonia lyase and that reactivation and activation occur through the exchange of modified coenzyme for free intact adenosylcobalamin.

The crystal structure of coenzyme B12-dependent glycerol dehydratase in complex with cobalamin and propane-1,2-diol

Recombinant glycerol dehydratase of Klebsiella pneumoniae was purified to homogeneity. The subunit composition of the enzyme was most probably alpha 2 beta 2 gamma 2. When (R)- and (S)-propane-1,2-diols were used independently as substrates, the rate with the (R)-enantiomer was 2.5 times faster than that with the (S)-isomer. In contrast to diol dehydratase, an isofunctional enzyme, the affinity of the enzyme for the (S)-isomer was essentially the same or only slightly higher than that for the (R)-isomer (Km(R)/Km(S) = 1.5). The crystal structure of glycerol dehydratase in complex with cyanocobalamin and propane-1,2-diol was determined at 2.1 A resolution. The enzyme exists as a dimer of the alpha beta gamma heterotrimer. Cobalamin is bound at the interface between the alpha and beta subunits in the so-called 'base-on' mode with 5,6-dimethylbenzimidazole of the nucleotide moiety coordinating to the cobalt atom. The electron density of the cyano group was almost unobservable, suggesting that the cyanocobalamin was reduced to cob(II)alamin by X-ray irradiation. The active site is in a (beta/alpha)8 barrel that was formed by a central region of the alpha subunit. The substrate propane-1,2-diol and essential cofactor K+ are bound inside the (beta/alpha)8 barrel above the corrin ring of cobalamin. K+ is hepta-coordinated by the two hydroxyls of the substrate and five oxygen atoms from the active-site residues. These structural features are quite similar to those of diol dehydratase. A closer contact between the alpha and beta subunits in glycerol dehydratase may be reminiscent of the higher affinity of the enzyme for adenosylcobalamin than that of diol dehydratase. Although racemic propane-1,2-diol was used for crystallization, the substrate bound to glycerol dehydratase was assigned to the (R)-isomer. This is in clear contrast to diol dehydratase and accounts for the difference between the two enzymes in the susceptibility of suicide inactivation by glycerol.

Mechanism of reactivation of coenzyme B12-dependent diol dehydratase by a molecular chaperone-like reactivating factor

The mechanism of reactivation of diol dehydratase by its reactivating factor was investigated in vitro by using enzyme. cyanocobalamin complex as a model for inactivated holoenzyme. The factor mediated the exchange of the enzyme-bound, adenine-lacking cobalamins for free, adenine-containing cobalamins through intermediate formation of apoenzyme. The factor showed extremely low but distinct ATP-hydrolyzing activity. It formed a tight complex with apoenzyme in the presence of ADP but not at all in the presence of ATP. Incubation of the enzyme.cyanocobalamin complex with the reactivating factor in the presence of ADP brought about release of the enzyme-bound cobalamin, leaving the tight apoenzyme-reactivating factor complex. Although the resulting complex was inactive even in the presence of added adenosylcobalamin, it dissociated by incubation with ATP, forming the apoenzyme, which was reconstitutable into active holoenzyme with added coenzyme. Thus, it was established that the reactivation of the inactivated holoenzyme by the factor in the presence of ATP and Mg2+ takes place in two steps: ADP-dependent cobalamin release and ATP-dependent dissociation of the apoenzyme.factor complex. ATP plays dual roles as a precursor of ADP in the first step and as an effector to change the factor into the low-affinity form for diol dehydratase. The enzyme-bound adenosylcobalamin was also susceptible to exchange with free adeninylpentylcobalamin, although to a much lesser degree. The mechanism for discrimination of adenine-containing cobalamins from adenine-lacking cobalamins was explained in terms of formation equilibrium constants of the cobalamin.enzyme.reactivating factor ternary complexes. We propose that the reactivating factor is a new type of molecular chaperone that participates in reactivation of the inactivated enzymes.

A reactivating factor for coenzyme B12-dependent diol dehydratase

Adenosylcobalamin-dependent diol dehydratase of Klebsiella oxytoca undergoes suicide inactivation by glycerol, a physiological substrate. The coenzyme is modified through irreversible cleavage of its cobalt-carbon bond, resulting in inactivation of the enzyme by tight binding of the modified coenzyme to the active site. Recombinant DdrA and DdrB proteins of K. oxytoca were co-purified to homogeneity from cell-free extracts of Escherichia coli overexpressing the ddrAB genes. They existed as a tight complex, i.e. a putative reactivating factor, with an apparent molecular weight of 150,000. The factor consists of equimolar amounts of the two subunits with Mr of 64,000 (A) and 14,000 (B), encoded by the ddrA and ddrB genes, respectively. Therefore, its subunit structure is most likely A2B2. The factor not only reactivated glycerol-inactivated and O2-inactivated holoenzymes but also activated enzyme-cyanocobalamin complex in the presence of free adenosylcobalamin, ATP, and Mg2+. The reactivating factor mediated ATP-dependent exchange of the enzyme-bound cyanocobalamin for free 5-adeninylpentylcobalamin in the presence of ATP and Mg2+, but the reverse was not the case. Thus, it can be concluded that the inactivated holoenzyme becomes reactivated by exchange of the enzyme-bound, adenine-lacking cobalamins for free adenosylcobalamin, an adenine-containing cobalamin.

Characterization, sequencing, and expression of the genes encoding a reactivating factor for glycerol-inactivated adenosylcobalamin-dependent diol dehydratase

Diol dehydratase undergoes suicide inactivation by glycerol during catalysis involving irreversible cleavage of the Co-C bond of adenosylcobalamin. In permeabilized Klebsiella oxytoca and Klebsiella pneumoniae cells, the glycerol-inactivated holoenzyme or the enzyme-cyanocobalamin complex is rapidly activated by the exchange of the inactivated coenzyme or cyanocobalamin for free adenosylcobalamin in the presence of ATP and Mg2+ (Honda, S., Toraya, T., and Fukui, S. (1980) J. Bacteriol. 143, 1458-1465; Ushio, K., Honda, S., Toraya, T., and Fukui, S. (1982) J. Nutr. Sci. Vitaminol. 28, 225-236). Permeabilized Escherichia coli cells co-expressing the diol dehydratase genes with two open reading frames in the 3'-flanking region were capable of reactivating glycerol-inactivated diol dehydratase as well as activating the enzyme-cyanocobalamin complex in situ in the presence of free adenosylcobalamin, ATP, and Mg2+. These open reading frames, designated as ddrA and ddrB genes, were identified as the genes of a putative reactivating factor for inactivated diol dehydratase. The genes encoded polypeptides consisting of 610 and 125 amino acid residues with predicted molecular weights of 64,266 and 13,620, respectively. Co-expression of the open reading frame in the 5'-flanking region was stimulatory but not obligatory for conferring the reactivating activity upon E. coli. Thus, the product of this gene was considered not an essential component of the reactivating factor.

Phenotypic diversity of anaerobic glycerol dissimilation shown by seven enterobacterial species

The anaerobic glycerol pathway was studied in seven enterobacterial species selected as representative of different behaviours in terms of anaerobic glycerol dissimilation. The presence of oxidative and reductive pathways of the dha regulon in Klebsiella pneumoniae enabled the cells to grow fermentatively on glycerol. The first two enzymes of the dha regulon (glycerol dehydrogenase type I and dihydroxyacetone kinase) represent the oxidative branch, while the latter two (glycerol dehydratase and 1,3-propanediol dehydrogenase) represent the reductive branch of glycerol fermentation. The slower utilization of glycerol by K. oxytoca was attributed to low production of 1,3-propanediol. K. oxytoca lacked glycerol dehydratase and demonstrated low 1,3-propanediol dehydrogenase activity. K. planticola and K. ozaenae differed from K. pneumoniae and K. oxytoca in lacking the ability to grow on glycerol. K. planticola lacked both enzymes of the reductive branch of glycerol fermentation, and K. ozaenae possessed glycerol dehydrogenase only. K. rhinoscleromatis and Hafnia alvei, like Escherichia coli, did not possess a dha regulon. The glycerol dehydrogenase type II of H. alvei was distinct from that of E. coli. The phenotypic diversity of anaerobic glycerol dissimilation may have taxonomic applications.

Anaerobic degradation of 1,2-propanediol by a new Desulfovibrio strain and D. alcoholovorans

A sulfate-reducing bacterium, strain HDv, was isolated from the anoxic soil of a ricefield using lactate as electron donor. Cells were gram-negative, motile, nonsporulating curved rods, with single polar flagella. Substrates were incompletely oxidized to acetate and included glycerol, 1,2- and 1,3-propanediol. Sulfate, sulfite, thiosulfate, elemental sulfur, fumarate, maleate, and malate were utilized as electron acceptors. Pyruvate, fumarate, maleate, malate and dihydroxyacetone were fermented. Desulfoviridin and c-type cytochromes were present. The DNA base composition was 66.6 +/- 0.3 mol% G+C. The isolate was identified as a Desulfovibrio sp.; its metabolic properties were somewhat different from those of previously described Desulfovibrio species. Comparative biochemical study of 1,2-propanediol dissimilation by the new isolate and Desulfovibrio alcoholovorans showed that NAD-dependent dehydrogenases play a key role in the catabolism of this substrate. The hypothetical pathways of 1,2-propanediol degradation by Desulfovibrio spp. are presented.

Optimizing aerobic conversion of glycerol to 3-hydroxypropionaldehyde

When cells of Klebsiella pneumoniae NRRL B-199 (ATCC 8724) were grown aerobically on a rich glycerol medium and then suspended in buffer supplemented with semicarbazide and glycerol, aerobic conversion of glycerol to 3-hydroxypropionaldehyde (3-HPA) ensued. Depending on conditions, 0.38 to 0.67 g of 3-HPA were formed per gram of glycerol consumed. This means that up to 83.8% of the carbon invested as glycerol could potentially be recovered as the target product, 3-HPA. Production of 3-HPA was sensitive to the age of cells harvested for resuspension and was nonexistent if cells were cultivated on glucose instead of glycerol as the sole carbon source. Compared with 24- and 72-h cells, 48-h cells produced 3-HPA at the highest rate and with the greatest yield. The cell biomass concentration present during the fermentation was never particularly critical to the 3-HPA yield, but initial fermentation rates and 3-HPA accumulation displayed a linear dependence on biomass concentration that faded when biomass exceeded 3 g/liter. Fermentation performance was a function of temperature, and an optimum initial specific 3-HPA productivity occurred at 32 degrees C, although the overall 3-HPA yield increased continuously within the 25 to 37 degrees C range studied. The pH optimum based on fermentation rate was different from that based on overall yield; 8 versus 7, respectively. Initial glycerol concentrations in the 20 to 50 g/liter range optimized initial 3-HPA productivity and yield.

Glycerol fermentation in Klebsiella pneumoniae: functions of the coenzyme B12-dependent glycerol and diol dehydratases

Glycerol and diol dehydratases are inducible, coenzyme B12-dependent enzymes found together in Klebsiella pneumoniae ATCC 25955 during anaerobic growth on glycerol. Mutants of this strain isolated by a novel procedure were separately constitutive for either dehydratase, showing the structural genes for the two enzymes to be under independent control in vivo. Glycerol dehydratase and a trimethylene glycol dehydrogenase were implicated as members of a pleiotropic control system that includes glycerol dehydrogenase and dihydroxyacetone kinase for the anaerobic dissimilation of glycerol (the "dha system"). The dehydratase and dehydrogenases were induced by dihydroxyacetone and were jointly constitutive in mutants isolated as constitutive for either the dha system or glycerol dehydratase. These data and the stimulation of growth by Co2+ suggested that glycerol dehydratase and trimethylene glycol dehydrogenase are obligatory enzymes for anaerobic growth on glycerol as the sole carbon source.

In situ reactivation of glycerol-inactivated coenzyme B12-dependent enzymes, glycerol dehydratase and diol dehydratase

The catalytic properties of coenzyme B12-dependent glycerol dehydratase and diol dehydratase were studied in situ with Klebsiella pneumoniae cells permeabilized by toluene treatment, since the in situ enzymes approximate the in vivo conditions of the enzymes more closely than enzymes in cell-free extracts or cell homogenates. Both dehydratases in situ underwent rapid "suicidal" inactivation by glycerol during catalysis, as they do in vitro. The inactivated dehydratases in situ, however, were rapidly and continually reactivated by adenosine 5'-triphosphate (ATP) and Mn2+ in the presence of free adenosylcobalamin, although in cell-free extracts or in cell homogenates they could not be reactivated at all under the same reaction conditions. ATP was partially replaced by cytidine 5'-triphosphate or guanosine 5'-triphosphate but not by the beta, gamma-methylene analog of ATP in the in situ reactivation. Mn2+ was fully replaced by Mg2+ but only partially by Co2+. Hydroxocoblamin could not replace adenosylcobalamin in reactivation mixtures. The ability to reactivate the glycerol-inactivated dehydratases in situ was only seen in cells grown anaerobically in glycerol-containing media. This suggests that some factor(s) required for in situ reactivation is subject to induction by glycerol. Of the two possible mechanisms of in situ reactivation, i.e., the regeneration of adenosylcobalamin by Co-adenosylation of the bound inactivated coenzyme moiety (B12-adenosylation mechanism) and the displacement of the bound inactivated coenzyme moiety by free adenosyl-cobalamin (B12-exchange mechanism), the former seems very unlikely from the experimental results.

Distribution of coenzyme B12-dependent diol dehydratase and glycerol dehydratase in selected genera of Enterobacteriaceae and Propionibacteriaceae

The presence of diol dehydratase and glycerol dehydratase was shown in several bacteria of Enterobacteriaceae grown anaerobically on 1,2-propanediol and on glycerol, respectively. Diol dehydratases of Enterobacteriaceae were immunologically similar, but distinct from that of Propionibacterium freudenreichii.

Resolution of the coenzyme B-12-dependent dehydratases of Klebsiella sp. and Citrobacter freundii

Diol dehydratase (1,2-propanediol hydro-lyase, EC 4.2.1.28) and glycerol dehydratase (glycerol hydro-lyase, EC 4.2.1.30) are shown to be distinct, separable enzymes that occur individually or together in different strains of Klebsiella sp. Anaerobic growth with propan-1,2-diol induces diol dehydratase alone, whereas glycerol fermentation induces both enzymes in K. pneumoniae ATCC 25955 and in Citrobacter freundii NCIB 3735. The dehydratases can be resolved by polyacrylamide-gel electrophoresis or separated by anion-exchange chromatography alone. Sucrose density gradient centrifugation failed to distinguish the enzymes and indicated a molecular weight of 1.9 . 10(5) for both. The enzymes can be assayed individually, even when present in the same crude extract, using the 67-fold difference in their Km values for coenzyme B-12. For both enzymes inactivation kinetics are observed with glycerol as substrated, and monovalent cations influence both the inactivation rate and catalytic rate of the reaction.

Fermentation of 1,2-propanediol with 1,2-ethanediol by some genera of Enterobacteriaceae, involving coenzyme B12-dependent diol dehydratase

Klebsiella pneumoniae (Aerobacter aerogenes) ATCC 8724 was able to grow anaerobically on 1,2-propanediol and 1,2-ethanediol as carbon and energy sources. Whole cells of the bacterium grown anaerobically on 1,2-propanediol or on glycerol catalyzed conversion of 1,2-diols and aldehydes to the corresponding acids and alcohols. Glucose-grown cells also converted aldehydes, but not 1,2-diols, to acids and alcohols. The presence of activities of coenzyme B(12)-dependent diol dehydratase, alcohol dehydrogenase, coenzyme-A-dependent aldehyde dehydrogenase, phosphotransacetylase, and acetate kinase was demonstrated with crude extracts of 1,2-propanediol-grown cells. The dependence of the levels of these enzymes on growth substrates, together with cofactor requirements in in vitro conversion of these substrates, indicates that 1,2-diols are fermented to the corresponding acids and alcohols via aldehydes, acyl-coenzyme A, and acyl phosphates. This metabolic pathway for 1,2-diol fermentation was also suggested in some other genera of Enterobacteriaceae which were able to grow anaerobically on 1,2-propanediol. When the bacteria were cultivated in a 1,2-propanediol medium not supplemented with cobalt ion, the coenzyme B(12)-dependent conversion of 1,2-diols to aldehydes was the rate-limiting step in this fermentation. This was because the intracellular concentration of coenzyme B(12) was very low in the cells grown in cobalt-deficient medium, since the apoprotein of diol dehydratase was markedly induced in the cells grown in the 1,2-propanediol medium. Better cell yields were obtained when the bacteria were grown anaerobically on 1,2-propanediol. Evidence is presented that aerobically grown cells have a different metabolic pathway for utilizing 1,2-propanediol.