A Theoretical Study on the Metabolic Requirements Resulting from α-Ketoglutarate-Dependent Cleavage of Phenoxyalkanoates

Abstract

The etherolytic cleavage of phenoxyalkanoic acids in various bacteria is catalyzed by an ␣-ketoglutaratedependent dioxygenase. In this reaction, the electron acceptor is oxidatively decarboxylated to succinate, whereas the proper substrate is cleaved by forming the oxidized alkanoic acid and the phenolic intermediate. The necessity of regenerating ␣-ketoglutarate and the consequences for the overall metabolism were investigated in a theoretical study. It was found that the dioxygenase mechanism is accompanied by a significant loss of carbon amounting to up to 62.5% in the assimilatory branch, thus defining the upper limit of carbon conversion efficiency. This loss in carbon is almost compensated for in comparison to a monooxygenasecatalyzed initial step when the dissimilatory efforts of the entire metabolism are included: the yield coefficients become similar. The ␣-ketoglutarate-dependent dioxygenase mechanism has more drastic consequences for microorganisms which are restricted in their metabolism to the first step of phenoxyalkanoate degradation by excreting the phenolic intermediate as a dead-end product. In the case of phenoxyacetate derivatives, the cleavage reaction would quickly cease due to the exhaustion of ␣-ketoglutarate and no growth would be possible. With the cleavage products of phenoxypropionate and phenoxybutyrate herbicides, i.e., pyruvate and succinate(semialdehyde), respectively, as the possible products, the regeneration of ␣-ketoglutarate will be guaranteed for stoichiometric reasons. However, the maintenance of the cleavage reaction ought to be restricted due to physiological factors owing to the involvement of other metabolic reactions in the pool of metabolites. These effects are discussed in terms of a putative recalcitrance of these compounds.

Uptake Kinetics of 2,4-Dichlorophenoxyacetate byDelftia acidovoransMC1 and Derivative Strains: Complex Characteristics in Response to pH and Growth Substrate

“…It should be noted that the pKa values of 2,4-D and 2,4-DP are 2.6 and 3.0 respectively, i.e., the non-dissociated acid was present only at about 0.01-0.1%, near neutral pH. In the case of strain MC1, the presence of -ketoglutarate was found to have great impact on degradative performance: a potential shortage of this compound due to starvation led to pseudo-recalcitrance of 2,4-D. 55,56) It should be emphasized that the cells used in this investigation were also characterized by a starvation state caused by the time required for biomass collection and preparation. Thus immediate onset of degradation is not to be expected, at least not at a rate that would correspond to the external substrate concentration.…”

“…The cleavage reaction depends on the presence ofketoglutarate, the supply of which is strongly coupled to the general metabolism and to the physiological state of the population. 55,56) Thus the kinetics of these enzymes in combination with the kinetics of the uptake proteins will finally determine the rates of 2,4-D utilization. This is the essence of the model of 2,4-D utilization by R. eutropha JMP134 presented by Leveau et al 27) Uptake in the higher concentration range was assumed by these authors to take place to a great extent by diffusion of the non-dissociated acid.…”

Delftia acidovoransMC1 Resists High Herbicide Concentrations — a Study of Nutristat Growth on (RS)-2-(2,4-Dichlorophenoxy)propionate and 2,4-Dichlorophenoxyacetate

“…-ketoglutarate which is an essential component of the initial degradative step of these herbicides 19) thus making the reaction (degradation) run. 12,13) Nevertheless, the rate found in situ on building rubble should only be faint due to a low titer of microbes. Any improvement of the rate by autocatalytic, i.e.…”

“…This may partly be due to their specific metabolism, which includes some potential recalcitrance. 12,13) But it may also result from the fact that the efficiency of productive degradation is reduced or even repressed by too high a level of the toxicant. This was shown to be the case with Alcaligenes eutrophus (Ralstonia eutropha) JMP134 during longterm exposure to 2,4-D. 14) When we studied herbicide degradation in polluted building rubble with herbicide concentrations of up to 5 mM and pH values > 11 in the aqueous eluates, i.e.…”

Pseudo-recalcitrance of Chlorophenoxyalkanoate Herbicides - Correlation to the Availability ofα-Ketoglutarate

“…The results showed that 2,4-dichlorophenoxyacetate (2,4-D) could hardly be cleaved under these conditions which is attributed to the inability to regenerate α-ketoglutarate from the cleavage products, i.e. succinate and glyoxylate [1]. With pyruvate, in contrast, liberated as the oxidized alkanoic acid from the cleavage of (RS)-2-(2,4-dichlorophenoxy)propionate (2,4-DP), the regeneration of α-ketoglutarate seems to be guaranteed from succinate as resulted from the utilization of 2,4-DP to a considerable amount under these conditions.…”