Threonine formation via the coupled activity of 2-amino-3-ketobutyrate coenzyme A lyase and threonine dehydrogenase.

The enzymes L-threonine dehydrogenase and 2-amino-3-ketobutyrate coenzyme A (CoA) lyase are known to catalyze the net conversion of L-threonine plus NAD+ plus CoA to NADH plus glycine plus acetyl-CoA. When homogeneous preparations of these two enzymes from Escherichia coli were incubated together for 40 min at 25 degrees C with glycine, acetyl-CoA, and NADH, a 36% decrease in the level of glycine (with concomitant NADH oxidation) was matched by formation of an equivalent amount of threonine, indicating that this coupled sequence of enzyme-catalyzed reactions is reversible in vitro. Several experimental factors that affect the efficiency of this conversion in vitro were examined. A constructed strain of E. coli, MD901 (glyA thrB/C tdh), was unable to grow unless both glycine and threonine were added to defined rich medium. Introduction of the plasmid pDR121 (tdh+kbl+) into this strain enabled the cells to grow in the presence of either added glycine or threonine, indicating that interconversion of these two amino acids occurred. Threonine that was isolated from the total pool of cellular protein of MD901/pDR121 had the same specific radioactivity as the [14C]glycine added to the medium, establishing that threonine was formed exclusively from glycine in this strain. Comparative growth rate studies with several strains of E. coli containing plasmid pDR121, together with the finding that kcat values of pure E. coli 2-amino-3-ketobutyrate CoA lyase favor the cleavage of 2-amino-3-ketobutyrate over its formation by a factor of 50, indicate that the biosynthesis of threonine is less efficient than glycine formation via the coupled threonine dehydrogenase-2-amino-3-ketobutyrate lyase reactions.     

Three-dimensional structure of 2-amino-3-ketobutyrate CoA ligase from Escherichia coli complexed with a PLP-substrate intermediate: inferred reaction mechanism

2-Amino-3-ketobutyrate CoA ligase (KBL, EC 2.3.1.29) is a pyridoxal phosphate (PLP) dependent enzyme, which catalyzes the second reaction step on the main metabolic degradation pathway for threonine. It acts in concert with threonine dehydrogenase and converts 2-amino-3-ketobutyrate, the product of threonine dehydrogenation by the latter enzyme, with the participation of cofactor CoA, to glycine and acetyl-CoA. The enzyme has been well conserved during evolution, with 54% amino acid sequence identity between the Escherichia coli and human enzymes. We present the three-dimensional structure of E. coli KBL determined at 2.0 A resolution. KBL belongs to the alpha family of PLP-dependent enzymes, for which the prototypic member is aspartate aminotransferase. Its closest structural homologue is E. coli 8-amino-7-oxononanoate synthase. Like many other members of the alpha family, the functional form of KBL is a dimer, and one such dimer is found in the asymmetric unit in the crystal. There are two active sites per dimer, located at the dimer interface. Both monomers contribute side chains to each active/substrate binding site. Electron density maps indicated the presence in the crystal of the Schiff base intermediate of 2-amino-3-ketobutyrate and PLP, an external aldimine, which remained bound to KBL throughout the protein purification procedure. The observed interactions between the aldimine and the side chains in the substrate binding site explain the specificity for the substrate and provide the basis for a detailed proposal of the reaction mechanism of KBL. A putative binding site of the CoA cofactor was assigned, and implications for the cooperation with threonine dehydrogenase were considered.     

Role of threonine dehydrogenase in Escherichia coli threonine degradation.

Threonine was used as nitrogen source by Escherichia coli K-12 through a pathway beginning with the enzyme threonine dehydrogenase. The 2-amino-3-ketobutyrate formed was converted to glycine, and the glycine was converted to serine, which acted as the actual nitrogen donor. The enzyme formed under anaerobic conditions and known as threonine deaminase (biodegradative) is less widespread than threonine dehydrogenase and may be involved in energy metabolism rather than in threonine degradation per se.     

Engineering of a novel biochemical pathway for the biosynthesis of L-2-aminobutyric acid in Escherichia coli K12

L-2-Aminobutyric acid was synthesised in a transamination reaction from L-threonine and L-aspartic acid as substrates in a whole cell biotransformation using recombinant Escherichia coli K12. The cells contained the cloned genes tyrB, ilvA and alsS which respectively encode tyrosine aminotransferase of E. coli, threonine deaminase of E. coli and alpha-acetolactate synthase of B. subtilis 168. The 2-aminobutyric acid was produced by the action of the aminotransferase on 2-ketobutyrate and L-aspartate. The 2-ketobutyrate is generated in situ from L-threonine by the action of the deaminase, and the pyruvate by-product is eliminated by the acetolactate synthase. The concerted action of the three enzymes offers significant yield and purity advantages over the process using the transaminase alone with an eight to tenfold increase in the ratio of product to the major impurity.     

Metabolic Basis for the Isoleucine, Pantothenate or Methionine Requirement of ilvG Strains of Salmonella typhimurium

Salmonella typhimurium strain DU501, which was found to be deficient in acetohydroxy acid synthase II (AHAS II) and to possess elevated levels of transaminase B and biosynthetic threonine deaminase, required isoleucine, methionine, or pantothenate for growth. This strain accumulated α-ketobutyrate and, to a lesser extent, α-aminobutyrate. We found that α-ketobutyrate was a competitive substrate for ketopantoate hydroxymethyltransferase, the first enzyme in pantothenate biosynthesis. This competition with the normal substrate, α-ketoisovalerate, limited the supply of pantothenate, which resulted in a requirement for methionine. Evidence is presented to support the conclusion that the ambivalent requirement for either pantothenate or methionine is related to a decrease in succinyl coenzyme A, which is produced from pantothenate and which is an obligatory precursor of methionine biosynthesis. The autointoxification by endogenously produced α-ketobutyrate could be mimicked in wild-type S. typhimurium by exogenously supplied α-ketobutyrate or salicylate, a known inhibitor of pantothenate biosynthesis. The accumulation of α-ketobutyrate was initiated by the inability of the residual AHAS activity provided by AHAS I to efficiently remove the α-ketobutyrate produced by biosynthetic threonine deaminase. The accumulation of α-ketobutyrate was amplified by the action of transaminase B, which decreased the isoleucine pool by catalyzing the formation of α-keto-β-methylvalerate and aminobutyrate from isoleucine and α-ketobutyrate; this resulted in release of threonine deaminase from end product inhibition and unbridled production of α-ketobutyrate. Isoleucine satisfied the auxotrophic requirement of the AHAS II-deficient strain by curtailing the activity of threonine deaminase. Additional lines of evidence based on genetic and physiological experiments are presented to support the basis for the autointoxification of strain DU501 as well as other nonpolarigenic ilvG mutant strains.     

Overproduction of noncanonical amino acids by Escherichia coli cells

Overproduction of noncanonical amino acids norvaline and norleucine by Escherichia coli with inactivated acetohydroxy acid synthases was demonstrated. The cultivation conditions for the overproduction of noncanonical amino acids were studied. The effect of the restoration of acetohydroxy acid synthase activity, increased expression of the leuABCD operon, and inactivation of the biosynthetic threonine deaminase on norvaline and norleucine synthesis was studied. When grown under valine limitation, E. coli cells with inactivated acetohydroxy acid synthases and an elevated level of expression of the valine operon were shown to accumulate norvaline and norleucine (up to 0.8 and 4 g/l, respectively). These results confirm the existing hypothesis of norvaline and norleucine formation from 2-ketobutyrate by leucine biosynthesis enzymes.     

Inhibition of Threonine Dehydratase Is Herbicidal

Threonine dehydratase, the first enzyme in isoleucine biosynthesis, catalyzes deamination and dehydration of threonine to produce 2-ketobutyrate and ammonia. An antimetabolite, 2-(1-cyclohexen-3(R)-yl)-S-glycine (CHG), inhibits the plant enzyme. CHG inhibits the growth of Black Mexican Sweet corn (Zea mays) cells and of Arabidopsis thaliana plants. The herbicidal effects of CHG can be reversed by 2-ketobutyrate, other intermediates of isoleucine biosynthesis, and by isoleucine itself. These results suggest that the herbicidal effects observed with CHG are a consequence of inhibition of threonine dehydratase. The enzyme could be a potential target site for an herbicide screening program.     

产L-苏氨酸重组大肠杆菌的构建和发酵性能

【背景】大肠杆菌由于生长性能优良、遗传背景清晰,常被用作苏氨酸生产菌。【目的】敲除大肠杆菌Escherichia coli THR苏氨酸合成途径的非必需基因,并异源表达苏氨酸合成必需的关键酶,构建一株苏氨酸高产菌株。【方法】利用FLP/FRT重组酶系统,敲除E. coli THR中lysC、pfkB和sstT,同时进行谷氨酸棒杆菌中lysC~(fbr)、thrE和丙酮丁醇梭菌中gapC的重组质粒构建并转化到宿主菌中。【结果】以E. coli THR为出发菌株,敲除其苏氨酸合成途径中表达天冬氨酸激酶Ⅲ (AKⅢ)的基因lysC、磷酸果糖激酶Ⅱ基因pfkB及苏氨酸吸收蛋白表达基因sstT,使菌株积累苏氨酸的产量达到75.64±0.35g/L,比出发菌株增加9.9%。随后异源表达谷氨酸棒杆菌中解除了反馈抑制的天冬氨酸激酶(lysC~(fbr))、苏氨酸分泌转运蛋白(thrE)及丙酮丁醇梭菌中由gapC编码的NADP+依赖型甘油醛-3-磷酸脱氢酶,获得重组菌株E. coli THR6菌株。该菌株积累苏氨酸的产量提高到105.3±0.5 g/L,糖酸转化率提高了43.20%,单位产酸能力提高到5.76 g/g DCW,最大生物量为18.26 g DCW/L。【结论】单独敲除某个基因或改造某个途径不能使苏氨酸大量合成和积累,对多个代谢途径共同改造是构建苏氨酸工程菌的最有效方法。     

Metabolic engineering of Escherichia coli for biosynthesis of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) from glucose

The Escherichia coli XL1-blue strain was metabolically engineered to synthesize poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] through 2-ketobutyrate, which is generated via citramalate pathway, as a precursor for propionyl-CoA. Two different metabolic pathways were examined for the synthesis of propionyl-CoA from 2-ketobutyrate. The first pathway is composed of the Dickeya dadantii 3937 2-ketobutyrate oxidase or the E. coli pyruvate oxidase mutant (PoxB L253F V380A) for the conversion of 2-ketobutyrate into propionate and the Ralstonia eutropha propionyl-CoA synthetase (PrpE) or the E. coli acetyl-CoA:acetoacetyl-CoA transferase for further conversion of propionate into propionyl-CoA. The second pathway employs pyruvate formate lyase encoded by the E. coli tdcE gene or the Clostridium difficile pflB gene for the direct conversion of 2-ketobutyrate into propionyl-CoA. As the direct conversion of 2-ketobutyrate into propionyl-CoA could not support the efficient production of P(3HB-co-3HV) from glucose, the first metabolic pathway was further examined. When the recombinant E. coli XL1-blue strain equipped with citramalate pathway expressing the E. coli poxB L253F V380A gene and R. eutropha prpE gene together with the R. eutropha PHA biosynthesis genes was cultured in a chemically defined medium containing 20 g/L of glucose as a sole carbon source, P(3HB-co-2.3 mol% 3HV) was produced up to the polymer content of 61.7 wt.%. Moreover, the 3HV monomer fraction in P(3HB-co-3HV) could be increased up to 5.5 mol% by additional deletion of the prpC and scpC genes, which are responsible for the metabolism of propionyl-CoA in host strains.     

Development of a continuous assay and steady-state characterization of Escherichia coli threonine synthase

Threonine synthase (TS) catalyzes the hydrolysis of O-phospho-L-homoserine (OPHS) to produce L-threonine (L-Thr) and inorganic phosphate. Here, we report a simplified purification protocol for the OPHS substrate and a continuous, coupled-coupled, spectrophotometric TS assay. The sequential actions of threonine deaminase (TD) and hydroxyisocaproate dehydrogenase (HO-HxoDH) convert the L-Thr product of TS to α-ketobutyrate (α-KB) and then to 2-hydroxybutyrate, respectively, and are monitored as the decrease in absorbance at 340 nm resulting from the concomitant oxidation of β-nicotinamide adenine dinucleotide (NADH) to NAD(+) by HO-HxoDH. The effect of pH on the activities of Escherichia coli TD and Lactobacillus delbrueckii HO-HxoDH was determined to establish this continuous assay as suitable for steady-state characterization and to facilitate the optimization of coupling enzyme concentrations under different assay conditions to enable studies of TS across phyla. To validate this assay, TS from E. coli was characterized. The kinetic parameters (k(cat)=4s(-1) and K(m)=0.34 mM) and the pH optimum of 8.7, determined using the continuous assay, are consistent with values reported for this enzyme based on the discontinuous malachite green assay. The k(cat)/K(m)(OPHS) versus pH profile of E. coli TS is bell-shaped, and the apparent pK(a) values for the acidic and basic limbs are 7.1 and 10.4, respectively.     

Role of 2-ketobutyrate as an alarmone in E. coli K12: inhibition of adenylate cyclase activity mediated by the phosphoenolpyruvate: glycose phosphotransferase transport system

Summary 2-ketobutyrate and its analogues were found to inhibit strongly and transiently the rate of -galactosidase synthesis in Escherichia coli K12. This effect was ascribed to a strong and transient inhibition of the adenylate cyclase activity. By using pts mutants, we showed, in agreement with our previous results (Daniel et al. 1983), that the likely target of 2-ketobutyrate and its analogues is the phosphoenolpyruvate: glycose phosphotransferase transport system (PTS). Furthermore, evidence for such a cascade effect caused by 2-ketobutyrate and its analogues allowed us to corroborate our previous proposal (Daniel et al. 1983) that 2-ketobutyrate, a precursor of isoleucine, acts as an E. coli alarmone monitoring the passage from anaerobic to aerobic growth conditions.     

Structure and function of enzymes involved in the anaerobic degradation of L-threonine to propionate

In Escherichia coli and Salmonella typhimurium, L-threonine is cleaved non-oxidatively to propionate via 2-ketobutyrate by biodegradative threonine deaminase, 2-ketobutyrate formate-lyase (or pyruvate formate-lyase), phosphotransacetylase and propionate kinase. In the anaerobic condition, L-threonine is converted to the energy-rich keto acid and this is subsequently catabolised to produce ATP via substrate-level phosphorylation, providing a source of energy to the cells. Most of the enzymes involved in the degradation of L-threonine to propionate are encoded by the anaerobically regulated tdc operon. In the recent past, extensive structural and biochemical studies have been carried out on these enzymes by various groups. Besides detailed structural and functional insights, these studies have also shown the similarities and differences between the other related enzymes present in the metabolic network. In this paper, we review the structural and biochemical studies carried out on these enzymes.     

Branched-Chain Amino Acid Biosynthesis in Salmonella typhimurium: a Quantitative Analysis

We report here the first quantitative study of the branched-chain amino acid biosynthetic pathway in Salmonella typhimurium LT2. The intracellular levels of the enzymes of the pathway and of the 2-keto acid intermediates were determined under various physiological conditions and used for estimation of several of the fluxes in the cells. The results led to a revision of previous ideas concerning the way in which multiple acetohydroxy acid synthase (AHAS) isozymes contribute to the fitness of enterobacteria. In wild-type LT2, AHAS isozyme I provides most of the flux to valine, leucine, and pantothenate, while isozyme II provides most of the flux to isoleucine. With acetate as a carbon source, a strain expressing AHAS II only is limited in growth because of the low enzyme activity in the presence of elevated levels of the inhibitor glyoxylate. A strain with AHAS I only is limited during growth on glucose by the low tendency of this enzyme to utilize 2-ketobutyrate as a substrate; isoleucine limitation then leads to elevated threonine deaminase activity and an increased 2-ketobutyrate/2-ketoisovalerate ratio, which in turn interferes with the synthesis of coenzyme A and methionine. The regulation of threonine deaminase is also crucial in this regard. It is conceivable that, because of fundamental limitations on the specificity of enzymes, no single AHAS could possibly be adequate for the varied conditions that enterobacteria successfully encounter.     

Properties of Threonine Deaminase From a Bacterium Able to Use Threonine as Sole Source of Carbon

A threonine deaminase susceptible to inhibition by isoleucine was purified over 3,000-fold from extracts of Pseudomonas multivorans, a bacterium able to use threonine or α-ketobutyrate as sole source of carbon and energy. The enzyme was characterized with respect to molecular weight, dissociation to subunits, and apparent affinities for threonine, isoleucine, and several other ligands. Certain features of the enzyme including its reversible dissociation to subunits, its high constitutive activity, its marked stability, and high apparent orders of binding for threonine and isoleucine were unusual compared to those of isoleucine-inhibitable enzymes from other bacteria. These findings suggested some relationship between properties of the enzyme and the ability of P. multivorans to use threonine as sole carbon source. However, mutant studies ruled out a direct role of the enzyme in threonine catabolism and indicated that another enzyme, threonine dehydrogenase, is essential for growth on threonine.     

Specialized peptide transport system in Escherichia coli.

Trileucine is utilized as a source of leucine for growth of strains of Escherichia coli K-12 that are deficient in the oligopeptide transport system (Opp). Trithreonine is toxic to E. coli K-12. Opp- mutants of E. coli K-12 retain complete sensitivity to this tripeptide. Moreover, E. coli W, which is resistant to trithreonine, can utlize this tripeptide as a threonine source and this capability is fully maintained in E. coli W (Opp-). A spontaneous trithreonine-resistant mutant of E. coli K-12 (Opp-) has been isolated that has an impaired growth response to trileucine and is resistant to trithreonine. Trileucine competes with the uptake of trithreonine as measured by its ability to relieve trithreonine toxicity in E. coli K-12. It is concluded that trileucine as well as trithreonine are transported into E. coli K-12 or W by a common uptake system that is distinct from the Opp system. Trimethionine can act as a competitor of trileucine or trithreonine-supported growth and as an antagonist of trithreonine toxicity in Opp- mutants. It is concluded that trimethionine is recognized by the trileucine-trithreonine transport system. Trithreonine, trimethionine, and trileucine are also transported by the Opp system, as they all relieve triornithine toxicity towards E. coli W and compete with tetralysine utilization as lysine source for growth of a lysine auxotroph of this strain.     

Inactivation of the RluD pseudouridine synthase has minimal effects on growth and ribosome function in wild-type Escherichia coli and Salmonella enterica

The Escherichia coli rluD gene encodes a pseudouridine synthase responsible for the pseudouridine (Ψ) modifications at positions 1911, 1915, and 1917 in helix 69 of 23S rRNA. It has been reported that deletion of rluD in K-12 strains of E. coli is associated with extremely slow growth, increased readthrough of stop codons, and defects in 50S ribosomal subunit assembly and 30S-50S subunit association. Suppressor mutations in the prfB and prfC genes encoding release factor 2 (RF2) and RF3 that restore the wild type-growth rate and also correct the ribosomal defects have now been isolated. These suppressors link helix 69 Ψ residues with the termination phase of protein synthesis. However, further genetic analysis reported here also reveals that the slow growth and other defects associated with inactivation of rluD in E. coli K-12 strains are due to a defective RF2 protein, with a threonine at position 246, which is present in all K-12 strains. This is in contrast to the more typical alanine found at this position in most bacterial RF2s, including those of other E. coli strains. Inactivation of rluD in E. coli strains containing the prfB allele from E. coli B or in Salmonella enterica, both carrying an RF2 with Ala246, has negligible effects on growth, termination, or ribosome function. The results indicate that, in contrast to those in wild bacteria, termination functions in E. coli K-12 strains carrying a partially defective RF2 protein are especially susceptible to perturbation of ribosome-RF interactions, such as that caused by loss of h69 Ψ modifications.     

Physiological implications of the specificity of acetohydroxy acid synthase isozymes of enteric bacteria.

The rates of formation of the two alternative products of acetohydroxy acid synthase (AHAS) have been determined by a new analytical method (N. Gollop, Z. Barak, and D. M. Chipman, Anal. Biochem., 160:323-331, 1987). For each of the three distinct isozymes of AHAS in Escherichia coli and Salmonella typhimurium, a specificity ratio, R, was defined: Formula: see text, which is constant over a wide range of substrate concentrations. This is consistent with competition between pyruvate and 2-ketobutyrate for an active acetaldehyde intermediate formed irreversibly after addition of the first pyruvate moiety to the enzyme. Isozyme I showed no product preference (R = 1), whereas isozymes II and III form acetohydroxybutyrate (AHB) at approximately 180- and 60-fold faster rates, respectively, than acetolactate (AL) at equal pyruvate and 2-ketobutyrate concentrations. R values higher than 60 represent remarkably high specificity in favor of the substrate with one extra methylene group. In exponentially growing E. coli cells (under aerobic growth on glucose), which contain about 300 microM pyruvate and only 3 microM 2-ketobutyrate, AHAS I would produce almost entirely AL and only 1 to 2% AHB. However, isozymes II and III would synthesize AHB (on the pathway to Ile) and AL (on the pathway to valine-leucine) in essentially the ratio required for protein synthesis. The specificity ratio R of any AHAS isozyme was affected neither by the natural feedback inhibitors (Val, Ile) nor by the pH. On the basis of the specificities of the isozymes, the known regulation of AHAS I expression by the catabolite repression system, and the reported behavior of bacterial mutants containing single AHAS isozymes, we suggest that AHAS I enables a bacterium to cope with poor carbon sources, which lead to low endogenous pyruvate concentrations. Although AHAS II and III are well suited to producing the branched-chain amino acid precursors during growth on glucose, they would fail to provide appropriate quantities of AL when the concentration of pyruvate is relatively low.     

亮氨酸脱氢酶C端Loop区域的理性设计及多酶级联高效合成l-2-氨基丁酸

亮氨酸脱氢酶 (Leucine dehydrogenase,LDH) 是制备l-2-氨基丁酸的关键限速酶,针对该酶的Loop区域进行改造以提高关键酶的酶活及稳定性从而高效合成l-2-氨基丁酸。通过亮氨酸脱氢酶的分子动力学模拟分析均方根涨落 (Root mean square fluctuation,RMSF) 值,对其波动非常明显的Loop区域合理设计以得到比酶活提高的截短突变体EsLDHD2,其比酶活为野生型的123.2%;此外,由于l-2-氨基丁酸制备过程中苏氨酸脱氨酶催化l-苏氨酸制备2-酮丁酸的速率过快导致多酶催化不平衡,因此双拷贝亮氨酸脱氢酶及甲酸脱氢酶以平衡多酶催化速率,构建多酶级联催化的单细胞E. coli BL21/pACYCDuet-RM,其摩尔转化率相较于E. coli BL21/pACYCDuet-RO提高74.6%;对菌株E. coli BL21/pACYCDuet-RM的全细胞转化条件进行优化,其最适pH、温度、底物浓度分别为7.5、35 ℃和80 g/L,此时摩尔转化率大于99%;在1 L转化体系和最适转化条件下分批加入l-苏氨酸80 g和40 g,l-2-氨基丁酸的产量达97.2 g。总之,该策略为l-2-氨基丁酸的制备提供了绿色、高效的合成方法,具有工业化制备药物前体的巨大潜力。     

Novel keto acid formate-lyase and propionate kinase enzymes are components of an anaerobic pathway in Escherichia coli that degrades L-threonine to propionate

An immunological analysis of an Escherichia coli strain unable to synthesize the main pyruvate formate-lyase enzyme Pfl revealed the existence of a weak, cross-reacting 85 kDa polypeptide that exhibited the characteristic oxygen-dependent fragmentation typical of a glycyl radical enzyme. Polypeptide fragmentation of this cross-reacting species was shown to be dependent on Pfl activase. Cloning and sequence analysis of the gene encoding this protein revealed that it coded for a new enzyme, termed TdcE, which has 82% identity with Pfl. On the basis of RNA analyses, the tdcE gene was shown to be part of a large operon that included the tdcABC genes, encoding an anaerobic threonine dehydratase, tdcD , coding for a propionate kinase, tdcF , the function of which is unknown, and the tdcG gene, which encodes a L -serine dehydratase. Expression of the tdcABCDEFG operon was strongly catabolite repressed. Enzyme studies showed that TdcE has both pyruvate formate-lyase and 2-ketobutyrate formate-lyase activity, whereas the TdcD protein is a new propionate/acetate kinase. By monitoring culture supernatants from various mutants using ¹H nuclear magnetic resonance (NMR), we followed the anaerobic conversion of L -threonine to propionate. These studies confirmed that 2-ketobutyrate, the product of threonine deamination, is converted in vivo by TdcE to propionyl-CoA. These studies also revealed that Pfl and an as yet unidentified thiamine pyrophosphate-dependent enzyme(s) can perform this reaction. Double null mutants deficient in phosphotransacetylase (Pta) and acetate kinase (AckA) or AckA and TdcD were unable to metabolize threonine to propionate, indicating that propionyl-CoA and propionyl-phosphate are intermediates in the pathway and that ATP is generated during the conversion of propionyl-P to propionate by AckA or TdcD.