[36] Oxaloacetate transcarboxylase from Propionibacterium

Responses of short-chain fatty acids production to the addition of various biocarriers to sludge anaerobic fermentation

This study aimed to evaluate the effects and explore the mechanisms of polyethylene (PE), polyurethane sponge (PUS), and granule activated carbon (GAC) on short-chain fatty acids (SCFAs) production from sludge anaerobic fermentation. Results showed that no matter the biocarrier type, addition of biocarriers increased the diversity of SCFAs. In contrast with GAC, addition of PE and PUS considerably facilitated the accumulation of the total SCFAs. Suspended PE and PUS might have stronger frictions with sludge particles which resulted in a better sludge disintegration. Other factors that contributed to the enhancement of PE and PUS include higher hydrolytic and acidogenic enzymes activities, lower methanogenic enzyme activity, more Firmicutes and less Proteobacteria. Consistent with enzymatic and microbial results, the PE and the PUS tests also showed greater abundance in all metabolic functions predicted with PICRUSt. This study provides a novel strategy for sludge anaerobic fermentation by using traditional wastewater biocarriers.

A new biological process for short-chain fatty acid generation from waste activated sludge improved by Clostridiales enhancement

Short-chain fatty acids (SCFAs), the carbon source of biological nutrient removal, can be produced by waste activated sludge (WAS) anaerobic fermentation. To get more SCFAs from sludge, most studies in literature focused on the mechanical process control or the structure of microbial community; little attention has been paid to the key microorganisms and their function related to SCFA generation. In this study, a different sludge pretreated method, i.e., pretreating sludge by proteinase K for 2 days followed by pretreating at pH 10 for 4 days, is reported, by which the proportion of Clostridiales was increased and SCFA generation was enhanced. First, the effects of different proteinase K concentrations and initial pH on sludge hydrolysis and SCFA generation were investigated. The optimal conditions showed the highest SCFA generation (352.91 mg COD per gram of volatile suspended solids), which was 2.89-fold of the blank (un-pretreated). Further, the new biological pretreatment process led to the conversion of other SCFAs to acetic acid. Acetic acid accounted for 60.8 % of total SCFAs with the new biological pretreatment process compared with 44.9 % in the blank test. Then, the investigation on the key microorganisms related to SCFA production with 16S rRNA gene clone library and fluorescence in situ hybridization (FISH) indicated that there were much greater active Clostridiales when SCFAs were generated with the proteinase K and pH 10 pretreated sludge. Further, the mechanisms for the optimal conditions significantly enhancing SCFA generation were investigated. It was found that pretreating sludge by proteinase K and pH 10 caused the greatest key enzyme activities, organic consumption, and inhibition of methane generation. Graphical abstract A new biological process for short-chain fatty acid generation from waste activated sludge improved by Clostridiales enhancement.

Waste activated sludge hydrolysis and acidification: A comparison between sodium hydroxide and steel slag addition

Alkaline treatment with steel slag and NaOH addition were investigated under different pH conditions for the fermentation of waste activated sludge. Better performance was achieved in steel slag addition scenarios for both sludge hydrolysis and acidification. More solubilization of organic matters and much production of higher VFA (volatile fatty acid) in a shorter time can be achieved at pH10 when adjusted by steel slag. Higher enzyme activities were also observed in steel slag addition scenarios under the same pH conditions. Phosphorus concentration in the supernatant increased with fermentation time and pH in NaOH addition scenarios, while in contrast most phosphorus was released and captured by steel slag simultaneously in steel slag addition scenarios. These results suggest that steel slag can be used as a substitute for NaOH in sludge alkaline treatment.

Enhancing the quantity and quality of short-chain fatty acids production from waste activated sludge using CaO2 as an additive

The effect of calcium peroxide (CaO2) addition on anaerobic fermentation of waste activated sludge (WAS) was investigated. The lab-scale experiments were conducted at 35 °C with CaO2 doses ranging from 0.05 to 0.3 g/g VSS. The performances of hydrolysis and acidification of WAS were significantly enhanced by CaO2 addition, whereas the production of methane was inhibited. Maximum total short-chain fatty acids (TSCFA) production (284 mg COD/g VSS) occurred at a CaO2 dose of 0.2 g/g VSS and fermentation time of 7 d, which was 3.9 times higher than the control tests. Further, CaO2 addition led to the conversion of other SCFAs to acetic acid. Acetic acid comprised 60.2% of TSCFA with the addition of 0.2 g CaO2/g VSS compared with 45.1% in the control tests. The mechanism of improved SCFAs generation was analyzed from the view of both chemical and biological effects. Chemical effect facilitated the disintegration of WAS, and improved the activities of both hydrolytic enzymes and acid-forming enzymes. Illumina MiSeq sequencing analysis revealed that bacteria within phylum Firmicutes increased significantly due to CaO2 addition, which played an important role in the hydrolysis and acidification of WAS. In addition, CaO2 oxidized most refractory organic contaminants, which were difficult to biodegrade under the ordinary anaerobic condition. Hydroxyl radicals were the most abundant reactive oxygen species released by CaO2, which played a key role in the removal of refractory organic compounds. We developed a promising technology to produce a valuable carbon source from WAS.

Waste-Activated Sludge Fermentation for Polyacrylamide Biodegradation Improved by Anaerobic Hydrolysis and Key Microorganisms Involved in Biological Polyacrylamide Removal

During the anaerobic digestion of dewatered sludge, polyacrylamide (PAM), a chemical conditioner, can usually be consumed as a carbon and nitrogen source along with other organic matter (e.g., proteins and carbohydrates in the sludge). However, a significant accumulation of acrylamide monomers (AMs) was observed during the PAM biodegradation process. To improve the anaerobic hydrolysis of PAM, especially the amide hydrolysis process, and to avoid the generation of the intermediate product AM, a new strategy is reported herein that uses an initial pH of 9, 200 mg COD/L of PAM and a fermentation time of 17 d. First, response surface methodology (RSM) was applied to optimize PAM removal in the anaerobic digestion of the sludge. The biological hydrolysis of PAM reached 86.64% under the optimal conditions obtained from the RSM. Then, the mechanisms for the optimized parameters that significantly improved the biological hydrolysis of PAM were investigated by the synergistic effect of the main organic compounds in the sludge, the floc size distribution, and the enzymatic activities. Finally, semi-continuous-flow experiments for a microbial community study were investigated based on the determination of key microorganisms involved in the biological hydrolysis of PAM.

Enhanced bio-hydrogen production from protein wastewater by altering protein structure and amino acids acidification type

Enhanced bio-hydrogen production from protein wastewater by altering protein structure and amino acids acidification type via pH control was investigated. The hydrogen production reached 205.2 mL/g-protein when protein wastewater was pretreated at pH 12 and then fermented at pH 10. The mechanism studies showed that pH 12 pretreatment significantly enhanced protein bio-hydrolysis during the subsequent fermentation stage as it caused the unfolding of protein, damaged the protein hydrogen bonding networks, and destroyed the disulfide bridges, which increased the susceptibility of protein to protease. Moreover, pH 10 fermentation produced more acetic but less propionic acid during the anaerobic fermentation of amino acids, which was consistent with the theory of fermentation type affecting hydrogen production. Further analyses of the critical enzymes, genes, and microorganisms indicated that the activity and abundance of hydrogen producing bacteria in the pH 10 fermentation reactor were greater than those in the control.

Improved bioproduction of short-chain fatty acids from waste activated sludge by perennial ryegrass addition

To improve short-chain fatty acids (SCFAs) production from waste activated sludge (WAS), studies that focus on pre-treatment methods, pH control or adding extra carbon like rice were reported. In this study, a kind of green waste (perennial ryegrass) was used as carbon source to adjust carbon to nitrogen ratio (C/N) in WAS to enhance SCFAs production. The effects of different C/N ratio ranging from 26/1 to 7/1 on SCFAs production and cellulose, hemicellulose, and lignin consumption were investigated in detail. It was observed that the maximal SCFAs yield was 368.71 g COD per kilogram of total solids (TS) at C/N 20/1 with fermentation time of 12 d, which was, respectively, over 4 and 12 times than that at C/N 26/1 (sole perennial ryegrass) and C/N 7/1 (sole WAS). Meanwhile, cellulose, hemicellulose and lignin consumptions were 109.12, 148.74, 20.90 g COD/kg TS, respectively, at C/N 20/1 with fermentation time of 12 d. The analysis of the composition of SCFAs showed that acetic acid ranked the first among other acids from C/N ratio of 26/1 to 18/1, whereas propionic acid was the dominant product from C/N ratio of 16/1 to 7/1. Because the results of this study were different from previous studies of SCFAs generation, the mechanism of improved SCFAs at C/N ratio of 20/1 by perennial ryegrass addition was investigated. Results showed that as soluble COD, soluble protein and soluble carbohydrate increased, more substrates were available for SCFAs production at C/N ratio of 20/1. In addition, with the drop of pH to 4.82, methane generating was inhabitant and the SCFAs production was therefore significantly enhanced. Additionally, the mechanism of improved SCFAs generation was analyzed from the view of enzyme activities and microbial community. The experiments revealed that at C/N ratio of 20/1 the activities of enzymes were the highest comparing with other C/N ratios and lignin was the rate-limiting steps in WAS anaerobic digestion by perennial ryegrass addition. The 16S rRNA gene clone library demonstrated that Clostridia, Spirochaetes, and Bacteroidetes were the dominant microbial community at C/N ratio of 20/1.

Purification and cellular localization of the Entamoeba histolytica transcarboxylase

Genome analysis of Entamoeba histolytica predicts the presence of acetyl-CoA carboxylase. Using Western blot, histochemistry, and confocal microscopy, we demonstrated the presence of a biotin-containing protein in the cytoplasm of E. histolytica, with a molecular weight of 136 kDa and biotin-carboxylase activity. This protein probably corresponds to a transcarboxylase that catalyzes the rate-limiting reaction leading to fatty acid elongation.

A new process for efficiently producing methane from waste activated sludge: alkaline pretreatment of sludge followed by treatment of fermentation liquid in an EGSB reactor

In the literature the production of methane from waste activated sludge (WAS) was usually conducted in a continuous stirred tank reactor (CSTR) after sludge was pretreated. It was reported in our previous publication that compared with other pretreatment methods the methane production in CSTR could be significantly enhanced when sludge was pretreated by NaOH at pH 10 for 8 days. In order to further improve methane production, this study reported a new process for efficiently producing methane from sludge, that is, sludge was fermented at pH 10 for 8 days, which was adjusted by Ca(OH)(2), and then the fermentation liquid was treated in an expanded granular sludge bed (EGSB) for methane generation. First, for comparing the methane production observed in this study with that reported in the literature, the conventional operational model was applied to produce methane from the pH 10 pretreated sludge, that is, directly using the pH 10 pretreated sludge to produce methane in a CSTR. It was observed that the maximal methane production was only 0.61 m(3)CH(4)/m(3)-reactor/day. Then, the use of fermentation liquid of pH 10 pretreated sludge to produce methane in the reactors of up-flow anaerobic sludge bed (UASB), anaerobic sequencing batch reactor (ASBR) and EGSB was compared. The maximal methane production in UASB, ASBR, and EGSB reached 1.41, 3.01, and 12.43 m(3)CH(4)/m(3)-reactor/day, respectively. Finally, the mechanisms for EGSB exhibiting remarkably higher methane production were investigated by enzyme, adenosine-triphosphate (ATP), scanning electron microscope (SEM) and fluorescence in situ hybridization (FISH) analyses. It was found that the granular sludge in EGSB had the highest conversion efficiency of acetic acid to methane, and the greatest activity of hydrolysis and acidification enzymes and general physiology with much more Methanosarcinaceae.

Ultrasonic enhancement of waste activated sludge hydrolysis and volatile fatty acids accumulation at pH 10.0

Volatile fatty acids (VFA), the preferred carbon source for biological nutrients removal, can be produced by waste activated sludge (WAS) anaerobic fermentation. However, because the rate of VFA accumulation is limited by that of WAS hydrolysis and VFA is always consumed by methanogens at acidic or neutral pHs, the ultrasonic pretreatment which can accelerate the rate of WAS hydrolysis, and alkaline adjustment which can inhibit the activities of methanogens, were, therefore, used to improve WAS hydrolysis and VFA accumulation in this study. Experiment results showed that the combination of ultrasonic pretreatment and alkaline adjustment caused significant enhancements of WAS hydrolysis and VFA accumulation. The study of ultrasonic energy density effect revealed that energy density influenced not only the total VFA accumulation but also the percentage of individual VFA. The maximal VFA accumulation (3109.8mg COD/L) occurred at ultrasonic energy density of 1.0kW/L and fermentation time of 72h, which was more than two times that without ultrasonic treatment (1275.0mg COD/L). The analysis of VFA composition showed that the percentage of acetic acid ranked the first (more than 40%) and those of iso-valeric and propionic acids located at the second and third places, respectively. Thus, the suitable ultrasonic conditions combined with alkaline adjustment for VFA accumulation from WAS were ultrasonic energy density of 1.0kW/L and fermentation time of 72h. Also, the key enzymes related to VFA formation exhibited the highest activities at ultrasonic energy density of 1.0kW/L, which resulted in the greatest VFA production during WAS fermentation at pH 10.0.

Enhancement of waste activated sludge protein conversion and volatile fatty acids accumulation during waste activated sludge anaerobic fermentation by carbohydrate substrate addition: the effect of pH

Volatile fatty acids (VFAs), the carbon source of biological nutrients removal, can be produced by waste activated sludge (WAS) anaerobic fermentation. However, because of high protein content and low carbon to nitrogen mass ratio (C/N) of WAS, the production of VFAs, especially propionic acid, a more preferred VFA than acetic acid for enhanced biological phosphorus removal (EBPR), is limited. After the addition of carbohydrate (rice was used as the model matter) to the WAS anaerobic fermentation system to balance the C/N ratio, the effect of pH on WAS protein conversion and VFAs production was investigated in this paper. Experimental results showed that the addition of carbohydrate matter caused a remarkable enhancement of WAS protein conversion and protease activity, and an apparent synergistic effect between WAS and carbohydrate matter was observed. The study of pH effect revealed that pH influenced not only the total VFAs production but the percentage of individual VFA. The maximal VFAs production (520.1 mg COD per gram of volatile suspended solids (VSS)) occurred at pH 8.0 and a fermentation time of 8 d, which was more than three times that at uncontrolled pH (150.2 mg COD/g VSS). The analysis of the composition of VFAs showed that propionic acid ranked first at pH 6.0-9.0 (around 50%) whereas acetic acid was the dominant product at other pHs investigated. Thus, the suitable conditions for propionic acid-enriched VFAs production were pH 8.0 and a time of 8 d. Further investigation showed that as there was more fermentation substrate consumption with lower biogas generation at pH 8.0, improved VFAs production was observed. Also, the key enzymes relevant to propionic acid formation exhibited the highest activities at pH 8.0, which resulted in the greatest propionic acid content in the fermentative VFAs. The 16S rRNA gene clone library demonstrated that Clostridia, beta-Proteobacteria, and Bacteroidetes were the dominant microbial community when the current anaerobic fermentation system was operated at pH 8.0. With the fermentative VFAs as the additional carbon source of municipal wastewater, the EBPR performance was significantly increased.

Enhanced propionic acid fermentation by Propionibacterium acidipropionici mutant obtained by adaptation in a fibrous-bed bioreactor

Fed-batch fermentations of glucose by P. acidipropionici ATCC 4875 in free-cell suspension culture and immobilized in a fibrous-bed bioreactor (FBB) were studied. The latter produced a much higher propionic acid concentration (71.8 +/- 0.8 g/L vs. 52.2 +/- 1.1 g/L), indicating enhanced tolerance to propionic acid inhibition by cells adapted in the FBB. Compared to the free-cell fermentation, the FBB culture produced 20-59% more propionate (0.40-0.65 +/- 0.02 g/g vs. 0.41 +/- 0.02 g/g), 17% less acetate (0.10 +/- 0.01 g/g vs. 0.12 +/- 0.02 g/g), and 50% less succinate (0.09 +/- 0.02 g/g vs. 0.18 +/- 0.03 g/g) from glucose. The higher propionate production in the FBB was attributed to mutations in two key enzymes, oxaloacetate transcarboxylase and propionyl CoA: succinyl CoA transferase, leading to the production of propionic acid from pyruvate. Both showed higher specific activity and lower sensitivity to propionic acid inhibition in the mutant than in the wild type. In contrast, the activity of PEP carboxylase, which converts PEP directly to oxaloacetate and leads to the production of succinate from glucose, was generally lower in the mutant than in the wild type. For phosphotransacetylase and acetate kinase in the acetate formation pathway, however, there was no significant difference between the mutant and the wild type. In addition, the mutant had a striking change in its morphology. With a threefold increase in its length and approximately 24% decrease in its diameter, the mutant cell had an approximately 10% higher specific surface area that should have made the mutant more efficient in transporting substrates and metabolites across the cell membrane. A slightly lower membrane-bound ATPase activity found in the mutant also indicated that the mutant might have a more efficient proton pump to allow it to better tolerate propionic acid. In addition, the mutant had more longer-chain saturated fatty acids (C17:0) and less unsaturated fatty acids (C18:1), both of which could decrease membrane fluidity and might have contributed to the increased propionate tolerance. The enhanced propionic acid production from glucose by P. acidipropionici was thus attributed to both a high viable cell density maintained in the reactor and favorable mutations resulted from adaptation by cell immobilization in the FBB.

Transcarboxylase 5S structures: assembly and catalytic mechanism of a multienzyme complex subunit

Transcarboxylase is a 1.2 million Dalton (Da) multienzyme complex from Propionibacterium shermanii that couples two carboxylation reactions, transferring CO(2)(-) from methylmalonyl-CoA to pyruvate to yield propionyl-CoA and oxaloacetate. Crystal structures of the 5S metalloenzyme subunit, which catalyzes the second carboxylation reaction, have been solved in free form and bound to its substrate pyruvate, product oxaloacetate, or inhibitor 2-ketobutyrate. The structure reveals a dimer of beta(8)alpha(8) barrels with an active site cobalt ion coordinated by a carbamylated lysine, except in the oxaloacetate complex in which the product's carboxylate group serves as a ligand instead. 5S and human pyruvate carboxylase (PC), an enzyme crucial to gluconeogenesis, catalyze similar reactions. A 5S-based homology model of the PC carboxyltransferase domain indicates a conserved mechanism and explains the molecular basis of mutations in lactic acidemia. PC disease mutations reproduced in 5S result in a similar decrease in carboxyltransferase activity and crystal structures with altered active sites.

Metabolic engineering of a novel propionate-independent pathway for the production of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) in recombinant Salmonella enterica serovar typhimurium

A pathway was metabolically engineered to produce poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), a biodegradable thermoplastic with proven commercial applications, from a single, unrelated carbon source. An expression system was developed in which a prpC strain of Salmonella enterica serovar Typhimurium, with a mutation in the ability to metabolize propionyl coenzyme A (propionyl-CoA), served as the host for a plasmid harboring the Acinetobacter polyhydroxyalkanoate synthesis operon (phaBCA) and a second plasmid with the Escherichia coli sbm and ygfG genes under an independent promoter. The sbm and ygfG genes encode a novel (2R)-methylmalonyl-CoA mutase and a (2R)-methylmalonyl-CoA decarboxylase, respectively, which convert succinyl-CoA, derived from the tricarboxylic acid cycle, to propionyl-CoA, an essential precursor of 3-hydroxyvalerate (HV). The S. enterica system accumulated PHBV with significant HV incorporation when the organism was grown aerobically with glycerol as the sole carbon source. It was possible to vary the average HV fraction in the copolymer by adjusting the arabinose or cyanocobalamin (precursor of coenzyme B12) concentration in the medium.

Primary structure of the monomer of the 12S subunit of transcarboxylase as deduced from DNA and characterization of the product expressed in Escherichia coli

Transcarboxylase from Propionibacterium shermanii is a complex biotin-containing enzyme composed of 30 polypeptides of three different types: a hexameric central 12S subunit to which 6 outer 5S subunits are attached through 12 1.3S biotinyl subunits. The enzyme catalyzes a two-step reaction in which methylmalonyl coenzyme A and pyruvate serve as substrates to form propionyl coenzyme A (propionyl-CoA) and oxalacetate, the 12S subunit specifically catalyzing one of the two reactions. We report here the cloning, sequencing, and expression of the 12S subunit. The gene was identified by matching amino acid sequences derived from isolated authentic 12S peptides with the deduced sequence of an open reading frame present in a cloned P. shermanii genomic fragment known to contain the gene encoding the 1.3S biotinyl subunit. The cloned 12S gene encodes a protein of 604 amino acids and of M(r) 65,545. The deduced sequence shows regions of extensive homology with the beta subunit of mammalian propionyl-CoA carboxylase as well as regions of homology with acetyl-CoA carboxylase from several species. Two genomic fragments were subcloned into pUC19 in an orientation such that the 12S open reading frame could be expressed from the lac promoter of the vector. Crude extracts prepared from these cells contained an immunoreactive band on Western blots (immunoblots) which comigrated with authentic 12S. The Escherichia coli-expressed 12S was purified to apparent homogeneity by a three-step procedure and compared with authentic 12S from P. shermanii. Their quaternary structures were identical by electron microscopy, and the E. coli 12S preparation was fully active in the reactions catalyzed by this subunit. We conclude that we have cloned, sequenced, and expressed the 12S subunit which exists in a hexameric active form in E.coli.

The iron-sulfur-cluster-containing L-serine dehydratase from Peptostreptococcus asaccharolyticus. Stereochemistry of the deamination of L-threonine

The stereochemistry of the deamination of L-threonine to 2-oxobutyrate, catalyzed by purified L-serine dehydratase of Peptostreptococcus asaccharolyticus, was elucidated. For this purpose the enzyme reaction was carried out with unlabelled L-threonine in 2H2O and in 3HOH, as well as with L-[3-3H]threonine in unlabelled water. Isotopically labelled 2-oxobutyrate thus formed was directly reduced in a coupled reaction with L- or D-lactate dehydrogenase and NADH. The (2R)- or (2S)-2-hydroxybutyrate species obtained were then subjected to configurational analyses of their labelled methylene group. The results from 1H-NMR spectroscopy and, after degradation of 2-hydroxybutyrate to propionate, the transcarboxylase assay consistently indicated that the deamination of L-threonine catalyzed by L-serine dehydratase of P. asaccharolyticus proceeds with inversion and retention in a 2:1 ratio. This partial racemization is the first ever to be observed for a reaction catalyzed by serine dehydratase, therefore confirming the distinction of the L-serine dehydratase of P. asaccharolyticus as an iron-sulfur protein from those dehydratases dependent on pyridoxal phosphate. For the latter enzymes exclusively, retention has been reported.

Pathway and sites for energy conservation in the metabolism of glucose by Selenomonas ruminantium

On the basis of enzyme activities detected in extracts of Selenomonas ruminantium HD4 grown in glucose-limited continuous culture, at a slow (0.11 h-1) and a fast (0.52 h-1) dilution rate, a pathway of glucose catabolism to lactate, acetate, succinate, and propionate was constructed. Glucose was catabolized to phosphoenol pyruvate (PEP) via the Emden-Meyerhoff-Parnas pathway. PEP was converted to either pyruvate (via pyruvate kinase) or oxalacetate (via PEP carboxykinase). Pyruvate was reduced to L-lactate via a NAD-dependent lactate dehydrogenase or oxidatively decarboxylated to acetyl coenzyme A (acetyl-CoA) and CO2 by pyruvate:ferredoxin oxidoreductase. Acetyl-CoA was apparently converted in a single enzymatic step to acetate and CoA, with concomitant formation of 1 molecule of ATP; since acetyl-phosphate was not an intermediate, the enzyme catalyzing this reaction was identified as acetate thiokinase. Oxalacetate was converted to succinate via the activities of malate dehydrogenase, fumarase and a membrane-bound fumarate reductase. Succinate was then excreted or decarboxylated to propionate via a membrane-bound methylmalonyl-CoA decarboxylase. Pyruvate kinase was inhibited by Pi and activated by fructose 1,6-bisphosphate. PEP carboxykinase activity was found to be 0.054 mumol min-1 mg of protein-1 at a dilution rate of 0.11 h-1 but could not be detected in extracts of cells grown at a dilution rate of 0.52 h-1. Several potential sites for energy conservation exist in S. ruminantium HD4, including pyruvate kinase, acetate thiokinase, PEP carboxykinase, fumarate reductase, and methylmalonyl-CoA decarboxylase. Possession of these five sites for energy conservation may explain the high yields reported here (56 to 78 mg of cells [dry weight] mol of glucose-1) for S. ruminantium HD4 grown in glucose-limited continuous culture.

Biotin-dependent carboxylation catalyzed by transcarboxylase is a stepwise process

To investigate the mechanism of the carboxylation of pyruvate to oxalacetate catalyzed by the enzyme transcarboxylase, we have measured the D(V/K) and 13(V/K) isotope effects. Comparison of the double-reciprocal plots of the initial velocities with [1H3]pyruvate and with [2H3]pyruvate as substrate yields a deuterium isotope effect on Vmax/Km of 1.39 +/- 0.04. The 13C kinetic isotope effect on the carboxylation of pyruvate to oxalacetate has been measured by the competitive method and is 1.0227 +/- 0.0008. To determine whether the removal of the proton from pyruvate and the addition of the carboxyl group occur in the same or in different steps, the double-isotope fractionation test has been used. When [2H3]pyruvate replaces [1H3]pyruvate as the substrate, the observed 13(V/K) isotope effect falls from 1.0227 to 1.0141 +/- 0.001. The latter value is in excellent agreement with the value of 1.0136, predicted for a stepwise pathway. We may conclude, therefore, that the carboxylation of pyruvate catalyzed by transcarboxylase proceeds by a stepwise mechanism involving the intermediate formation of the substrate carbanion.

Fluorine NMR studies on stereochemical aspects of reactions catalyzed by transcarboxylase, pyruvate kinase, and enzyme I

The stereochemistry of the transcarboxylase-catalyzed carboxylation of 3-fluoropyruvate has been studied by using fluorine NMR of unpurified reaction mixtures. When the product 3-fluorooxaloacetate was trapped by using malate dehydrogenase, only the 2R,3R diastereomer of 3-fluoromalate was formed. The fluoromethyl group of fluoropyruvate does not take up deuterium label from the solvent during the reaction. These results confirm and extend those obtained previously by Walsh and co-workers [Goldstein, J. A., Cheung, Y. F., Marletta, M. A., & Walsh, C. (1978) Biochemistry 17, 5567-5575] showing that transcarboxylase is specific for one of the two prochiral hydrogens in fluoropyruvate. Transcarboxylase, coupled to malate dehydrogenase, has been used to analyze samples of chiral fluoropyruvate obtained by dephosphorylation of (Z)-fluorophosphoenolpyruvate in D2O in the presence of either pyruvate kinase or enzyme I from the Escherichia coli sugar transport systems. Analysis of the fluoromalate produced showed that fluoroenolpyruvate is deuterated from opposite faces by these two enzymes: enzyme I protonates (deuterates) fluoroenolpyruvate exclusively from the 2-re face and pyruvate kinase does so mainly from the 2-si face. Fluoropyruvate is carboxylated by transcarboxylase with absolute retention of configuration.

Acyl coenzyme A carboxylase of Propionibacterium shermanii: detection and properties

An acyl coenzyme A (CoA) carboxylase, which catalyzes the adenosine triphosphate-dependent fixation of CO2 into acetyl-, propionyl-, and butyryl-CoA, was detected in fractionated cell extracts of Propionibacterium shermanii. Catalytic activity was inhibited by avidin but was unaffected by avidin pretreated with excess biotin. The carboxylase levels detected were relatively small and were related to cellular growth. Maximal carboxylase activity was detected in cells grown for about 96 h. Thereafter, the activity declined rapidly. Optimal CO2 fixation occurred at pH 7.5. Other parameters of the assay system were optimized, and the apparent Km values for substrates were determined. The end product of the reaction (with acetyl-CoA as the substrate) was identified as malonyl-CoA. The stoichiometry of the reaction was such that, for every mole of acetyl-CoA and adenosine triphosphate consumed, 1 mol each of malonyl-CoA, adenosine diphosphate, and orthophosphate was formed. These data provide the first evidence for the presence of another biotin-containing enzyme, an acyl-CoA carboxylase, in these bacteria in addition to the well-characterized methylmalonyl-CoA carboxyltransferase.

Pathway of succinate and propionate formation in Bacteroides fragilis

Cell suspensions of Bacteroides fragilis were allowed to ferment glucose and lactate labeled with (14)C in different positions. The fermentation products, propionate and acetate, were isolated, and the distribution of radioactivity was determined. An analysis of key enzymes of possible pathways was also made. The results of the labeling experiments showed that: (i) B. fragilis ferments glucose via the Embden-Meyerhof pathway; and (ii) there was a randomization of carbons 1, 2, and 6 of glucose during conversion to propionate, which is in accordance with propionate formation via fumarate and succinate. The enzymes 6-phosphofrucktokinase (pyrophosphate-dependent), fructose-1,6-diphosphate aldolase, phosphoenolpyruvate carboxykinase, malate dehydrogenase, fumarate reductase, and methylmalonyl-coenzyme A mutase could be demonstrated in cell extracts. Their presence supported the labeling results and suggested that propionate is formed from succinate via succinyl-, methylmalonyl-, and propionyl-coenzyme A. From the results it also is clear that CO(2) is necessary for growth because it is needed for the formation of C4 acids. There was also a randomization of carbons 1, 2, and 6 of glucose during conversion to acetate, which indicated that pyruvate kinase played a minor role in pyruvate formation from phosphoenolpyruvate. Phosphoenolpyruvate carboxykinase, oxaloacetate decarboxylase, and malic enzyme (nicotinamide adenine dinucleotide phosphate-dependent) were present in cell extracts of B. fragilis, and the results of the labeling experiments agreed with pyruvate synthesis via oxaloacetate and malate if these acids are in equilibrium with fumarate. The conversion of [2-(14)C]- and [3-(14)C]lactate to acetate was not associated with a randomization of radioactivity.

Stereochemistry of propionyl-coenzyme A and pyruvate carboxylations catalyzed by transcarboxylase

The stereochemistry of the two half-reactions catalyzed by the biotin-containing enzyme, transcarboxy-lase from Propionobacteria shermanii, has been determined. The pro-R hydrogen at C-2 of propionyl-coenzyme A is replaced by CO2 in formation of the S isomer of methylmalonyl-CoA, defining the process as retention of configuration. This C-2 hydrogen is abstracted at a rate identical with product formation. For the other half-reaction, pyruvate to oxalacetate, the chiral methyl group methodology of Rose (I. A. Rose (1970), J. Biol. Chem. 245, 6052) was employed. First, it was determined with [3-2-He]pyruvate that a kinetic deuterium isotope effect of 2.1 occurs at Vmax in this carboxyl transfer, indicating that the necessary requirement for discrimination against heavy isotopes of hydrogen existed. Then, 3(S)-[3-2-H,3-H]pyruvate, generated from 3(S)-]E-2-H,3-H]phosphoglycerate, was carboxylated and the oxalacetate trapped as [3030H]malate using malate dehydrogenase. Exhaustive incubation of the tritiated malate (3-H/14-C = 1.95) with fumarase to labilize the pro-R hydrogen at C-3 resulted in release of 65% of the tritium into water. Reisolation of the malate after fumarase action yielded a 30H/14-C ration of 0.67, indicating 34% retention as expected. The theoretical enantiotopic distribution for the observed k1H/k2H of 2.1 is 68:32. Selective enrichment of tritium in the pro-R position at C-3 of malate indicates enzymatic carboxylation of pyruvate with retention of configuration in this half-reaction also.

Production of antibodies that bind biotin and inhibit biotin containing enzymes

Methods were developed for the coupling of biotin to bovine serum albumin and bovine gamma-globulin using a water-soluble carbodimide. The use of [14-C]biotin as a tracer allowed quantitation of the incorporation of biotin into the conjugates: 2.55 mol of biotin was incorporated per mol of gamma-globulin and 7-9 mol of biotin was incorporated per mol of serum albumin in different preparations. These conjugates were highly immunogenic in the rabbit and anti-bodies reactive with the biotinyl group itself could be detected by their ability to precipitate the heterologous biotinated carrier but not the unmodified heterologous carrier. There antisera rapidly inactivated transcarboxylase and pyruvate carboxylase and this inactivation could be blocked by pretreatment of the antisera with biotin or biocytin. Using enzyme inhibition to detect free antibody, the binding constant for biotin was found to be 5.0 x 10- minus 8 M and that for biocytin 3.5 x 10- minus 8 M.

Isolation of peptides from the carboxyl carrier subunit of transcarboxylase. Role of the non-biotinyl peptide in assembly

Transcarboxylase is made up of a central hexameric subunit (S20,W similar 12 S), three peripheral dimeric metallo subunits (S20,W similar to 5 S), and six biotinyl carboxyl carrier subunits (S20,W similar to 1.3 S). The results presented here show that the carboxyl carrier subunit is required for assembly of the 12S and 5S subunits into the oligomer. However, only a portion of the subunit is required for assembly. On treatment of transcarboxylase briefly with trypsin at pH 6.3 extremely susceptible peptide bonds of the carboxyl carrier protein are cleaved releasing biotinyl peptides of about similar to 66 and similar to 40 residues. The resulting trypsinized transcarboxylase, though enzymatically inactive, remains essentially intact as judged by its hydrodynamic and molecular sieving properties. The modified enzyme can be dissociated at pH 8 to the central 12S subunit and peripheral 5S subunit to which the residual portion(s) of the cleaved carboxyl carrier protein is still attached. These components can then be separated by molecular sieving. The residual portion of the carboxyl carrier protein (non-biotinyl peptide) can then be isolated by dissociation of the 5S subunit complex at pH 9 and by chromatography over Bio-Gel A-1.5m. The isolated non-biotinyl peptide has been shown to contain the combining domain of the 1.3SE carboxyl carrier protein since it causes combination of the 12S and 5S subunits. Active enzyme is formed by combination of the intact carboxyl carrier protein and the 12S and 5S subunits and an inactive oligomer of similar size is formed if the non-biotinyl peptide is used in place of the carboxyl carrier protein. The similar to 66- and similar to 40-residue biotinyl peptides, which are released by the trypsin treatment, apparently occur on an exposed portion of the enzyme. This portion of the carboxyl carrier protein apparently serves to place the biotinyl group adjacent to the two substrate sites of the enzyme, one of which is on the peripheral subunit and the other on the central subunit. Thus the carboxyl carrier protein has two functions: one portion holds the 12S and 5S subunits in juxtaposition and the other portion orients the biotinyl group adjacent to the substrate sites so that it may function as a carboxyl carrier between the sites.

Evidence that the two partial reactions of transcarboxylation are catalyzed by two dissimilar subunits of transcarboxylase

The results presented here show that isolated subunits of transcarboxylase specifically catalyze the two partial reactions of transcarboxylation as shown in eq 1-3. The 12S central subunit is active in the transcarboxylation with methylmalonyl-CoA but inactive with oxalacetate and the peripheral metallo 5S subunit is active in the transcarboxylation with oxalacetate but inactive with methylmalonyl-CoA. These subunits, likewise, are specific for the reverse partial reactions; the central subunit catalyzing transfer from the carboxylated biotinyl group to propionyl-CoA to yield methylmalonyl-CoA and the peripheral subunit to pyruvate to yield oxalacetate. Thus, the central subunit contains the sites for the CoA esters (methylmalonyl-CoA and propionyl-CoA) and the peripheral metallo subunits for the keto acids (oxalacetate and pyruvate). In the overall reaction the biotinyl carboxyl carrier protein acts as a shuttle to carry the carboxyl groups between the two subunits. Biotin and certain biotin analogs are inactive in these partial reactions but the similar to 40- or similar to 66-residue biotinyl peptides, which are derived from the carboxyl carrier protein, are active. Transcarboxylase can be reconstituted from its isolated subunits and a comparison was made of the rate of the overall reaction when the subunits were assembled, as in the intact enzyme, with that obtained when the reaction was catalyzed by the nonassembled subunits. In the latter case, since the biotinyl carboxyl carrier subunit must diffuse from one subunit to the other, the overall reaction is much slower than with the assembled subunits. The reaction with trypsinized transcarboxylase from which the similar to 66-residue and similar to 40-residue biotinyl peptides have been stripped, likewise, was slow even though the biotinyl peptides were added to the reconstitution mixture. The 12SH and 5SE subunits remain assembled after trypsin treatment but the biotinyl peptides apparently do not combine firmly or properly with the trypsinized enzyme and the biotinyl group apparently must oscillate as a carboxyl carrier between the two sites on the subunits by diffusion.