Purification of α-acetolactate decarboxylase from Lactobacillus casei DSM 2547

Structure and catalytic mechanistic insight into Enterobacter aerogenes acetolactate decarboxylase

α-Acetolactate decarboxylase (ALDC) catalyses α-acetolactate into acetoin (3-hydroxy-2-butanone, AC) and is considered to be the rate-limiting enzyme in the synthesis of 2,3-butanediol. In this work, the enzymatic activity of ALDC from Enterobacter aerogenes ALDC (E.a.-ALDC) was fully characterized with enzyme kinetics, indicating a K_m of 14.83 ± 0.87 mM and a k_cat of 0.81 ± 0.09 s^-1. However, compared with the activities of ALDCs reported from other bacteria, the activity of E.a.-ALDC was determined to present a relatively lower value of 849.08 ± 35.21 U/mg. The enzyme showed maximum activity at pH 5.5. In addition, the activity of E.a.-ALDC was promoted by Mg²⁺. The crystal structure of E.a.-ALDC firstly solved by X-ray crystallography at resolution of 2.4 Å revealed a chelated zinc ion with conserved His199, His201, His212, Glu70 and Glu259. In the active center, the conservative Arg150 was particularly proven to deviate from the zinc ion of the active centre, by adopting a flexible conformational change, resulting in a weak interaction network of the enzyme and the substrate. Further in silico docking of E.a.-ALDC with two enantiomers, (R)-acetolactate and (S)-acetolactate, unaltered the interaction network of E.a.-ALDC from the apo structure, which confirmed the weakened role of Arg150 in the catalytic properties of E.a.-ALDC. Our results reveal a unique structure-function relationship of acetolactate decarboxylase and provide a fundamental basis for the enzymatic synthesis of acetoin.

Studies on structure-function relationships of acetolactate decarboxylase from Enterobacter cloacae

Acetoin is an important bio-based platform chemical with wide applications. Among all bacterial strains, Enterobacter cloacae is a well-known acetoin producer via α-acetolactate decarboxylase (ALDC), which converts α-acetolactate into acetoin and is identified as the key enzyme in the biosynthetic pathway of acetoin. In this work, the enzyme properties of Enterobacter cloacae ALDC (E.c.-ALDC) were characterized, revealing a K m value of 12.19 mM and a k cat value of 0.96 s-1. Meanwhile, the optimum pH of E.c.-ALDC was 6.5, and the activity of E.c.-ALDC was activated by Mn2+, Ba2+, Mg2+, Zn2+ and Ca2+, while Cu2+ and Fe2+ significantly inhibited ALDC activity. More importantly, we solved and reported the first crystal structure of E.c.-ALDC at 2.4 Å resolution. The active centre consists of a zinc ion coordinated by highly conserved histidines (199, 201 and 212) and glutamates (70 and 259). However, the conserved Arg150 in E.c.-ALDC orients away from the zinc ion in the active centre of the molecule, losing contact with the zinc ion. Molecular docking of the two enantiomers of α-acetolactate, (R)-acetolactate and (S)-acetolactate allows us to further investigate the interaction networks of E.c.-ALDC with the unique conformation of Arg150. In the models, no direct contacts are observed between Arg150 and the substrates, which is unlikely to maintain the stabilization function of Arg150 in the catalytic reaction. The structure of E.c.-ALDC provides valuable information about its function, allowing a deeper understanding of the catalytic mechanism of ALDCs.

Production of acetoin from hydrothermally pretreated oil mesocarp fiber using metabolically engineered Escherichia coli in a bioreactor system

Acetoin is used in the biochemical, chemical and pharmaceutical industries. Several effective methods for acetoin production from petroleum-based substrates have been developed, but they all have an environmental impact and do not meet sustainability criteria. Here we describe a simple and efficient method for acetoin production from oil palm mesocarp fiber hydrolysate using engineered Escherichia coli. An optimization of culture conditions for acetoin production was carried out using reagent-grade chemicals. The final concentration reached 29.9gL^-1 with a theoretical yield of 79%. The optimal pretreatment conditions for preparing hydrolysate with higher sugar yields were then determined. When acetoin was produced using hydrolysate fortified with yeast extract, the theoretical yield reached 97% with an acetoin concentration of 15.5gL^-1. The acetoin productivity was 10-fold higher than that obtained using reagent-grade sugars. This approach makes use of a compromise strategy for effective utilization of oil palm biomass towards industrial application.

Fermentative Pyruvate and Acetyl-Coenzyme A Metabolism

Pyruvate and acetyl-CoA form the backbone of central metabolism. The nonoxidative cleavage of pyruvate to acetyl-CoA and formate by the glycyl radical enzyme pyruvate formate lyase is one of the signature reactions of mixed-acid fermentation in enterobacteria. Under these conditions, formic acid accounts for up to one-third of the carbon derived from glucose. The further metabolism of acetyl-CoA to acetate via acetyl-phosphate catalyzed by phosphotransacetylase and acetate kinase is an exemplar of substrate-level phosphorylation. Acetyl-CoA can also be used as an acceptor of the reducing equivalents generated during glycolysis, whereby ethanol is formed by the polymeric acetaldehyde/alcohol dehydrogenase (AdhE) enzyme. The metabolism of acetyl-CoA via either the acetate or the ethanol branches is governed by the cellular demand for ATP and the necessity to reoxidize NADH. Consequently, in the absence of an electron acceptor mutants lacking either branch of acetyl-CoA metabolism fail to cleave pyruvate, despite the presence of PFL, and instead reduce it to D-lactate by the D-lactate dehydrogenase. The conversion of PFL to the active, radical-bearing species is controlled by a radical-SAM enzyme, PFL-activase. All of these reactions are regulated in response to the prevalent cellular NADH:NAD+ ratio. In contrast to Escherichia coli and Salmonella species, some genera of enterobacteria, e.g., Klebsiella and Enterobacter, produce the more neutral product 2,3-butanediol and considerable amounts of CO2 as fermentation products. In these bacteria, two molecules of pyruvate are converted to α-acetolactate (AL) by α-acetolactate synthase (ALS). AL is then decarboxylated and subsequently reduced to the product 2,3-butandiol.

Purification and characterization of α-acetolactate decarboxylase (ALDC) from newly isolated Lactococcus lactis DX

BACKGROUND

Diacetyl (2,3-butanedione) is a common flavor aroma from fermented dairy products. There is a need to screen new microorganisms that can efficiently produce large amounts of diacetyl.

RESULTS

A new lactic acid bacterium that produced high concentrations of diacetyl was identified based on Gram staining, microscopic examination and 16S rDNA sequence analysis as Lactococcus lactis DX. Its α-acetolactate decarboxylase (ALDC) was purified using 0.45 g mL(-1) ammonium sulfate precipitation, Sephacryl S-300 and S-200 HR and native-PAGE. The purified ALDC displayed a monomer structure and had a molecular mass of about 73.1 kDa, which was estimated using SDS-PAGE. IR analysis showed that the ALDC had a typical protein structure. The optimal temperature and pH for ALDC activity were 40 °C and 6.5 respectively. The ALDC of L. lactis DX was activated by Fe(2+) , Zn(2+) , Mg(2+) , Ba(2+) and Ca(2+) , while Cu(2+) significantly inhibited ALDC activity. Leucine, valine and isoleucine activated the ALDC.

CONCLUSION

A strain that had high ability to produce diacetyl was identified as L. lactis DX. The difference in diacetyl production may be due to the ALDC, which is different from other ALDCs.

Purification and characterisation of acetolactate decarboxylase from Leuconostoc lactis NCW1

A two-step strategy involving DEAE-cellulose and POROS PI anion exchange chromatography has been developed for rapid purification of acetolactate decarboxylase (ALD) from Leuconostoc lactis NCW1. This results in 5333-fold purification with a yield of 30%. Purified ALD is a dimer of 49-kDa subunits, has a pH optimum of 6.0, a pI of 4.2 and its activity is independent of metals or branched chain amino acids. At the optimum pH, the K(m) for 2-acetolactate (ALA) was found to be 1.3 mM and the turnover number was 4000 min(-1). N-terminal sequence comparison with other ALDs showed little sequence conservation in this region. Purified ALD does not catalyse direct production of diacetyl from ALA, unlike the crude extract.

Purification and properties of the alpha-acetolactate decarboxylase from Lactococcus lactis subsp. lactis NCDO 2118

alpha-Acetolactate decarboxylase from Lactococcus lactis subsp. lactis NCDO 2118 was expressed at low levels in cell extracts and was also unstable. The purification was carried out from E. coli in which the enzyme was expressed 36-fold higher. The specific activity was 24-fold enhanced after purification. The main characteristics of alpha-acetolactate decarboxylase were: (i) activation by the three branched chain amino acids leucine, valine and isoleucine; (ii) allosteric properties displayed in absence and Michaelis kinetics in the presence of leucine. The enzyme is composed of six identical subunits of 26,500 Da.

Characterization of the genes of the 2,3-butanediol operons from Klebsiella terrigena and Enterobacter aerogenes

The genes involved in the 2,3-butanediol pathway coding for alpha-acetolactate decarboxylase, alpha-acetolactate synthase (alpha-ALS), and acetoin (diacetyl) reductase were isolated from Klebsiella terrigena and shown to be located in one operon. This operon was also shown to exist in Enterobacter aerogenes. The budA gene, coding for alpha-acetolactate decarboxylase, gives in both organisms a protein of 259 amino acids. The amino acid similarity between these proteins is 87%. The K. terrigena genes budB and budC, coding for alpha-ALS and acetoin reductase, respectively, were sequenced. The 559-amino-acid-long alpha-ALS enzyme shows similarities to the large subunits of the Escherichia coli anabolic alpha-ALS enzymes encoded by the genes ilvB, ilvG, and ilvI. The K. terrigena alpha-ALS is also shown to complement an anabolic alpha-ALS-deficient E. coli strain for valine synthesis. The 243-amino-acid-long acetoin reductase has the consensus amino acid sequence for the insect-type alcohol dehydrogenase/ribitol dehydrogenase family and has extensive similarities with the N-terminal and internal regions of three known dehydrogenases and one oxidoreductase.

Recombinant brewer's yeast strains suitable for accelerated brewing

Four brewer's yeast strains carrying the alpha-ald gene of Klebsiella terrigena (ex. Aerobacter aerogenes) or of Enterobacter aerogenes on autonomously replicating plasmids were constructed. The alpha-ald genes were linked either to the ADC1 promoter or to the PGK1 promoter of yeast Saccharomyces cerevisiae. In pilot scale brewing (50 l) with three of these recombinant yeasts the formation of diacetyl in beer was so low during fermentation that lagering was not required. All other brewing properties of the strains were unaffected and the quality of finished beers was as good as that of finished beer prepared with the control strain. The total process time of beer production could therefore be reduced to 2 weeks, in contrast to about 5 weeks required in the conventional process.