Nonenzymatic acetolactate oxidation to diacetyl by flavin, nicotinamide and quinone coenzymes

The chemical mechanisms of the enzymes in the branched-chain amino acids biosynthetic pathway and their applications

l-Valine, l-isoleucine, and l-leucine are three key proteinogenic amino acids, and they are also the essential amino acids required for mammalian growth, possessing important and to some extent, special physiological and biological functions. Because of the branched structures in their carbon chains, they are also named as branched-chain amino acids (BCAAs). This review will highlight the advance in studies of the enzymes involved in the biosynthetic pathway of BCAAs, concentrating on their chemical mechanisms and applications in screening herbicides and antibacterial agents. The uses of some of these enzymes in lab scale organic synthesis are also discussed.

Powering the next industrial revolution: transitioning from nonrenewable energy to solar fuels via CO2 reduction

Solar energy has been used for decades for the direct production of electricity in various industries and devices; however, harnessing and storing this energy in the form of chemical bonds has emerged as a promising alternative to fossil fuel combustion. The common feedstocks for producing such solar fuels are carbon dioxide and water, yet only the photoconversion of carbon dioxide presents the opportunity to generate liquid fuels capable of integrating into our existing infrastructure, while simultaneously removing atmospheric greenhouse gas pollution. This review presents recent advances in photochemical solar fuel production technology. Although efforts in this field have created an incredible number of methods to convert carbon dioxide into gaseous and liquid fuels, these can generally be classified under one of four categories based on how incident sunlight is utilised: solar concentration for thermoconversion (Category 1), transformation toward electroconversion (Category 2), natural photosynthesis for bioconversion (Category 3), and artificial photosynthesis for direct photoconversion (Category 4). Select examples of developments within each of these categories is presented, showing the state-of-the-art in the use of carbon dioxide as a suitable feedstock for solar fuel production.

Solar energy has been used for decades for the direct production of electricity in various industries and devices. However, harnessing and storing this energy in the form of chemical bonds has emerged as a promising alternative to fossil fuels.

Determination of Diacetyl in Beer by a Precolumn Derivatization-HPLC-UV Method Using 4-(2,3-Dimethyl-6-quinoxalinyl)-1,2-benzenediamine as a Derivatizing Reagent

Diacetyl is an important flavoring compound in many foods, especially in beer. In the present study, we developed and validated a new precolumn derivatization HPLC-UV method for the determination of diacetyl using 4-(2,3-dimethyl-6-quinoxalinyl)-1,2-benzenediamine as a novel derivatizing reagent. After derivatization with the reagent at a pH value 4.0 at ambient temperature for 10 min, diacetyl was analyzed on an ODS column and detected at 254 nm. The results show that the correlation coefficient of the method is 0.9991 in the range of 0.10 to 100.0 μM diacetyl, and the limit of detection is 0.02 μM. The method was further evaluated in the analysis of beer samples with the recoveries ranging from 94.4 to 102.6% and RSDs from 1.36 to 3.33%. The concentrations of diacetyl in 8 beer samples were determined in the range of 0.19 to 0.42 μM. The method established in this study may be well suitable for the determination of diacetyl in beer.

The influence of shear on the metabolite yield of Lactobacillus rhamnosus biofilms

A tubular recycle bioreactor was employed to ensure homogeneous shear conditions on the biofilm surface. Superficial liquid velocities of 0.19 ms(-1), 0.37 ms(-1), 0.55 ms(-1) and 3.65 ms(-1) were used. The highest velocity resulted in negligible cell attachment (chemostat) while the ratio of attached-to-total cell mass escalated as the superficial velocity decreased. The lactic acid yield on glucose increased from 0.75 g g(-1) to 0.90 g g(-1) with declining shear while the corresponding acetoin yield on glucose decreased from 0.074 g g(-1) to 0.017 g g(-1). Redox analysis of the catabolites revealed a net consumption of NADH in the anabolism, while the extent of NADH consumption decreased when shear was reduced. This was attributed to the formation of more extracellular polymeric substance (EPS) at low shear conditions. A simplified metabolic flux model was used to estimate the EPS content of the biomass as a function of the shear velocity. Rate data supported the notion of increased EPS at lower shear.

Cyanobacterial conversion of carbon dioxide to 2,3-butanediol

Conversion of CO(2) for the synthesis of chemicals by photosynthetic organisms is an attractive target for establishing independence from fossil reserves. However, synthetic pathway construction in cyanobacteria is still in its infancy compared with model fermentative organisms. Here we systematically developed the 2,3-butanediol (23BD) biosynthetic pathway in Synechococcus elongatus PCC7942 as a model system to establish design methods for efficient exogenous chemical production in cyanobacteria. We identified 23BD as a target chemical with low host toxicity, and designed an oxygen-insensitive, cofactor-matched biosynthetic pathway coupled with irreversible enzymatic steps to create a driving force toward the target. Production of 23BD from CO(2) reached 2.38 g/L, which is a significant increase for chemical production from exogenous pathways in cyanobacteria. This work demonstrates that developing strong design methods can continue to increase chemical production in cyanobacteria.

Cytosolic re-localization and optimization of valine synthesis and catabolism enables inseased isobutanol production with the yeast Saccharomyces cerevisiae

BACKGROUND

The branched chain alcohol isobutanol exhibits superior physicochemical properties as an alternative biofuel. The yeast Saccharomyces cerevisiae naturally produces low amounts of isobutanol as a by-product during fermentations, resulting from the catabolism of valine. As S. cerevisiae is widely used in industrial applications and can easily be modified by genetic engineering, this microorganism is a promising host for the fermentative production of higher amounts of isobutanol.

RESULTS

Isobutanol production could be improved by re-locating the valine biosynthesis enzymes Ilv2, Ilv5 and Ilv3 from the mitochondrial matrix into the cytosol. To prevent the import of the three enzymes into yeast mitochondria, N-terminally shortened Ilv2, Ilv5 and Ilv3 versions were constructed lacking their mitochondrial targeting sequences. SDS-PAGE and immunofluorescence analyses confirmed expression and re-localization of the truncated enzymes. Growth tests or enzyme assays confirmed enzymatic activities. Isobutanol production was only increased in the absence of valine and the simultaneous blockage of the mitochondrial valine synthesis pathway. Isobutanol production could be even more enhanced after adapting the codon usage of the truncated valine biosynthesis genes to the codon usage of highly expressed glycolytic genes. Finally, a suitable ketoisovalerate decarboxylase, Aro10, and alcohol dehydrogenase, Adh2, were selected and overexpressed. The highest isobutanol titer was 0.63 g/L at a yield of nearly 15 mg per g glucose.

CONCLUSION

A cytosolic isobutanol production pathway was successfully established in yeast by re-localization and optimization of mitochondrial valine synthesis enzymes together with overexpression of Aro10 decarboxylase and Adh2 alcohol dehydrogenase. Driving forces were generated by blocking competition with the mitochondrial valine pathway and by omitting valine from the fermentation medium. Additional deletion of pyruvate decarboxylase genes and engineering of co-factor imbalances should lead to even higher isobutanol production.

2,3-butanediol production by acetogenic bacteria, an alternative route to chemical synthesis, using industrial waste gas

2,3-Butanediol (23BD) is a high-value chemical usually produced petrochemically but which can also be synthesized by some bacteria. To date, the best microbial 23BD production rates have been observed using pathogenic bacteria in fermentation systems that depend on sugars as the carbon and energy sources for product synthesis. Here we present evidence of 23BD production by three nonpathogenic acetogenic Clostridium species-Clostridium autoethanogenum, C. ljungdahlii, and C. ragsdalei-using carbon monoxide-containing industrial waste gases or syngas as the sole source of carbon and energy. Through an analysis of the C. ljungdahlii genome, the complete pathway from carbon monoxide to 23BD has been proposed. Homologues of the genes involved in this pathway were also confirmed for the other two species investigated. A gene expression study demonstrates a correlation between mRNA accumulation from 23BD biosynthetic genes and the onset of 23BD production, while a broader expression study of Wood-Ljungdahl pathway genes provides a transcription-level view of one of the oldest existing biochemical pathways.

Acetoin metabolism in bacteria

Acetoin is an important physiological metabolite excreted by many microorganisms. The excretion of acetoin, which can be diagnosed by the Voges Proskauer test and serves as a microbial classification marker, has its vital physiological meanings to these microbes mainly including avoiding acification, participating in the regulation of NAD/NADH ratio, and storaging carbon. The well-known anabolism of acetoin involves alpha-acetolactat synthase and alpha-acetolactate decarboxylase; yet its catabolism still contains some differing views, although much attention has been focused on it and great advances have been achieved. Current findings in catabolite control protein A (CcpA) mediated carbon catabolite repression may provide a fuller understanding of the control mechanism in bacteria. In this review, we first examine the acetoin synthesis pathways and its physiological meanings and relevancies; then we discuss the relationship between the two conflicting acetoin cleavage pathways, the enzymes of the acetoin dehydrogenase enzyme system, major genes involved in acetoin degradation, and the CcpA mediated acetoin catabolite repression pathway; in the end we discuss the genetic engineering progresses concerning applications. To date, this is the first integrated review on acetoin metabolism in bacteria, especially with regard to catabolic aspects. The apperception of the generation and dissimilation of acetoin in bacteria will help provide a better understanding of microbial strategies in the struggle for resources, which will consequently better serve the utilization of these microbes.

α,β-dicarbonyl reduction by Saccharomyces d-arabinose dehydrogenase

An alpha,beta-dicarbonyl reductase activity was purified from Saccharomyces cerevisiae and identified as the cytosolic enzyme D-Arabinose dehydrogenase (ARA1) by MALDI-TOF/TOF. Size exclusion chromatography analysis of recombinant Ara1p revealed that this protein formed a homodimer. Ara1p catalyzed the reduction of the reactive alpha,beta-dicarbonyl compounds methylglyoxal, diacetyl, and pentanedione in a NADPH dependant manner. Ara1p had apparent Km values of approximately 14 mM, 7 mM and 4 mM for methylglyoxal, diacetyl and pentanedione respectively, with corresponding turnover rates of 4.4, 6.9 and 5.9 s(-1) at pH 7.0. pH profiling showed that Ara1p had a pH optimum of 4.5 for the diacetyl reduction reaction. Ara1p also catalyzed the NADP+ dependant oxidation of acetoin; however this back reaction only occurred at alkaline pH values. That Ara1p was important for degradation of alpha,beta-dicarbonyl substrates was further supported by the observation that ara1-Delta knockout yeast mutants exhibited a decreased growth rate phenotype in media containing diacetyl.

Effect of alpha-acetolactate decarboxylase inactivation on alpha-acetolactate and diacetyl production by Lactococcus lactis subsp. lactis biovar diacetylactis

Strains of Lactococcus lactis subsp. lactis biovar diacetylactis deficient in alpha-acetolactate decarboxylase produce alpha-acetolactate. This unstable compound is a precursor of acetoin and an aromatic compound, diacetyl. Following random mutagenesis of strain CNRZ 483, alpha-acetolactate decarboxylase-negative mutant 483 M1 was selected. When grown in milk, its growth and acidification characteristics were similar to those of the parental strain. In anaerobic conditions, the parental strain produced 2.10 mM acetoin and less than 0.05 mM diacetyl. The mutant accumulated up to 2.11 mM alpha-acetolactate, which spontaneously degraded to acetoin and diacetyl. After 24 h of culture, the alpha-acetolactate concentration was only 0.49 mM and the acetoin and diacetyl concentrations reached 1.50 mM and 0.26 mM, respectively. Diacetyl production by both strains increased in aerobic conditions, as well as when citrate was added. In contrast to cultures of the parental strain, however, diacetyl and acetoin concentrations in mutant cultures continued to increase without reaching a plateau. The results also showed that diacetyl production by wild type L. lactis subsp. lactis biovar diacetylactis strains cannot be explained uniquely by the spontaneous decarboxylation of the alpha-acetolactate produced in the culture medium.

Natural-abundance isotope ratio mass spectrometry as a means of evaluating carbon redistribution during glucose-citrate cofermentation by Lactococcus lactis

The cometabolism of citrate and glucose by growing Lactococcus lactis ssp. lactis bv. diacetylactis was studied using a natural-abundance stable isotope technique. By a judicious choice of substrates differing slightly in their 13C/12C ratios, the simultaneous metabolism of citrate and glucose to a range of compounds was analysed. These end-products include lactate, acetate, formate, diacetyl and acetoin. All these products have pyruvate as a common intermediate. With the objective of estimating the degree to which glucose and citrate metabolism through pyruvate may be differentially regulated, the delta13C values of the products accumulated over a wide range of concentrations of citrate and glucose were compared. It was found that, whereas the relative accumulation of different products responds to both the substrate concentration and the ratio between the substrates, the delta13C values of the products primarily reflect the availability of the two substrates over the entire range examined. It can be concluded that in actively growing L. lactis the maintenance of pyruvate homeostasis takes precedence over the redox status of the cells as a regulatory factor.

Constructing an amylolytic brewing yeast Saccharomyces pastorianus suitable for accelerated brewing

An amylolytic brewing yeast Saccharomyces pastorianus, free of vector sequences and drug-resistance genes, was constructed by disrupting the alpha-acetolactate synthase gene and introducing the alpha-amylase gene as a selective marker. The resulting recombinant strain was able to utilize starch as the sole carbon source and its alpha-acetolactate synthase activity was lowered by 30%. Fermentation tests confirmed that the diacetyl concentration and the residual oligosaccharide were reduced by 70% and 25%, respectively, in fermented wort by the recombinant strain, while the brewing performance of the recombinant strain was retained.

Expression vectors for Methanococcus maripaludis: overexpression of acetohydroxyacid synthase and beta-galactosidase

A series of integrative and shuttle expression vectors was developed for use in Methanococcus maripaludis. The integrative expression vectors contained the Methanococcus voltae histone promoter and multiple cloning sites designed for efficient cloning of DNA. Upon transformation, they can be used to overexpress specific homologous genes in M. maripaludis. When tested with ilvBN, which encodes the large and small subunits of acetohydroxyacid synthase, transformants possessed specific activity 13-fold higher than that of the wild type. An expression shuttle vector, based on the cryptic plasmid pURB500 and the components of the integrative vector, was also developed for the expression of heterologous genes in M. maripaludis. The beta-galactosidase gene from Escherichia coli was expressed to approximately 1% of the total cellular protein using this vector. During this work, the genes for the acetohydroxyacid synthase (ilvBN) and phosphoenolpyruvate synthase (ppsA) were sequenced from a M. maripaludis genomic library.

Cytosolic enzymes with a mitochondrial ancestry from the anaerobic chytrid Piromyces sp. E2

The anaerobic chytrid Piromyces sp. E2 lacks mitochondria, but contains hydrogen-producing organelles, the hydrogenosomes. We are interested in how the adaptation to anaerobiosis influenced enzyme compartmentalization in this organism. Random sequencing of a cDNA library from Piromyces sp. E2 resulted in the isolation of cDNAs encoding malate dehydrogenase, aconitase and acetohydroxyacid reductoisomerase. Phylogenetic analysis of the deduced amino acid sequences revealed that they are closely related to their mitochondrial homologues from aerobic eukaryotes. However, the deduced sequences lack N-terminal extensions, which function as mitochondrial leader sequences in the corresponding mitochondrial enzymes from aerobic eukaryotes. Subcellular fractionation and enzyme assays confirmed that the corresponding enzymes are located in the cytosol. As anaerobic chytrids evolved from aerobic, mitochondria-bearing ancestors, we suggest that, in the course of the adaptation from an aerobic to an anaerobic lifestyle, mitochondrial enzymes were retargeted to the cytosol with the concomitant loss of their N-terminal leader sequences.