Xylitol production by immobilized recombinantSaccharomyces cerevisiae in a continuous packed-bed bioreactor

Intelligent self-control of carbon metabolic flux in SecY-engineered Escherichia coli for xylitol biosynthesis from xylose-glucose mixtures

Xylitol is a salutary sugar substitute that has been widely used in the food, pharmaceutical, and chemical industries. Co-fermentation of xylose and glucose by metabolically engineered cell factories is a promising alternative to chemical hydrogenation of xylose for commercial production of xylitol. Here, we engineered a mutant of SecY protein-translocation channel (SecY [ΔP]) in xylitol-producing Escherichia coli JM109 (DE3) as a passageway for xylose uptake. It was found that SecY (ΔP) channel could rapidly transport xylose without being interfered by XylB-catalyzed synthesis of xylitol-phosphate, which is impossible for native XylFGH and XylE transporters. More importantly, with the coaction of SecY (ΔP) channel and carbon catabolite repression (CCR), the flux of xylose to the pentose phosphate (PP) pathway and the xylitol synthesis pathway in E. coli could be automatically controlled in response to glucose, thereby ensuring that the mutant cells were able to fully utilize sugars with high xylitol yields. The E. coli cell factory developed in this study has been proven to be applicable to a broad range of xylose-glucose mixtures, which is conducive to simplifying the mixed-sugar fermentation process for efficient and economical production of xylitol.

Enzymes, In Vivo Biocatalysis, and Metabolic Engineering for Enabling a Circular Economy and Sustainability

Since the industrial revolution, the rapid growth and development of global industries have depended largely upon the utilization of coal-derived chemicals, and more recently, the utilization of petroleum-based chemicals. These developments have followed a linear economy model (produce, consume, and dispose). As the world is facing a serious threat from the climate change crisis, a more sustainable solution for manufacturing, i.e., circular economy in which waste from the same or different industries can be used as feedstocks or resources for production offers an attractive industrial/business model. In nature, biological systems, i.e., microorganisms routinely use their enzymes and metabolic pathways to convert organic and inorganic wastes to synthesize biochemicals and energy required for their growth. Therefore, an understanding of how selected enzymes convert biobased feedstocks into special (bio)chemicals serves as an important basis from which to build on for applications in biocatalysis, metabolic engineering, and synthetic biology to enable biobased processes that are greener and cleaner for the environment. This review article highlights the current state of knowledge regarding the enzymatic reactions used in converting biobased wastes (lignocellulosic biomass, sugar, phenolic acid, triglyceride, fatty acid, and glycerol) and greenhouse gases (CO₂ and CH₄) into value-added products and discusses the current progress made in their metabolic engineering. The commercial aspects and life cycle assessment of products from enzymatic and metabolic engineering are also discussed. Continued development in the field of metabolic engineering would offer diversified solutions which are sustainable and renewable for manufacturing valuable chemicals.

Continuous Laminaribiose Production Using an Immobilized Bienzymatic System in a Packed Bed Reactor

The first continuous production system of laminaribiose from sucrose and glucose in a bienzymatic reaction is reported in this study. Immobilized laminaribiose phosphorylase and sucrose phosphorylase were used in a packed bed reactor system comprising of a 3-cm glass column at 35 °C with a steady feeding flow rate of 0.1 ml/min. Factors affecting product formation including enzyme ratio, peal concept (both enzymes in one pearl or in separate pearls), and pearl size were studied. An enzyme ratio of 2:1 of laminaribiose phosphorylase (LP) to sucrose phosphorylase (SP) when encapsulated separately in bigger size peals resulted in higher concentration of product. Laminaribiose (0.4 g/(L h)) is produced in the optimized system at steady state. The reaction system proved to be operationally stable throughout 10 days of continuous processing. A half-life time of more than 9 days was observed for both biocatalysts.

Green synthesis of isopropyl myristate in novel single phase medium Part II: Packed bed reactor (PBR) studies

Isopropyl myristate is a useful functional molecule responding to the requirements of numerous fields of application in cosmetic, pharmaceutical and food industry. In the present work, lipase-catalyzed production of isopropyl myristate by esterification of myristic acid with isopropyl alcohol (molar ratio of 1:15) in the homogenous reaction medium was performed on a bench-scale packed bed reactors, in order to obtain suitable reaction performance data for upscaling. An immobilized lipase B from Candida antartica was used as the biocatalyst based on our previous study. The process intensification resulted in a clean and green synthesis process comprising a series of packed bed reactors of immobilized enzyme and water dehydrant. In addition, use of the single phase reaction system facilitates efficient recovery of the product with no effluent generated and recyclability of unreacted substrates. The single phase reaction system coupled with a continuous operating bioreactor ensures a stable operational life for the enzyme.

Sweeteners

Polyols as sugar substitutes, intense sweeteners and some new carbohydrates are increasingly used in foods and beverages. Some sweeteners are produced by fermentation or using enzymatic conversion. Many studies for others have been published. This chapter reviews the most important sweeteners.

Optimization of CDT-1 and XYL1 expression for balanced co-production of ethanol and xylitol from cellobiose and xylose by engineered Saccharomyces cerevisiae

Production of ethanol and xylitol from lignocellulosic hydrolysates is an alternative to the traditional production of ethanol in utilizing biomass. However, the conversion efficiency of xylose to xylitol is restricted by glucose repression, causing a low xylitol titer. To this end, we cloned genes CDT-1 (encoding a cellodextrin transporter) and gh1-1 (encoding an intracellular β-glucosidase) from Neurospora crassa and XYL1 (encoding a xylose reductase that converts xylose into xylitol) from Scheffersomyces stipitis into Saccharomyces cerevisiae, enabling simultaneous production of ethanol and xylitol from a mixture of cellobiose and xylose (main components of lignocellulosic hydrolysates). We further optimized the expression levels of CDT-1 and XYL1 by manipulating their promoters and copy-numbers, and constructed an engineered S. cerevisiae strain (carrying one copy of PGK1p-CDT1 and two copies of TDH3p-XYL1), which showed an 85.7% increase in xylitol production from the mixture of cellobiose and xylose than that from the mixture of glucose and xylose. Thus, we achieved a balanced co-fermentation of cellobiose (0.165 g/L/h) and xylose (0.162 g/L/h) at similar rates to co-produce ethanol (0.36 g/g) and xylitol (1.00 g/g).

Microbial production of xylitol from D-xylose and sugarcane bagasse hemicellulose using newly isolated thermotolerant yeast Debaryomyces hansenii

A thermotolerant yeast capable of fermenting xylose to xylitol at 40°C was isolated and identified as a strain of Debaryomyces hansenii by ITS sequencing. This paper reports the production of xylitol from D-xylose and sugarcane bagasse hemicellulose by free and Ca-alginate immobilized cells of D. hansenii. The efficiency of free and immobilized cells were compared for xylitol production from D-xylose and hemicellulose in batch culture at 40°C. The maximum xylitol produced by free cells was 68.6 g/L from 100 g/L of xylose, with a yield of 0.76 g/g and volumetric productivity 0.44 g/L/h. The yield of xylitol and volumetric productivity were 0.69 g/g and 0.28 g/L/h respectively from hemicellulosic hydrolysate of sugarcane bagasse after detoxification with activated charcoal and ion exchange resins. The Ca-alginate immobilized D. hansenii cells produced 73.8 g of xylitol from 100 g/L of xylose with a yield of 0.82 g/g and volumetric productivity of 0.46 g/L/h and were reused for five batches with steady bioconversion rates and yields.

A review on the effects of supercritical carbon dioxide on enzyme activity

Different types of enzymes such as lipases, several phosphatases, dehydrogenases, oxidases, amylases and others are well suited for the reactions in SC-CO₂. The stability and the activity of enzymes exposed to carbon dioxide under high pressure depend on enzyme species, water content in the solution and on the pressure and temperature of the reaction system. The three-dimensional structure of enzymes may be significantly altered under extreme conditions, causing their denaturation and consequent loss of activity. If the conditions are less adverse, the protein structure may be largely retained. Minor structural changes may induce an alternative active protein state with altered enzyme activity, specificity and stability.

Efficient regeneration of NADPH using an engineered phosphite dehydrogenase

The in situ regeneration of reduced nicotinamide cofactors (NAD(P)H) is necessary for practical synthesis of many important chemicals. Here, we report the engineering of a highly stable and active mutant phosphite dehydrogenase (12x-A176R PTDH) from Pseudomonas stutzeri and evaluation of its potential as an effective NADPH regeneration system in an enzyme membrane reactor. Two practically important enzymatic reactions including xylose reductase-catalyzed xylitol synthesis and alcohol dehydrogenase-catalyzed (R)-phenylethanol synthesis were used as model systems, and the mutant PTDH was directly compared to the commercially available NADP(+)-specific Pseudomonas sp. 101 formate dehydrogenase (mut Pse-FDH) that is widely used for NADPH regeneration. In both model reactions, the two regeneration enzymes showed similar rates of enzyme activity loss; however, the mutant PTDH showed higher substrate conversion and higher total turnover numbers for NADP(+) than mut Pse-FDH. The space-time yields of the product with the mutant PTDH were also up to fourfold higher than those with mut Pse-FDH. In particular, a space-time yield of 230 g L(-1) d(-1) xylitol was obtained with the mutant PTDH using a charged nanofiltration membrane, representing the highest productivity compared to other existing biological processes for xylitol synthesis based on yeast D-xylose converting strains or similar in vitro enzyme membrane reactor systems.

Repeated-Batch Xylitol Bioproduction Using Yeast Cells Entrapped in Polyvinyl Alcohol–Hydrogel

Xylose-to-xylitol conversion was investigated in a bench-scale bioreactor using Candida guilliermondii cells entrapped within polyvinyl alcohol-hydrogel beads in a system operated in repeated-batch mode with cell recycling. Yeast-viable cells were immobilized in the support using the freezing-thawing method. Bioconversion assays were performed in a stirred tank reactor operated at 400-rpm agitation speed, 30 degrees C temperature, and 1.04-vvm air flow rate. The system was explored during six successive cycles, and a small decrease in the conversion performance in the fifth cycle was observed, but the biocatalytic activity of the microorganism was recovered in the sixth cycle after washing the particles. During the process, the hydrogel beads maintained their shape and size without appreciable deterioration. Xylitol production, yield factor, and volumetric productivity increased with progressive recycling of cells and achieved their maximum values (P(F) = 39.7 g l(-1); Y(P/S) = 0.77 g g(-1); Q(P) = 0.53 g l(-1) h(-1), respectively) after the third cell recycling, probably because of cells' adaptation to the medium.

EngineeringEscherichia colifor xylitol production from glucose-xylose mixtures

The range of value-added chemicals produced by Escherichia coli from simple sugars has been expanded to include xylitol. This was accomplished by screening the in vivo activity of a number of heterologous xylitol-producing enzymes. Xylose reductases from Candida boidinii (CbXR), Candida tenuis (CtXR), Pichia stipitis (PsXR), and Saccharmoyces cerivisiae (ScXR), and xylitol dehydrogenases from Gluconobacter oxydans (GoXDH) and Pichia stipitis (PsXDH) were all functional in E. coli to varying extents. Replacement of E. coli's native cyclic AMP receptor protein (CRP) with a cyclic AMP-independent mutant (CRP*) facilitated xylose uptake and xylitol production from mixtures of glucose and xylose, with glucose serving as the growth substrate and source of reducing equivalents. Of the enzymes tested, overexpression of NADPH-dependent CbXR produced the highest concentrations of xylitol in shake-flask cultures (approximately 275 mM in LB cultures, approximately 180 mM using minimal medium). Expression of CbXR in strain PC09 (crp*, DeltaxylB) in a 10-L controlled fermentation containing minimal medium resulted in production of approximately 250 mM xylitol (38 g/L), with concomitant utilization of approximately 150 mM glucose. The ratio of moles xylitol produced (from xylose) per mole glucose consumed was improved to > 3.7:1 using metabolically active "resting" cells.