Redox potential control and applications in microaerobic and anaerobic fermentations

Many fermentation products are produced under microaerobic or anaerobic conditions, in which oxygen is undetectable by dissolved oxygen probe, presenting a challenge for process monitoring and control. Extracellular redox potentials that can be detected conveniently affect intracellular redox homeostasis and metabolism, and consequently control profiles of fermentation products, which provide an alternative for monitoring and control of these fermentation processes. This article reviews updated progress in the impact of redox potentials on gene expression, protein biosynthesis and metabolism as well as redox potential control strategies for more efficient production of fermentation products, taking ethanol fermentation by the yeast Saccharomyces under microaerobic conditions and butanol production by the bacterium Clostridium under anaerobic conditions as examples.     

Cellulase–lignin interactions—The role of carbohydrate-binding module and pH in non-productive binding

Non-productive cellulase adsorption onto lignin is a major inhibitory mechanism preventing enzymatic hydrolysis of lignocellulosic feedstocks. Therefore, understanding of enzyme–lignin interactions is essential for the development of enzyme mixtures and processes for lignocellulose hydrolysis. We have studied cellulase–lignin interactions using model enzymes, Melanocarpus albomyces Cel45A endoglucanase (MaCel45A) and its fusions with native and mutated carbohydrate-binding modules (CBMs) from Trichoderma reesei Cel7A. Binding of MaCel45A to lignin was dependent on pH in the presence and absence of the CBM; at high pH, less enzyme bound to isolated lignins. Potentiometric titration of the lignin preparations showed that negatively charged groups were present in the lignin samples and that negative charge in the samples was increased with increasing pH. The results suggest that electrostatic interactions contributed to non-productive enzyme adsorption: Reduced enzyme binding at high pH was presumably due to repulsive electrostatic interactions between the enzymes and lignin. The CBM increased binding of MaCel45A to the isolated lignins only at high pH. Hydrophobic interactions are probably involved in CBM binding to lignin, because the same aromatic amino acids that are essential in CBM–cellulose interaction were also shown to contribute to lignin-binding.     

kLa of stirred tank bioreactors revisited

By means of improved feedback control k_La measurements become possible at a precision and reproducibility that now allow a closer look at the influences of power input and aeration rate on the oxygen mass transfer. These measurements are performed online during running fermentations without a notable impact on the biochemical conversion processes. A closer inspection of the mass transfer during cultivations showed that at least the number of impellers influences mass transfer and mixing: On the laboratory scale, two hollow blade impellers clearly showed a larger k_La than the usually employed three impeller versions when operated at the same agitation power and aeration rate. Hollow blade impellers are preferable under most operational conditions because of their perfect gas handling capacity. Mixing time studies showed that these two impeller systems are also preferable with respect to mixing. Furthermore the widths of the baffle bars depict a significant influence on the k_La. All this clearly supports the fact that it is not only the integral power density that finally determines k_La.     

Complete genome sequence of the hydrogenotrophic Archaeon Methanobacterium sp. Mb1 isolated from a production-scale biogas plant

Methanobacterium sp. Mb1, a hydrogenotrophic methanogenic Archaeon, was isolated from a rural biogas plant producing methane-rich biogas from maize silage and cattle manure in Germany. Here we report the complete genome sequence of the novel methanogenic isolate Methanobacterium sp. Mb1 harboring a 2,029,766 bp circular chromosome featuring a GC content of 39.74%. The genome encodes two rRNA operons, 41 tRNA genes and 2021 coding sequences and represents the smallest genome currently known within the genus Methanobacterium.     

A modified metabolic model for mixed culture fermentation with energy conserving electron bifurcation reaction and metabolite transport energy

A modified metabolic model for mixed culture fermentation (MCF) is proposed with the consideration of an energy conserving electron bifurcation reaction and the transport energy of metabolites. The production of H₂ related to NADH/NAD⁺ and Fdred/Fdox is proposed to be divided in three processes in view of energy conserving electron bifurcation reaction. This assumption could fine‐tune the intracellular redox balance and regulate the distribution of metabolites. With respect to metabolite transport energy, the proton motive force is considered to be constant, while the transport rate coefficient is proposed to be proportional to the octanol–water partition coefficient. The modeling results for a glucose fermentation in a continuous stirred tank reactor show that the metabolite distribution is consistent with the literature: (1) acetate, butyrate, and ethanol are main products at acidic pH, while the production shifts to acetate and propionate at neutral and alkali pH; (2) the main products acetate, ethanol, and butyrate shift to ethanol at higher glucose concentration; (3) the changes for acetate and butyrate are following an increasing hydrogen partial pressure. The findings demonstrate that our modified model is more realistic than previous proposed model concepts. It also indicates that inclusion of an energy conserving electron bifurcation reaction and metabolite transport energy for MCF is sound in the viewpoint of biochemistry and physiology. Biotechnol. Bioeng. 2013; 110: 1884–1894. © 2013 Wiley Periodicals, Inc.     

酿酒酵母产苹果酸的还原TCA路径构建及发酵调控

苹果酸广泛应用于食品、化工行业。文中通过在酿酒酵母内敲除丙酮酸脱羧酶PDC1,并通过构建胞质内还原TCA的路径,即超表达丙酮酸羧化酶和苹果酸脱氢酶,成功地实现了苹果酸的生产。在野生型菌株中基本检测不到苹果酸的生成,而在工程菌株,苹果酸发酵浓度达到了45 mmol /L,同时副产物乙醇的产量也降低了18%。进一步通过发酵调控提高第二信使Ca2+的浓度使苹果酸的产量提高了7 %,在此基础上提高丙酮酸羧化酶的辅酶生物素浓度,使苹果酸的产量达到52.5 mmol /L,较原始菌株提高了16%。     

阿魏蘑液体发酵过程菌体形态变化对产漆酶的影响

本文旨在研究阿魏蘑菌体形态与漆酶产量之间的关系。结果显示,玻璃珠的添加可改变发酵过程中菌体形态,漆酶产量在球状菌体条件下高于丝状、絮状菌丝：直径分布在0.2~0.4 mm范围的菌球对漆酶的合成具有明显的促进作用;适合的葡萄糖、玉米粉和麸皮添加量,对直径在0.2~0.4 mm范围的菌球形成具有重要影响。此外,添加惰性载体同样可以控制菌球的直径分布,但对漆酶的合成无促进作用。     

Synthesis and Physiological Activity of Methyl 6′6′-Didemethyl Abscisate and New Chiral Analogs

Methyl 6′, 6′-didemethyl abscisate (5) was synthesized and assayed to elucidate the physiological activity of methyl substituents on the cyclohexene ring of abscisic acid (ABA). During this study two new chiral stereoisomeric analogs 6 and 7 were synthesized from l-and d-carvone. The rice seedling assay and germination assay of garden radish showed that 6′-methyl groups of ABA were not important in biological activity and that 5′-isopropenyl analogs 6 and 7 were inactive.     

Repeated Batch Production of Theanine by Coupled Fermentation with Energy Transfer Using Membrane-Enclosed γ-Glutamylmethylamide Synthetase and Dried Yeast Cells

γ-Glutamylmethylamide synthetase and dried baker’s yeast cells were enclosed together in a dialysis membrane tube to produce theanine repeatedly by coupled fermentation with energy transfer. The membrane-enclosed enzyme preparation (M-EEP) formed approximately 600 mM theanine from glutamic acid and ethylamine at a 100% conversion rate. M-EEP maintained its productivity of theanine during six consecutive reactions in a mixture containing NAD⁺.     

The Effect of Cocoa and Polydextrose on Bacterial Fermentation in Gastrointestinal Tract Simulations

Effects of cocoa mass and supplemented dietary fiber (polydextrose) on microbial fermentation were studied by combining digestion simulations of stomach and small intestine with multi-staged colon simulations. During the four phases of digestion, concentrations of available soluble proteins and reducing sugars reflected in vivo absorption of nutrients in small intestine. In colon simulation vessels, addition of polydextrose to digested cocoa mass significantly increased concentrations of total short-chain fatty acids and butyric acid, from 103 to 468 mM (P<0.01) and from 12 to 22 mM (P<0.01), respectively. Long-chain fatty acid concentrations (decreasing from 1,222 to 240 mM) were mainly affected by the presence of digested cocoa mass. Cocoa mass with or without polydextrose addition significantly decreased production of cadaverine (P<0.02) and branched-chain fatty acids compared to control during colon simulations. Results indicate beneficial effects on metabolism of colonic microbiota after digestion of cocoa mass, and even more so with polydextrose addition.