Jouni Jokela

Growth characteristics and metabolic flux analysis of Candida milleri

The growth characteristics of the sourdough yeast Candida milleri was studied in a carbon-limited aerobic chemostat culture on defined medium. The effect of glucose, xylose, and glucose-xylose mixture on metabolite production and on key enzyme activities was evaluated. Xylose as a sole carbon source was not metabolized by C. milleri. Glucose as a sole carbon source produced only biomass and carbon dioxide. When a glucose-xylose mixture (125:125 C-mM) was used as a carbon source, a small amount of xylose was consumed and a low concentration of xylitol was produced (7.20 C-mM). Enzymatic assays indicated that C. milleri does not possess xylitol dehydrogenase activity and its xylose reductase is exclusively NADPH-dependent. In glucose medium both NAD(+)- and NADP(+)-dependent aldehyde dehydrogenase activities were found, whereas in a glucose-xylose medium only NADP(+)-dependent aldehyde dehydrogenase activity was detected. The developed metabolic flux analysis corresponded well with the experimentally measured values of metabolite production, oxygen consumption (OUR), and carbon dioxide production (CER). Turnover number in generation and consumption of ATP, mitochondrial and cytosolic NADH, and cytosolic NADPH could be calculated and redox balance was achieved. Constraints were imposed on the flux estimates such that the directionality of irreversible reactions is not violated, and cofactor dependence of the measured enzyme activities were taken into account in constructing the metabolic flux network.

Enhanced glucose to fructose conversion in acetone with xylose isomerase stabilized by crystallization and cross-linking.

The effects of acetone and ethanol on glucose to fructose conversion catalyzed by soluble and cross‐linked crystalline (CLXIC) xylose isomerase were studied. Relative to pure buffer solvent, the fructose production rate was more than doubled in 50% acetone. The same kind of increase in the isomerization rate was not seen with ethanol. Increase both in acetone and in ethanol concentration in the reaction solvent enhanced the production of fructose. At 50 °C in pure buffer solvent the reaction mixture contained 49% fructose in equilibrium and in 90% acetone the fructose equilibrium content was 64%. Furthermore, CLXIC was relatively stable in the presence of high concentration of acetone: 70–80% of activity was left after incubation for 24 h at 50 °C in buffer solutions (pH 7.2) containing 10–90% acetone. In buffer containing 50% ethanol only 2% of the initial activity of CLXIC was retained after 24 h at 50 °C. Soluble xylose isomerase was considerably less stable than CLXIC in both acetone‐ and ethanol‐containing solutions. These results show that the addition of acetone enhances the production of fructose from glucose by enhancing the reaction rate and shifting the equilibrium toward fructose. However, xylose isomerase must be in the form of cross‐linked crystals for maximal activity and stability.

Cross-linked glucose isomerase crystals as a liquid chromatographic separation material

Cross-linked crystals of glucose isomerase (CLGI) were characterized as a liquid chromatographic separation material. The experiments were done with crystals having an average diameter of 83 µm. Porosity (epsilon(p)) and pore size distribution of the CLGI crystals were measured with size exclusion chromatography using D(2)O and polyethylene glycols as probes. CLGI material was capable of separating <1000 g/mol polyethylene glycols. Fifty two percent (epsilon(p) = 0.47) of the total crystal volume was in pores. Pore size measurement showed that CLGI crystals were microporous material, having an average apparent pore diameter of 29 +/- 0.08 Å. CLGI material separated n-alcohols C(1) to C(8) based on the hydrophobic interaction between the protein material and the carbon chain of the alcohols. Height equivalent to a theoretical plate (HETP, in millimeters) ranged from 1.6 to 0.89 for the C(1) to C(7) n-alcohol series. Despite the large crystal size, CLGI as a chirally active phase effectively separated D- and L-arabitol (R(s) = 0.58) and showed potential for chiral separation of amino acids.

Simultaneous catalysis and product separation by cross-linked enzyme crystals

Crystalline cross-linked xylose isomerase (CLXI, EC 5.3.1.5) and xylanase (CLX, EC 3.2.1.8) were studied in a packed-bed reactor for simultaneous catalytic reaction and separation of substrates from reaction products. Streptomyces rubiginosus xylose isomerase catalyzed a slow isomerization of L-arabinose to L-ribulose and an epimerization to L-ribose. In equilibrium the reaction mixture contained 52.5% arabinose, 22.5% ribulose, and 25% ribose. In a packed-bed column filled with CLXI, a simultaneous reaction and separation resulted in fractions where arabinose concentration varied between 100-0%, ribulose between 0-55%, and ribose between 0-100%. Trichoderma reesei xylanase II hydrolyzed and transferred xylotetraose mainly to xylotriose and xylobiose. In a packed-bed column filled with CLX, xylotetraose rapidly reacted to xylobiose and xylose by a mechanism that is not yet fully understood.

Biodegradation of VOCs from printing press air by an on-site pilot plant bioscrubber and laboratory scale continuous yeast cultures.

The volatile organic compound composition (VOCs) of printing press air wasfound to contain mostly ethanol, but also ethyl acetate, 1-propanol, 2-propanol,1-methoxy-2-propanol and 3-ethoxy-1-propanol. A pilot plant bioscrubber inoculatedwith a mixed microbial population was constructed on-site. The bioscrubber wasable to treat the polluted gas efficiently. It, however, suffered from strong wall growthand blockages in the column. The efficiencies of the pilot plant and a bioreactor iscompared. The yeasts Candida guilliermondii and Saccharomyces cerevisiae known to tolerate ethanol were selected instead of mixed population to avoid the wall growth and blockages in the bioreactor. The removal of the VOCs both individually and as a complex mixture was tested in a microcultivation system and in continuous chemostat cultures with and without cell recycling. The Candida yeast could use all the compounds as a carbon source while growth of S. cerevisiae was markedly slower on the methoxylated and ethoxylated propanols. Best total removal of the VOCs was 99% and achieved by C. guilliermondii. The only compound that was not totally removable in the chemostat experiment with C. guilliermondii was 1-methoxy-2-propanol. In laboratory scale the total and volumetric removal of VOCs by C. guilliermondii was more efficient compared to the pilot plant encouraging to scale up and applying the yeast bioreactor to real field conditions.

Total Hydrolysis of Xylotetraose and Xylobiose by Soluble and Cross-linked Crystalline Xylanase II from Trichoderma reesei

The ability of Trichoderma reesei xylanase II (EC 3.2.1.8) to hydrolyse the small xylo-oligomer substrates, xylotetraose and xylobiose, was studied. Xylanase was used in both soluble and cross-linked enzyme crystal (CLEC) form. Hydrolysis reactions with crystalline xylanase cross-linked with glutaraldehyde and lysine were performed in a column reactor. By using appropriate combination of column packing length and flow rate, xylotetraose and xylobiose (initial concentrations 10 mg ml m 1 ) were hydrolysed completely to xylose in less than 1 h. The observed reaction rate in the column depended substantially on the flow rate of the eluent, probably due to an enhanced mass-transfer with higher flow rates. With soluble xylanase, using extended reaction times of 24 h and extremely high enzyme/substrate ratios of 20 (w/w) or above, the hydrolysis reaction reached completion with both xylotetraose and xylobiose as substrates. Even with the lowest flow rate, the reaction in the column appeared to be faster than soluble enzyme hydrolysis with comparable enzyme/substrate ratios.

Enantioselective Affinity Chromatography of a Chiral Drug by Crystalline and Carrier-Bound Antibody Fab Fragment

An antibody Fab fragment, ENA5His, capable of enantioselective affinity chromatographic separation of a chiral drug, finrozole, was stabilized against organic solvents by chemical cross‐linking. High concentration of methanol is needed to release the bound drug from the antibody fragment. However, in native form the antibody fragment is unstable at these conditions. We used cross‐linked protein crystal technology to stabilize the antibody fragment molecule. Glutaraldehyde cross‐linked ENA5His crystals (CLAC) packed in a column separated pure enantiomers from the racemic mixture of the drug. CLAC was totally stable at the elution conditions, enabling reuse of the immunoaffinity column packed with CLAC. However, the specific drug enantiomer binding capacity of CLAC was only 50% of the corresponding capacity of immobilized ENA5His. We were also able to cross‐link immobilized ENA5His by glutaraldehyde. This method produced a protein matrix with high activity and stability in the elution conditions.