The application of functionalised magnetic adsorbent particles in combination with magnetic separation techniques has received considerable attention in recent years. The magnetically responsive nature of such adsorbent particles permits their selective manipulation and separation in the presence of other suspended solids. Thus, it becomes possible to magnetically separate selected target species directly out of crude biological process liquors (e.g. fermentation broths, cell disruptates, plasma, milk, whey and plant extracts) simply by binding them on magnetic adsorbents before application of a magnetic field. By using magnetic separation in this way, the several stages of sample pretreatment (especially centrifugation, filtration and membrane separation) that are normally necessary to condition an extract before its application on packed bed chromatography columns, may be eliminated. Magnetic separations are fast, gentle, scaleable, easily automated, can achieve separations that would be impossible or impractical to achieve by other techniques, and have demonstrated credibility in a wide range of disciplines, including minerals processing, wastewater treatment, molecular biology, cell sorting and clinical diagnostics. However, despite the highly attractive qualities of magnetic methods on a process scale, with the exception of wastewater treatment, few attempts to scale up magnetic operations in biotechnology have been reported thus far. The purpose of this review is to summarise the current state of development of protein separation using magnetic adsorbent particles and identify the obstacles that must be overcome if protein purification with magnetic adsorbent particles is to find its way into industrial practice.

Fermentation of the pentose sugar xylose to ethanol in lignocellulosic biomass would make bioethanol production economically more competitive. Saccharomyces cerevisiae, an efficient ethanol producer, can utilize xylose only when expressing the heterologous genesXYL1(xylose reductase) andXYL2(xylitol dehydrogenase). Xylose reductase and xylitol dehydrogenase convert xylose to its isomer xylulose. The geneXKS1encodes the xylulose-phosphorylating enzyme xylulokinase. In this study, we determined the effect ofXKS1overexpression on two differentS. cerevisiaehost strains, H158 and CEN.PK, also expressingXYL1andXYL2. H158 has been previously used as a host strain for the construction of recombinant xylose-utilizingS. cerevisiaestrains. CEN.PK is a new strain specifically developed to serve as a host strain for the development of metabolic engineering strategies. Fermentation was carried out in defined and complex media containing a hexose and pentose sugar mixture or a birch wood lignocellulosic hydrolysate.XKS1overexpression increased the ethanol yield by a factor of 2 and reduced the xylitol yield by 70 to 100% and the final acetate concentrations by 50 to 100%. However,XKS1overexpression reduced the total xylose consumption by half for CEN.PK and to as little as one-fifth for H158. Yeast extract and peptone partly restored sugar consumption in hydrolysate medium. CEN.PK consumed more xylose but produced more xylitol than H158 and thus gave lower ethanol yields on consumed xylose. The results demonstrate that strain background and modulation ofXKS1expression are important for generating an efficient xylose-fermenting recombinant strain ofS. cerevisiae.

Recombinant S. cerevisiae strains, with elevated levels of the enzymes of lower glycolysis (glyceraldehyde‐3‐phosphate dehydrogenase, phosphoglycerate mutase, phosphoglycerate kinase, enolase, pyruvate kinase, pyruvate decarboxylase and alcohol dehydrogenase) were physiologically characterized. During growth on glucose the enzyme levels in the recombinant strains (YHM4 and YHM7) were 1.1–3.4‐fold higher than in the host strain (CEN.PK.K45). The recombinant strains were grown in aerobic or anaerobic batch cultures on glucose or a mixture of glucose and galactose. The specific ethanol production rates in the recombinant strains were the same as for the host strain and the physiological behaviour of the recombinant strains and the host strain was similar. When the cellular demand for ATP was increased by means of glucose pulses (final concentrations of 3.9 g/l or 2.0 g/l, respectively) to aerobic chemostat cultures maintained at a dilution rate of 0.08/h, the specific carbon dioxide production rate (qCO₂) of CEN.PK.K45 accelerated at 6×10⁻³ mmol/g/min² during the first 15 min, whereas during the same time period the qCO₂ of YHM7 accelerated twice as fast at 12×10⁻³ mmol/g/min², indicating a higher fermentative capacity in the recombinant strain. Copyright © 2000 John Wiley & Sons, Ltd.