SPOTLIGHT

Theory and Practice Unveiledoday there is an excellent theoretical understanding of

& Towards the Isolation of Antibodies toSurface Biomarkers

The isolation of antibodies that bind specifically to cellsurface molecules is of great interest for drug discovery.Therapeutically important antibodies that bind to cellsurface molecules include Rituxan1 and Herceptin1 usedfor the treatment of Non-Hodgkin’s lymphoma and breastcancer, respectively. Jung and co-workers now show that theAnchored Periplasmic Expression (APEx) used for the displayand screening of combinatorial libraries in Escherichia coli issuitable for the discovery of binders to immobilized proteins,possibly including cell surface antigens. In APEx, proteins aredisplayed on the surface of the bacterial periplasmic mem-brane. The cells are then converted into spheroplasts, that is,they are chemically treated to remove the outer membrane ofthe bacterium, exposing the inner membrane displayedantibody to antigen. The authors show that spheroplastsdisplaying antibodies in the appropriate fashion bind toantigen immobilized on beads and that they can be easilyenriched by flow cytometry from spheroplasts displayingantibodies with unrelated antigen specificity. This workdemonstrates the feasibility of the APEx display technologyfor the isolation of antibodies specific to immobilized ligandsand potentially to biomarkers expressed on cell surfaces fromcombinatorial libraries. Page 39

DOI: 10.1002/bit.21605

DOI: 10.1002/bit.21608

& Direct Assessment of Cellulase Accessibilityin Pretreated Biomass

A key technical barrier to commercializing fuels and chemi-cals from biomass is the high cost and relative inefficiency ofproducing fermentable sugars from lignocellulosics. Althoughpretreatment is commonly used to improve biomass conver-sion efficiency, questions remain as to how specific chemicalor physical alterations of biomass conferred by pretreatmentimpact enzyme saccharification efficiency. Jeoh and co-workers report on a method to assess the cellulase accessibi-lity of biomass by directly measuring cellulase binding andactivity on pretreated substrates. The method uses afluorescence-labeled purified cellobiohydrolase (Cel7A) iso-lated from a commercial Trichoderma reesei preparation.Using this method it was found that substrate drying decre-ased enzyme binding and digestibility, whereas removal ofxylan and decreasing cellulose crystallinity increased bindingand digestibility. This study demonstrates that changes inbiomass characteristics that improve cellobiohydrolaseaccessibility lead to improved cellulose conversion efficiency.Page 112DOI: 10.1002/bit.21606

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e practical behavior of ion-exchange chromatography(Yamamoto et al., 1983. Biotechnol Bioeng 25:1465–1483and 1373–1391). However, this knowledge has not beenused in the development, optimization, and support oflarge-scale industrial separation processes. A recent paper(Shene et al., 2006. Biotechnol Bioeng 95:704–713) gives agood example on how theoretical models can be used in theoptimization of large-scale operations. The article presentedin this issue by Kaltenbrunner and coworkers, whichcorresponds to an excellent collaboration between academiaand industry, demonstrates how a sophisticated and com-plex model of ion-exchange chromatography can bepractically used by industry to gain additional under-standing of process behavior while reducing resourcesrelative to those currently used in established industrialpractice. The application of this recognized model—developed in an academic setting—to satisfy a crucial andimportant industrial need is an excellent example of thedynamic interplay between theoretical bioengineering andindustrial biotechnology. Page 201

DOI: 10.1002/bit.21607

& Steady as They Grow, but for How Long?

Have you ever felt a little unsteady when crowded by yournearest neighbors? This is a question that underlies the workof Meadows and co-workers, who developed a 2D cellularautomaton model to describe the growth of anchorage-dependent mammalian cells. Of particular interest is themodel’s utility in designing experiments for quantifying cellmetabolism with metabolic flux analysis, which typicallyinvolves isotopically labeled substrates and assumes meta-bolic and isotopic steady states. The model was used tostudy the impact of contact inhibition on the growthrate, specific extracellular flux rates, and 13C isotopic labelingof lactate in MCF7 breast cancer cells, and successfullydefined the time over which cell growth was exponential andisotopic labeling was at steady state. The cellular automatonmodel is therefore a new and useful tool for designingexperiments and selecting conditions to obtain metabolicallyhomogeneous cell populations suitable for metabolic fluxanalysis. Page 221

Biotechnology and Bioengineering, Vol. 98, No. 1, September 1, 2007