It soon became evident that the mere introduction ofpentose utilization path­ways in S. cerevisiae was not enough to render the recombinant strains traits for efficient ethanol fermentation [43] (strain RWB202, Tables 3 and 4), [47, 48,78,79] (strain TMB3001, Tables 1-3). A number of metabolic engin­eering strategies to enhance ethanolic xylose (and arabinose) fermentation in S. cerevisiae have been explored, the most important of which will be discussed below. The initial xylose utilization pathway, the cellular redox metabolism, and the flux of central carbon metabolism have been the main targets of these engineering strategies. Figure 3 highlights the metabolic re­actions that have been engineered to improve ethanolic xylose fermentation by S. cerevisiae.

4.1