Figure 6. A comparison of different pathways for alcohol biosynthesis. (A-D) The range of average chain lengths afforded by alternative alcohol pathways: FAS supplemented with (A) TesA, CAR, and AHR, (B) acyl-ACP reductase (ATR), (C) acyl-ACP thioesterase specific for medium chains (TesA), a fatty-acid-CoA ligase (FadD), and an acyl-CoA reductase (ACR2), and (D) TesA, FadD, a differentacyl-CoA reductase (ACR1), and an aldehyde reductase (AHR). At each production level, the shaded region denotes the average chain lengths achievable by changing enzyme concentrations; the range of lengths narrows as production increases. Dataset S1 contains the source data and enzyme compositions for plots A-D.
length and production. We explored this effect by repeating the ATR analysis with a much higher enzyme concentration (~ninefold). This adjustment broadened the range of accessible chain lengths at intermediate production levels. We note, however, that our model does not account for the potential for ATR to bind holo-ACP, an interaction that can inhibit fatty acid synthesis and may limit the usefulness of ATR overexpression.
A sensitivity analysis of all pathways examined in this work suggests that, in general, product profiles are most sensitive to concentrations of acyl-ACP thioesterases and β-ketoacyl-ACP synthases (Fig. S11). This sensitivity, however, does not imply that all pathways are compatible with concentration-based control strategies; the narrow substrate specificities of ATR and ACR1, for example, are limiting. Rather, our focused analysis of the alcohol pathways suggest that coordinated changes in enzyme concentration provide a versatile means of tuning the product profiles of pathways with promiscuous enzymes downstream of the FAS.