Activating the <scp>d</scp>-Tagatose Production Capacity of <i>Escherichia coli</i> with Structural Insights into C4 Epimerase Specificity
Dileep Sai Kumar Palur, J. Taylor, Bryant Luu, Ian C. Anderson, Augustine Arredondo, Trevor Gannalo, Bryan A. Skorka, Pamela R. Denish, John Didzbalis, Justin B. Siegel, Shota Atsumi
Abstract
High Resolution Image Download MS PowerPoint Slide d -Tagatose, a rare low-calorie sweetener, is ideal for beverages due to its high solubility and low viscosity. Current enzymatic production methods from d -galactose or d -galactitol are limited by reaction reversibility, affecting the yield and purity. This study demonstrates that Escherichia coli harbors a thermodynamically favorable pathway for producing d -tagatose from d -glucose via phosphorylation–epimerization–dephosphorylation steps. GatZ and KbaZ, annotated as aldolase chaperones, exhibit C4 epimerization activity, converting d -fructose-6-phosphate to d -tagatose-6-phosphate. Structural analysis reveals active site differences between these enzymes and class II aldolases, indicating functional divergence. By exploiting the strains’ inability to metabolize d -tagatose, carbon starvation was applied to remove sugar byproducts. The engineered strains converted 45 g L –1 d -glucose to d -tagatose, achieving a titer of 7.3 g L –1 and a productivity of 0.1 g L –1 h –1 under test tube conditions. This approach highlights E. coli as a promising host for efficient d -tagatose production.