3.1 Designing the oxidoreductive pathways forD-tagatose production
We constructed the oxidoreductive pathways for producing tagatose from
lactose. The synthesis pathway was composed of three modules, as shown
in Figure 1: (1) lactose hydrolysis module (withβ -galactosidase); (2) galactose reduction module (with aldose
reductase, EC: 1.1.1.307); and (3) galactitol oxidation module (with
galactitol dehydrogenase, EC: 1.1.1.14). A three-enzyme cascade
converting lactose into tagatose (Pathway I) was designed usingβ -galactosidase, NADPH-preferring Ps XR from P.
stipites (GenBank ID: XP_001385181), NAD+-dependentPd PDH from P.
denitrificans (GenBank ID: WP_011751060), or the
NAD+-dependent Rl GDH from R.
leguminosarum (GenBank ID: WP_011650422.1) (Figure 1A). The coenzyme
dependence of Ps XR and Rl GDH or Pd PDH is different.
Furthermore, we designed a novel Pathway II for constructing a cofactor
cycle between the reduction and oxidation modules (Figure 1B).