An Engineered Multi-Enzyme Cascade with Low-Cost ATP Regeneration for Efficient D-Allulose Production from D-Fructose

D-Allulose is a promising low-calorie rare sugar with significant health benefits. However, its industrial production is hindered by the low catalytic efficiency (≤33% conversion) and unfavorable equilibrium of the key enzyme, D-allulose 3-epimerase (DAE). To overcome this thermodynamic bottleneck, an in vitro synthetic enzymatic cascade based on a phosphorylation–dephosphorylation strategy was constructed. This engineered system comprises four synergistically operating enzymes: D-allulose-3-epimerase (DAE), L-rhamnulose kinase (RhaB), polyphosphate kinase (PPK), and acid phosphatase (AP). Through rational design and systematic optimization, the cascade achieved an exceptional 84.5% conversion yield from 50 mM D-fructose. Importantly, the system also maintained high conversion rates of 64.4% and 61.1% at high D-fructose loadings (50–100 g L−1). This performance, together with the integration of a low-cost PolyP6–PPK ATP regeneration module, underscores the potential industrial applicability of the proposed cascade strategy.