Engineering Escherichia coli for Optimized Fermentation Conditions in Whole-Cell Catalytic Synthesis of D-Allulose

Introduction

D-Allulose, also known as D-psicose, is a rare sugar with approximately 70% of the sweetness of sucrose but only 0.3% of its caloric value. It has gained significant attention in the food and pharmaceutical industries due to its health benefits, including anti-obesity, anti-diabetic, and antioxidant properties. However, the natural abundance of D-allulose is extremely low, making its large-scale production challenging. Biotechnological approaches, particularly microbial whole-cell catalysis, have emerged as a promising solution for the cost-effective and sustainable synthesis of D-allulose. Among microbial hosts, Escherichia coli has been widely used due to its well-characterized genetics, rapid growth, and ease of genetic manipulation. This article explores the engineering of E. coli for optimized fermentation conditions to enhance the whole-cell catalytic synthesis of D-allulose.

The Role of Whole-Cell Catalysis in D-Allulose Production

Whole-cell catalysis involves using living microbial cells as biocatalysts to convert substrates into desired products. In the case of D-allulose production, the key reaction is the isomerization of D-fructose to D-allulose, catalyzed by D-allulose 3-epimerase (DAE). Whole-cell systems offer several advantages over purified enzyme systems, including reduced production costs, enzyme stabilization within the cellular environment, and the ability to regenerate cofactors if required.

However, the efficiency of whole-cell catalysis depends on several factors, including the expression level of the target enzyme, substrate uptake, intracellular metabolic balance, and fermentation conditions. Engineering E. coli to optimize these factors is critical for achieving high yields of D-allulose.

Strategies for Engineering E. coli for D-Allulose Production

1. Heterologous Expression of D-Allulose 3-Epimerase

The first step in engineering E. coli for D-allulose production is the introduction of a gene encoding DAE. This enzyme is typically derived from thermophilic or mesophilic microorganisms such as Clostridium cellulolyticum, Agrobacterium tumefaciens, or Flavonifractor plautii. To achieve high expression levels, strong promoters (e.g., T7 or lac promoters) and optimized ribosome binding sites are used. Codon optimization is also performed to ensure efficient translation in E. coli.

2. Improving Substrate Uptake and Transport

Efficient uptake of D-fructose, the substrate for D-allulose synthesis, is essential for high catalytic activity. Native E. coli transporters, such as the phosphotransferase system (PTS), can be engineered to enhance D-fructose uptake

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