Construction of Engineered Escherichia coli and Optimization of Conditions for Carcinine Synthesis via Multi-Enzyme Cascade Catalysis

Abstract

Carcinine is an imidazole dipeptide with potent antioxidant and antiglycation properties, although its chemical synthesis currently relies on severely environmentally harmful processes. In this work, a multi-enzyme cascade biotransformation system comprising 4′-phosphopantetheinyl transferase and non-ribosomal peptide synthetase was constructed. To overcome the limitations arising from stochastic spatial distribution and suboptimal mass transfer associated with independent enzymes, a fusion protein strategy was adopted. The two enzymes were fused via a flexible genetic linker within plasmid pET28a-SFP-L-Ebony, which enabled robust soluble expression in Escherichia coli. Concurrently, the endogenous peptidase genes (pepA, pepB, pepD, and pepN) were systematically knocked out using CRISPR/Cas9-mediated gene editing. This quadruple protease-deficient strain (designated SFP-L-Ebony-ΔpepABDN) effectively suppressed product degradation. Subsequent optimization revealed that optimal catalytic performance occurred at 25 °C and pH 7.0. The highest biotransformation efficiency was achieved using 15 g/L crude enzymes, in the presence of 2 mM ATP and 10 mM MgCl2. Through a fed-batch substrate feeding strategy in a 50 mL reaction system, the final carcinine titer reached 7.0 g/L after 48 h. This study, therefore, provides an efficient and sustainable technological pathway for the green biomanufacturing of carcinine as well as other high-value dipeptides.