Rational engineering of homospermidine synthase for enhanced catalytic efficiency toward spermidine synthesis

Spermidine is a naturally occurring polyamine widely utilized in the prevention and treatment of various diseases. Current spermidine biosynthetic methods have problems such as low efficiency and complex multi-enzyme catalysis. Based on sequence-structure-function relationships, we engineered the widely studied homospermidine synthase from Blastochloris viridis (BvHSS) and obtained mutants that could catalyze the production of spermidine from 1,3-diaminopropane and putrescine. The specific activities of BvHSS and the mutants D361E and E232D + D361E (E232D-D) were 8.72, 46.04 and 48.30 U/mg, respectively. The optimal pH for both mutants was 9.0, and the optimal temperature was 50 °C. Molecular docking and dynamics simulations revealed that mutating aspartic acid at position 361 to glutamic acid narrowed the substrate binding pocket, promoting stable spermidine production. Conversely, mutating glutamic acid at position 232 to aspartic acid enlarged the substrate channel entrance, facilitating substrate entry into the active pocket and enhancing spermidine generation. In whole-cell catalysis lasting 6 h, D361E and E232D-D synthesized 725.3 and 933.5 mg/L of spermidine, respectively. This study offers a practical approach for single-enzyme catalyzed spermidine synthesis and sheds light on the crucial residues influencing homospermidine synthase catalytic activity in spermidine production.