Metabolic Engineering of Pichia pastoris for S-Adenosylmethionine Overproduction via Transcriptomic-Guided Energy Reallocation.

S-Adenosylmethionine (SAM) is a high-value biomolecule with critical applications in nutraceuticals, pharmaceuticals, and health supplements. However, the detailed metabolic mechanism by which sodium citrate promotes SAM production in Pichia pastoris has not yet been elucidated. Here, a comprehensive time-series transcriptomic analysis revealed that sodium citrate profoundly influences gene expression across multiple pathways. Sodium citrate supplementation redirects carbon flux toward oxidative energy metabolism by upregulating amino acid biosynthesis, translation, and glycolysis. Crucially, the transcriptional upregulation of argininosuccinate synthase (ARG1) and argininosuccinate lyase (ARG4) was identified as a key node driving fumarate-mediated TCA cycle anaplerosis and ATP supply. This foundational analysis led to the identification of ARG4 as a key metabolic engineering target. Overexpression of ARG4 significantly improved SAM production, achieving a 56.46% increase in shake flasks and 9.06 g/L SAM (a 22.10% improvement) in 500 mL fermenters compared to control strains. Integrated physiological and metabolic flux analysis (MFA) demonstrated that ARG4 overexpression redirects metabolic flow, channeling energy substrates toward SAM synthesis, effectively maintaining cellular respiratory metabolism and alleviating energy limitations in the late fermentation phase. This work establishes a novel strategy for enhancing product synthesis by restructuring energy allocation rather than merely increasing overall energy supply.