Enzyme-constrained Metabolic Model of Identified Glycerol-3-phosphate Dehydrogenase as an Alternate Electron Sink.

, the causative agent of syphilis, poses a significant global health threat. Its strict intracellular lifestyle and challenges in cultivation have impeded detailed metabolic characterization. In this study, we present iTP251, the first genome-scale metabolic model of , reconstructed and extensively curated to capture its unique metabolic features. These refinements included the curation of key reactions such as pyrophosphate-dependent phosphorylation and pathways for nucleotide synthesis, amino acid synthesis, and cofactor metabolism. The model demonstrated high predictive accuracy, validated by a MEMOTE score of 92%. To further enhance its predictive capabilities, we developed ec-iTP251, an enzyme-constrained version of iTP251, incorporating enzyme turnover rate and molecular weight information for all reactions having gene-protein-reaction associations. Ec-iTP251 provides detailed insights into protein allocation across carbon sources, showing strong agreement with proteomics data (Pearson's correlation of 0.88) in the central carbon pathway. Moreover, the thermodynamic analysis revealed that lactate uptake serves as an additional ATP-generating strategy to utilize unused proteomes, albeit at the cost of reducing the driving force of the central carbon pathway by 27%. Subsequent analysis identified glycerol-3-phosphate dehydrogenase as an alternative electron sink, compensating for the absence of a conventional electron transport chain while maintaining cellular redox balance. These findings highlight 's metabolic adaptations for survival and redox balance in intracellular environments, providing a foundation for future research into its unique bioenergetics.