AHCY: A metabolic gatekeeper at the interface of methylation, redox balance, and cellular stress response

S-Adenosylhomocysteinase (AHCY, also known as SAHH) is a highly conserved enzyme that catalyzes the reversible hydrolysis of SAH into adenosine and homocysteine. As the sole enzyme capable of catalyzing this reaction, AHCY modulates cellular methylation potential required for DNA, RNA, and protein methyltransferase activity. Recent discoveries, however, expand its role well beyond this canonical function, positioning AHCY as a metabolic gatekeeper that integrates one-carbon metabolism with epigenetic regulation, RNA processing, nucleotide balance, and redox signaling. This review brings together mechanistic, structural, and regulatory insights into AHCY while critically evaluating diverse biochemical and biophysical methods for assaying its activity. Comparative structural analyses uncover conserved tetrameric organization alongside species-specific adaptations in oligomeric state, NAD⁺ pocket accessibility, and C-terminal dynamics that shape enzyme catalytic efficiency and regulation. AHCY function is further fine-tuned through a wide spectrum of posttranslational modifications and small-molecule interactions, linking it to transcriptional control, stress adaptation, and viral infection. By linking SAH turnover to methylation capacity and adenosine/homocysteine flux, AHCY coordinates metabolism with chromatin regulation and stress responses. These cross-cutting roles highlight how a single metabolic enzyme bridges catalysis, regulation, and disease. In doing so, AHCY exemplifies the broader principle that metabolic enzymes can have a central role as regulators of metabolic flux and cellular regulation, offering both mechanistic depth and translational promise as a therapeutic target.