Biosynthesis of L-Pipecolic Acid and Its Hydroxylated Derivatives: Enzyme, Engineering, and Synthesis Method

l-Pipecolic acid (l-PA) and its hydroxylated derivatives (hydroxypipecolic acids, HPAs) are non-proteinogenic amino acids that serve as valuable chiral building blocks for pharmaceuticals, antibiotics, and natural products. Conventional chemical synthesis of these compounds often suffers from operational complexity, poor environmental compatibility, and insufficient stereochemical control, driving a shift toward biosynthetic approaches. This review covers recent advances in enzyme engineering and synthetic biology aimed at enabling sustainable and efficient production of l-PA and HPAs. For l-PA biosynthesis, various metabolic engineering strategies to enhance its production in microbes are introduced, and enzyme cascades, single enzyme strategy, and immobilized enzyme strategy involved in l-PA production are discussed. Regarding HPAs biosynthesis, which involves the regioselective hydroxylation of l-PA, their structural features, catalytic mechanisms, and recent progress in the biosynthesis of diverse HPAs, the protein engineering of proline hydroxylase is emphasized. Finally, we present future perspectives to accelerate the biosynthetic production of l-PA and HPAs.