What is it about?
Enzymes are protein-based biocatalysts that living organisms use to carry out highly selective metabolic transformations. Most enzymes are substrate-specific—often described by the “lock and key” model; ensuring that each enzyme acts on a particular molecule to maintain metabolic precision. However, some enzymes exhibit promiscuity, meaning they can act on a broader range of substrates, often with partial stereoselectivity. This study explores the substrate flexibility of alditol-2-dehydrogenases, a class of promiscuous enzymes capable of transforming various sugar alcohols (alditols) into enantiopure ketoses. Using absolute and relative configuration of alditols along with GC/MS and NMR spectroscopy, an organo-analytical chemistry approach combined with enzymology, we evaluated the degree of enzyme promiscuity and identified stereochemical factors that influence these transformations. Our findings offer new insights into rare ketose synthesis and expand the potential use of promiscuous enzymes in biocatalysis and biotechnology.
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Why is it important?
This research demonstrates significant interdisciplinary potential, integrating concepts from biocatalysis, synthetic biology, and industrial biotechnology. By emphasizing enzyme promiscuity, a key feature of bio-systemic evolution, it deepens our understanding of remarkable adaptability of molecular systems in terms of shape, size and 3D disposition. Moving forward, both conceptual and experimental investigations could translate these insights into engineered “biocatalytic machines,” with broad applications across biotechnological platforms, advancing the frontiers of applied biocatalysis. More importantly, this work may pave the way to the research of evolutionary biochemistry using stereochemistry or shape of molecules.
Perspectives
Promiscuous alditol-2-dehydrogenases can oxidize multiple sugar alcohols (alditols) to enantiopure rare ketoses without full stereospecificity, guided primarily by C2–C3 absolute/relative configurations rather than the entire molecule. This strategic exploitation of partial stereoselectivity could transform rare sugar synthesis, offering streamlined routes in biotech that align with the Izumoring hypothesis for generating diverse hexoses. Through GC/MS and NMR, we confirmed that three enzymes, galactitol-2-dehydrogenase (G2DH), D-altritol-5-dehydrogenase (D A5DH), and D-sorbitol-2-dehydrogenase (D S2DH), successfully produced all enantiopure ketoses from a set of 10 hexitols. Notably, G2DH exhibited the broadest substrate scope, oxidizing both symmetric (meso) and asymmetric alditols, including galactitol (~55% conversion) and L talitol (~2% conversion). The study uncovers a new dimension of "type II promiscuity" beyond C2–C3 matching possibly driven by molecular symmetry, bond rotation, and induced fit mechanisms. Harnessing these promiscuous pathways can simplify workflows, enabling one enzyme to produce multiple valuable ketoses, thereby reducing costs and process complexity. The findings illuminate evolutionary trajectories, how enzymes evolve to accommodate diverse substrates and maintain metabolic plasticity. Insights from this stereochemical basis could aid enzyme engineering efforts, designing next generation "biocatalytic machines" that convert varied substrates with predictable outcomes. To build on the current findings, a three-pronged strategy is envisioned: 1) Integrate kinetic and structural data (e.g., X-ray crystallography) to deepen mechanistic understanding and guide rational enzyme design. 2) Expand substrate conformation (chair, boat) based investigation of promiscuity of kinases, glycosyltransferases and glycosidases. 3) Develop enzyme variants with enhanced yields and selectivity, tailored for industrial-scale synthesis of rare ketoses. Overall, this work advances a powerful concept that by mapping stereochemical keys, not whole molecular structures, enzymatic promiscuity can be rationally leveraged. It's a blueprint for more efficient, flexible, and predictive biocatalytic processes targeting high-value rare sugars, bridging fundamental biochemistry with industrial innovation.
Prithwiraj De
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This page is a summary of: Harnessing enzyme promiscuity of alditol-2-dehydrogenases for oxidation of alditols to enantiopure ketoses, PLOS One, June 2025, PLOS,
DOI: 10.1371/journal.pone.0325955.
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