Maiti, Biplab K.Moura, IsabelMoura, José J.G.2025-09-042025-09-042025-071420-3049PURE: 128681909PURE UUID: 520216b0-e8f3-431c-9492-fab61781ce2aScopus: 105011619862PubMed: 40733288http://hdl.handle.net/10362/187551This research was funded by DST-SERB for the CRG grant (file no CRG/2022/005673) Copyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland.The nitrogen cycle (N-cycle) is a cornerstone of global biogeochemistry, regulating nitrogen availability and affecting atmospheric chemistry, agricultural productivity, and ecological balance. Central to this cycle is the reversible interplay between nitrate (NO3−) and nitrite (NO2−), mediated by molybdenum-dependent enzymes—Nitrate reductases (NARs) and Nitrite oxidoreductases (NXRs). Despite catalyzing opposite reactions, these enzymes exhibit remarkable structural and mechanistic similarities. This review aims to elucidate the molecular underpinnings of nitrate reduction and nitrite oxidation by dissecting their enzymatic architectures, redox mechanisms, and evolutionary relationships. By focusing on recent structural, spectroscopic, and thermodynamic data, we explore how these two enzyme families represent “two sides of the same coin” in microbial nitrogen metabolism. Special emphasis is placed on the role of oxygen atom transfer (OAT) as a unifying mechanistic principle, the influence of environmental redox conditions, and the emerging evidence of bidirectional catalytic potential. Understanding this dynamic enzymatic interconversion provides insight into the flexibility and resilience of nitrogen-transforming pathways, with implications for environmental management, biotechnology, and synthetic biology.3073919engmo-dependent enzymesnitrate reductasenitrite oxidoreductasenitrogen-biocycleAnalytical ChemistryChemistry (miscellaneous)Molecular MedicinePharmaceutical ScienceDrug DiscoveryPhysical and Theoretical ChemistryOrganic ChemistrySDG 2 - Zero Hungerreview10.3390/molecules30143023https://www.scopus.com/pages/publications/105011619862