After the introduction of molecular oxygen in the Earth's atmosphere during the Great Oxidation Event (2.4–2.1 bln years ago), all the living organisms needed to adapt to this potent oxidant, or perish. It is hypothesised that only few anaerobes unable at handling oxygen survived, mostly relegated to specific anoxic niches indeed striving in extreme environments (Canfield et al., 2013; Inupakutika et al., 2016; Luo et al., 2016; Olejarz et al., 2021). If the presence of oxygen worked as poison for many anaerobes, it had pushed others in evolving sophisticated metabolic pathways exploiting its oxidative power, eventually as ultimate electron sink for organic carbon respiration. Such dramatic metabolic shift at planetary scale did not happen overnight, millions of years of evolution took place, and probably progressed unevenly. This is most likely the basis for the great variety of oxidoreductases present today: different catalytic strategies and affinity for oxygen and reactive oxygen species (ROS), or the resistance at its oxidative damages might reflect the different level of oxygen and environmental conditions in which these enzymes evolved.

Across all domains of life, oxidoreductases represent nearly 44% of the entries amid enzymatic classes annotated by the Enzyme Commission (EC 1.x.x.x) (Schroder & Meilleur, 2021; Webb, 1992). By harnessing the electron's transfer among acceptor and donor molecules, these enzymes play crucial roles equally in both the aerobic and anaerobic metabolisms of several organisms and are capable of catalysing reactions ranging from proton/hydride extraction and transfer, to oxyfunctionalizations. Moreover, a wide substrate scope has been described involving both organic and inorganic compounds (Younus, 2019).

.Traditionally, the activation of inert CH, CC, CC bonds and selective oxyfunctionalizations of organic substrates represent some of the most valuable and challenging reactions in biochemistry and in total chemical synthesis (Davies & Morton, 2016; Torres Pazmino et al., 2010). Although novel, elegant routes have been developed recently, such reactions are traditionally carried exploiting inorganic catalysts and harsh condition of acidity, temperature and pressure. Therefore, the application of biological catalysts capable of performing chemo-, regio- an stereoselective oxyfunctionalizations in aqueous, milder conditions is greatly attractive (Wu et al., 2023).

.Oxygen is so significant for oxidoreductases due to some unique features: its high reactivity due to the electronegativity of 3.14 -s only to fluorine-, its relative abundance in the air (21%) and relative solubility in water, as well as the ability to diffuse across biological membranes and cellular compartments. These unparalleled characteristics make molecular oxygen (O2) and its reactive species (ROS; e.g. hydroxyl radical HO•, superoxide radical O2•−, or the hydroperoxide anion HOO−) as the ideal electron sink and/or co-substrate for many oxidoreductases. In recent years, the interest towards the application of oxidoreductases for biotechnological purposes has been growing exponentially. Especially enzymes capable of exploiting O2 while harbouring either iron and copper ions into their catalytic sites have been widely investigated. Some examples are laccases (EC 1.10.3.2), peroxidases (EC 1.11.1.13, −14,-16), various monooxygenases (EC 1.13.x.x and EC 1.14.x.x) – unspecific peroxygenases (EC 1.11.2.1) (Janusz et al., 2013; Kinner et al., 2021; Rodriguez Couto & Toca Herrera, 2006; Sellami et al., 2022; Urlacher & Girhard, 2019).

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For decades, the heme-based enzymes P450 monooxygenases (P450 or CYP) have been on the spotlight in regards to their application for synthetic purposes (Grandi et al., 2023; Renata, 2023; Urlacher & Girhard, 2019). Only to be challenged nowadays by the rise of unspecific peroxygenases (UPO) (Beltran-Nogal et al., 2022; Monterrey et al., 2023; Xu et al., 2023). Similarly, the copper-based lytic polysaccharide monooxygenases (LPMO) recently took the stage after being recognized as key elements in the oxidative cleavage of recalcitrant polysaccharides (Bissaro & Eijsink, 2023; Karnaouri et al., 2022; Munzone et al., 2024; Vandhana et al., 2022). In contrast, methane monooxygenases (MMO) attracted the scientific community for a long time by offering the chance of using CH4 as feedstock for the production of desirable biofuels and methanol-based chemicals. However, the heavily complex nature of both the cytoplasmic, diiron-based sMMO and the transmembrane, copper-based pMMO hampered their understanding and thus their application (Banerjee et al., 2019; Chan et al., 2022; Koo & Rosenzweig, 2021; Lawton & Rosenzweig, 2016). For different reasons, however, the implementation of these enzymes has been quite limited.

For each one of the enzymes introduced above, a vast volume of literature is emerging continuously. More and more detailed and comprehensive reviews – such as the references quoted in the above paragraph- describe and compare precise aspects of these enzymes. To date, though, broader overviews are rarer and are often focusing on specific aspects of their catalysis. These enzymes all possess the capability of overcoming the barrier of above ~100 kcal mol−1 of CH bonds by taming oxygen reactive species during their catalytic cycles. Overall, these enzymes evolved different strategies to generate similar reactive compounds, with equivalent intermediate steps and analogous drawbacks. The aim of this work is therefore to explicitly broaden the comparison between these Fe- and Cu-dependent enzymes which represent hot-topics in very diverse fields of expertise. Within this contribution the structural and activity diversity of these enzymes will be considered first (2.1 to 2.4), followed by an overview where the mechanistic aspects of their catalysis cycles will be compared (3.1 to 3.5). Finally, the role of enzymatic and/or molecular redox partners in the tuning of the oxyfunctionalization activity, will be underscored in the last section of this account (4.1 to 4.4), aiming to further develop ideas to exploit such biocatalysts in the nascent environmental and sustainable biorefinery, as well as the production of fine chemicals within industrial manufacturing.