The increased emissions of greenhouse gases (GHGs) play a crucial role in global warming, which have become a primary topic of research in last decade. The conversion GHGs into valuable products may offer a promising solution for mitigating climate change and addressing current shortages in feedstock. Recently, both CO2 and CH4 have been demonstrated as a substrate for the biosynthesis of carbohydrates and proteins, which are significant sources of human diets or functional feed additives in the biomanfacturing (Cai et al., 2021; Gao et al., 2024). However, achieving efficient conversion of CO2 and CH4 in biocatalysis presents challenges due to the high energy barrier required for their activation. CO2 as a linear and centrosymmetric molecule contains fully oxidized carbon atoms, while CH4 is a symmetrical tetrahedral compound with highly reduced carbon. The stable structure of these molecules results in high dissociation energy for the CO bond in CO2 and the CH bond in CH4, posing significant obstacles for their transformation (Aurnob et al., 2023).

Despite these challenges, efforts have been devoted in exploring methods for efficient conversion of CO2 and CH4 (Fig. 1). Traditional thermocatalysis, a well-established technology for CO2 and CH4 conversion, has been industrially implemented for a considerable period. However, this process often involves extensive energy consumption, harsh reaction conditions, catalyst deactivation, and presence of side reactions (Gao et al., 2023; Jiang et al., 2023). Alternatively, photocatalytic and electrocatalytic pathways offer promising avenues for upcycling CO2 and CH4 in terms of mild operating conditions, utilization of renewable energy sources, and precise reaction control as shown in Fig. 1 (Gao et al., 2022; Li et al., 2022). Nevertheless, both approaches face significant obstacles concerning conversion efficiency, catalyst lifespan, and product selectivity.

Various microorganisms possess natural metabolic pathways enabling the utilization of CO2 and CH4 for synthesizing valuable metabolites. Microalgae are able to convert CO2 via photosynthesis into glucose and oxygen, while methanotrophs can utilize CH4 as a sole carbon and energy source for cell growth (Guo et al., 2022; Huang et al., 2022). The bioconversion of CO2 and CH4 into desired products involves complex metabolic pathways and a cascade of multiple enzymes and reactions. Certain specialized enzymes are essential in facilitating the assimilation of CO2 and CH4 by microorganisms for carbon metabolism. These enzymes are key nodes determining the conversion efficiency, central metabolism rate, and yield of target products. The intrinsic characteristics present in living organisms make the biocatalysis of CO2 and CH4 easily adaptable into versatile cellular factories for the production of valuable products. And enzymatic reaction has inherent advantages such as mild conditions, low energy consumption, environmental benignity, and high selectivity (Fig. 1). Consequently, the study of these enzymes has garnered significant attention in recent years, leading to numerous significant discoveries.

This review summarizes the enzymes and associated reactions involved in the biocatalysis of CO2 and CH4. Existing challenges are discussed such as unsatisfactory enzyme catalytic efficiency, uncertainties of enzyme structures, unclear reaction mechanisms, reliance on living cells, costly cofactor requirements, and poor enzyme stability. Additionally, potential research pathways for enhancing enzyme catalytic performance and elucidating enzyme structures and reaction mechanisms are highlighted with a focus on developing robust systems for CO2 and CH4 fixation beyond traditional cellular boundaries.