Design, construction, and application of noncanonical redox cofactor infrastructures

Design, Synthesis and Activity Evaluation of Methylene-H4MPT Mimics

Tetrahydromethanopterin (H4MPT) is a specialized coenzyme found in methanogenic archaea and methylotrophic bacteria, essential for one-carbon (C1) transfer and redox reactions in processes like methanogenesis. However, its structural complexity and limited availability hinder its use in studying archaeal metabolism and H4MPT-dependent enzymes. Inspired by the success of nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) biomimetic compounds, we employed a similar structural simplification strategy for methylene-H4MPT, designing four mimics (mimics 1-4) that retain the crucial hydride-donating imidazolidine unit while modifying the pterin structure and hydrophilic tail. Structural and crystallographic analysis of methylene-H4MPT-bound enzymes, including H4MPT-dependent formaldehyde-activating enzyme MtdA and H4MPT-dependent formaldehyde-activating enzyme Fae, highlighted the importance of the pterin group in hydrogen bonding and active site interactions. Activity evaluation of the mimics in hydride transfer assays and relay reactions with MtdA and NADPH-dependent ketoreductase CgKR1 showed that the pterin structure is critical for enzymatic activity, and improving water solubility via hydrophilic tail modification enhances performance. This biomimetic approach offers functional substitutes for methylene-H4MPT and potentially valuable tools for studying archaeal metabolism and synthetic microbial systems for C1 compound utilization.

Seven critical challenges in synthetic one-carbon assimilation and their potential solutions

Synthetic C1 assimilation holds the promise of facilitating carbon capture while mitigating greenhouse gas emissions, yet practical implementation in microbial hosts remains relatively limited. Despite substantial progress in pathway design and prototyping, most efforts stay at the proof-of-concept stage, with frequent failures observed even under in vitro conditions. This review identifies seven major barriers constraining the deployment of synthetic C1 metabolism in microorganisms and proposes targeted strategies for overcoming these issues. A primary limitation is the low catalytic activity of carbon-fixing enzymes, particularly carboxylases, which restricts the overall pathway performance. In parallel, challenges in expressing multiple heterologous genes-especially those encoding metal-dependent or oxygen-sensitive enzymes-further hinder pathway functionality. At the systems level, synthetic C1 pathways often exhibit poor flux distribution, limited integration with the host metabolism, accumulation of toxic intermediates, and disruptions in redox and energy balance. These factors collectively reduce biomass formation and compromise product yields in biotechnological setups. Overcoming these interconnected challenges is essential for moving synthetic C1 assimilation beyond conceptual stages and enabling its application in scalable, efficient bioprocesses towards a circular bioeconomy.

Harnessing noncanonical redox cofactors to advance synthetic assimilation of one-carbon feedstocks

One-carbon (C1) feedstocks, such as carbon monoxide (CO), formate (HCO2H), methanol (CH3OH), and methane (CH4), can be obtained either through stepwise electrochemical reduction of CO2 with renewable electricity or via processing of organic side streams. These C1 substrates are increasingly investigated in biotechnology as they can contribute to a circular carbon economy. In recent years, noncanonical redox cofactors (NCRCs) emerged as a tool to generate synthetic electron circuits in cell factories to maximize electron transfer within a pathway of interest. Here, we argue that expanding the use of NCRCs in the context of C1-driven bioprocesses will boost product yields and facilitate challenging redox transactions that are typically out of the scope of natural cofactors due to inherent thermodynamic constraints.

Recent developments in enzymatic and microbial biosynthesis of flavor and fragrance molecules

Flavors and fragrances are an important class of specialty chemicals for which interest in biomanufacturing has risen during recent years. These naturally occurring compounds are often amenable to biosynthesis using purified enzyme catalysts or metabolically engineered microbial cells in fermentation processes. In this review, we provide a brief overview of the categories of molecules that have received the greatest interest, both academically and industrially, by examining scholarly publications as well as patent literature. Overall, we seek to highlight innovations in the key reaction steps and microbial hosts used in flavor and fragrance manufacturing.