Black William B, Saleh Samer, Perea Sean, Luu Emma, Cui Youtian, Sun Jiasi, Wang Zhaoxi, Lambrecht Shyanne, Awachi Salman, Hayworth Declan, Wang Andrea, Chuayiuso Chloe, Hagerty Raine, Gilcrease Patrick C, Jiao Feng, He Zhen, Siegel Justin B, Li Han
Department of Chemical and Biomolecular Engineering, University of California, Irvine, Irvine, USA.
Genome Center, University of California, Davis, Davis, CA, USA.
bioRxiv. 2025 Jul 28:2025.07.28.667276. doi: 10.1101/2025.07.28.667276.
Advances in organic and gas waste valorization have enabled high-yield production of carboxylic acids, positioning them as promising feedstocks for the bioeconomy. However, carboxylic acids must be activated before downstream use, typically requiring ATP, CoA, or reduced ferredoxin to overcome unfavorable thermodynamics. These activators are costly to generate and divert carboxylic acids into CO-releasing pathways, reducing carbon efficiency. Here, we demonstrate that aldehyde dehydrogenases (ALDHs) can directly reduce carboxylic acids to aldehydes without prior activation, a process previously thought to be biologically inaccessible. Screening 133 ALDHs revealed that this activity is remarkably widespread within the protein family, enabling production of aliphatic aldehydes and alcohols, diols, and aromatic alcohols, at titers >1 g/L, in some cases, after optimization of thermodynamic driving forces. Additionally, we applied this system to upgrade waste-derived carboxylic acid effluent streams from wastewater sludge, food waste, and waste gas (CO). This activation-free process, termed "reverse aldehyde oxidation" (rAOX), establishes a broadly applicable, energy-efficient platform for carboxylic acid valorization at 100% carbon yield. Analogous to the reverse tricarboxylic acid cycle (rTCA) and reverse β-oxidation (rBOX), rAOX exemplifies that metabolic reactions classically defined as unidirectional may have unexpected plasticity to operate in reverse and open new avenues in biomanufacturing.
有机和气体废物增值技术的进步使得羧酸能够高产，使其成为生物经济中很有前景的原料。然而，羧酸在下游使用之前必须被激活，通常需要ATP、辅酶A或还原型铁氧化还原蛋白来克服不利的热力学因素。这些激活剂的生成成本高昂，并且会使羧酸进入释放CO的途径，从而降低碳效率。在此，我们证明醛脱氢酶（ALDHs）可以直接将羧酸还原为醛，而无需事先激活，这一过程以前被认为在生物学上是无法实现的。对133种ALDHs进行筛选后发现，这种活性在蛋白质家族中非常普遍，在优化热力学驱动力后，能够以>1 g/L的滴度生产脂肪醛、醇、二醇和芳香醇。此外，我们应用该系统对来自废水污泥、食物垃圾和废气（CO）的废物衍生羧酸流出物进行升级。这种无需激活的过程，称为“反向醛氧化”（rAOX），建立了一个广泛适用、节能的平台，用于以100%的碳产率实现羧酸增值。与反向三羧酸循环（rTCA）和反向β-氧化（rBOX）类似，rAOX表明，传统上定义为单向的代谢反应可能具有意想不到的可塑性，可以反向运行并为生物制造开辟新途径。
