Ancient Metabolism and the Wood Ljunghal Pathway
Carbon dioxide (CO2) is one of the primary greenhouse gases on earth and its continuous emission is leading to a rise in atmospheric temperature, causing a global climate crisis with catastrophic consequences for mankind and the biodiversity of the planet. The utilisation of autotrophic organisms that can fix gaseous CO2 to generate fuel ethanol from abundant waste gas resources is one way to reduce the carbon footprint. Acetogens are a specialised group of strictly anaerobic bacteria that thrive on the formation of acetic acid from CO2 with electrons coming from molecular hydrogen (H2) or carbon monoxide (CO). Basically, the gas mixture of industrial exhaust gases making them a promising production platform for the production of biofuels or industrial bulk chemicals.
For CO2 reduction acetogens use the acetyl-coenzyme A or Wood-Ljungdahl pathway (WLP), the only pathway known to couple CO2 fixation with the generation of ATP. Thus, arguably the WLP is considered as one of the first or even the first biochemical pathway on earth. While there are some mechanistic insights in single enzymes of the WLP from methanogenic archaea, nearly nothing is known about the completely different enzymatic machinery of the acetogenic WLP.
The direct reduction of CO2 by the hydrogen-dependent CO2 reductase (HDCR) is the first step of the WLP. It directly converts the volatile gases hydrogen and carbon dioxide to formate. A reaction that has aroused considerable interest as a liquid electron carrier for hydrogen, due to its increased volumetric energy density and lower hazard compared to molecular hydrogen. Whereas most catalysts generally struggle to reduce the thermodynamically stable CO2 molecule, HDCR class enzymes performs this reaction with a higher activity than any other bio- and chemical catalysts known today. However, the structural and mechanistic basis of this high catalytic turnover rate remains unknown.
The HDCR complex contains four different subunits, two of them are small iron-sulfur proteins and two of them have catalytic activities. One catalytic active subunit has hydrogenase activity (HydA2) that oxidizes H2 to 2 H+ + 2 e- and the other has formate dehydrogenase / CO2 reductase activity (FdhF), that reduces CO2 + 2 e- + 2 H+ to HCOOH. Under physiological conditions the enzyme works in both directions: during lithotrophic growth it catalyses CO2 reduction and during growth on reduced C1 substrates such as formate or methanol it works as formate dehydrogenase. Hydrogen oxidation and CO2 reduction proceed spatially separated from each other in different proteins which begs the questions how the electrons manage to bridge the distance between the active sites.
In many acetogens the HDCR forms long filaments but the physiological or biochemical advantage of filamentation is poorly understood A structure and detailed mechanistic study of this enzyme is urgently needed to understand its unsurpassed catalytic turnover rate and unlock its potential as powerful biocatalysts for CO2 reduction and hydrogen storage. We are cooperating with the Müller Lab (Goethe University Frankfurt) on structural aspects of this biotechnological important CO2 fixation enzyme.