CO Metabolism in the Acetogen Acetobacterium woodii

Abstract

The Wood-Ljungdahl pathway allows acetogenic bacteria to grow on a number of one-carbon substrates, such as carbon dioxide, formate, methyl groups, or even carbon monoxide. Since carbon monoxide alone or in combination with hydrogen and carbon dioxide (synthesis gas) is an increasingly important feedstock for third-generation biotechnology, we studied CO metabolism in the model acetogen Acetobacterium woodii. When cells grew on H 2 -CO 2 , addition of 5 to 15% CO led to higher final optical densities, indicating the utilization of CO as a cosubstrate. However, the growth rate was decreased by the presence of small amounts of CO, which correlated with an inhibition of H 2 consumption. Experiments with resting cells revealed that the degree of inhibition of H 2 consumption was a function of the CO concentration. Since the hydrogen-dependent CO 2 reductase (HDCR) of A. woodii is known to be very sensitive to CO, we speculated that cells may be more tolerant toward CO when growing on formate, the product of the HDCR reaction. Indeed, addition of up to 25% CO did not influence growth rates on formate, while the final optical densities and the production of acetate increased. Higher concentrations (75 and 100%) led to a slight inhibition of growth and to decreasing rates of formate and CO consumption. Experiments with resting cells revealed that the HDCR is a site of CO inhibition. In contrast, A. woodii was not able to grow on CO as a sole carbon and energy source, and growth on fructose-CO or methanol-CO was not observed. There is a still-growing demand for sustainable biotechnological processes to cope with increasing needs for food, water, and energy for humankind. First-generation biotechnological processes all use food-based crops that compete for scarce cropland, fresh water, and fertilizers (1). Second-generation processes rely on biomass residues from forestry and agriculture that are used through lignocellulose fermentation. This is considered to play a major role in meeting the increasing demand, but it is still in its infancy (2). However, the process does produce the greenhouse gas CO 2 along with nonfermentable waste products. In an effort to combine reduction of greenhouse gases and global warming with the production of biocommodities, acetogenic bacteria have come into focus. They can convert CO 2 , H 2 , and CO to acetyl coenzyme A (acetyl-CoA), which is a starter molecule for various chemicals such as ethanol, acetate, 2,3 butanediol, and butanol (3-6). CO 2 , H 2 , and CO are the main components of synthesis gas (syngas), but the amounts of CO 2 , H 2 , and CO differ considerably, depending on the source. By gasification, virtually any organic matter can be converted to syngas. In addition, industrial waste gases contain syngas. Fermentation of syngas to ethanol is already used on a precommercial scale (7).Acetogenic bacteria have in common that they reduce CO 2 to acetic acid via the Wood-Ljungdahl pathway (WLP) (8). CO 2 reduction to acetyl-CoA proceeds via formate, formyl-tetrahydrofolic acid (TH...

CO Metabolism in the Thermophilic Acetogen Thermoanaerobacter kivui

“…Thus, the monofunctional CODH seems to play a superior role during CO metabolism, making it quite feasible that the ability to grow on CO is at least partly attributable to the monofunctional CODH. However, a monofunctional CODH besides the CODH/ACS is also encoded in the genome of A. woodii, and since this acetogen cannot grow solely on CO (24), it is more likely that a more elaborate adaptation is required.…”

“…Several acetogenic representatives have been shown to use CO as the sole energy source, such as Butyribacterium methylotrophicum (CO strain) (20), Eubacterium limosum (21), Blautia producta (22), Clostridium thermoautotrophicum (11), Moorella thermoacetica (12), and Clostridium ljungdahlii (23). The extent of CO tolerance and consumption varies greatly between different species, and some closely related bacteria cannot utilize CO as the sole energy source at all (20) or can utilize it only in combination with H 2 plus CO 2 , as is the case, for example, for Acetobacterium woodii (24). The circumstances defining the constraint of using CO as a sole energy source are still widely unresolved.…”

Energy Conservation Associated with Ethanol Formation from H 2 and CO 2 in Clostridium autoethanogenum Involving Electron Bifurcation

“…The 200-l mixture was then incubated at 40°C for 10 min with shaking at 1,400 rpm in a Thermomixer (type 5436; Eppendorf, Germany) to remove all 14 CO 2 , leaving behind the [ 14 C]formate formed. Subsequently, 100 l of the mixture was added to 5 ml of Quicksave A scintillation fluid (Zinsser Analytic, Frankfurt, Germany) and analyzed for 14 C radioactivity in a Beckman LS6500 liquid scintillation counter (Fullerton, CA).…”

“…C. pasteurianum ferredoxin reduction was monitored at 430 nm (⌬ε ox-red Ϸ 13.1 mM Ϫ1 cm Ϫ1 ), and benzyl viologen reduction was monitored at 578 nm (ε ϭ 8.6 mM Ϫ1 cm Ϫ1 ). The reduction of 14 CO 2 to [ 14 C]formate was followed by quantifying 14 C radioactivity in a Beckman LS6500 liquid scintillation counter (Fullerton, CA). One unit was defined as the transfer of 2 micromoles of electrons per minute.…”

“…The three clostridia differ in this respect from the well-studied Acetobacterium woodii, which has a pH growth optimum near 7 and reduces CO 2 with H 2 to acetic acid (reaction 1) but not to ethanol (reaction 2) (12). The three Clostridium species also differ from A. woodii in that A. woodii is dependent on sodium ions, whereas the three clostridial species are not, and in that A. woodii is unable to grow on CO in minimal medium (13) unless cultures are provided with formate (14). A common feature, however, is the lack of cytochromes and of menaquinone, which distinguish these acetogens from Moorella thermoacetica, which contains cytochromes and menaquinone (12,15).…”