Decarboxylation of Pyruvate to Acetaldehyde for Ethanol Production by Hyperthermophiles

Abstract

Pyruvate decarboxylase (PDC encoded by pdc) is a thiamine pyrophosphate (TPP)-containing enzyme responsible for the conversion of pyruvate to acetaldehyde in many mesophilic organisms. However, no pdc/PDC homolog has yet been found in fully sequenced genomes and proteomes of hyper/thermophiles. The only PDC activity reported in hyperthermophiles was a bifunctional, TPP- and CoA-dependent pyruvate ferredoxin oxidoreductase (POR)/PDC enzyme from the hyperthermophilic archaeon Pyrococcus furiosus. Another enzyme known to be involved in catalysis of acetaldehyde production from pyruvate is CoA-acetylating acetaldehyde dehydrogenase (AcDH encoded by mhpF and adhE). Pyruvate is oxidized into acetyl-CoA by either POR or pyruvate formate lyase (PFL), and AcDH catalyzes the reduction of acetyl-CoA to acetaldehyde in mesophilic organisms. AcDH is present in some mesophilic (such as clostridia) and thermophilic bacteria (e.g., Geobacillus and Thermoanaerobacter). However, no AcDH gene or protein homologs could be found in the released genomes and proteomes of hyperthermophiles. Moreover, no such activity was detectable from the cell-free extracts of different hyperthermophiles under different assay conditions. In conclusion, no commonly-known PDCs was found in hyperthermophiles. Instead of the commonly-known PDC, it appears that at least one multifunctional enzyme is responsible for catalyzing the non-oxidative decarboxylation of pyruvate to acetaldehyde in hyperthermophiles.

Umpolung in reactions catalyzed by thiamine pyrophosphate dependent enzymes

“…This is then converted to ethanol by alcohol dehydrogenase (Hanson, 1987;Nelson et al, 2013) reoxidizing NADH to sustain glycolysis (Nelson et al, 2013;Tylicki et al, 2018). Even though the mechanism of PDC is similar to that of PDH, KGDH and BCKDH, the enamine produced is protonated at the C2 α-position, releasing acetaldehyde (Eram & Ma, 2013). Note: The resulting carbanion from the reaction of TPP with pyruvic acid and the non-oxidative decarboxylation (43), is protonated (44) and released from TPP as acetaldehyde (45).…”

Production of Bioethanol from Bore (Alocasia macrorrhiza)

“…Source: [25] Pyruvate is decarboxylated due to the enzyme, pyruvate decarboxylase, found in yeast, which is subsequently formed by acetaldehyde. This is reduced to ethanol by the presence of the reducing agent NADH, by the action of the enzyme alcohol dehydrogenase (Figure 7) [26].…”

Bioprospecting thermophilic/thermotolerant microbes for production of lignocellulosic ethanol: A future perspective

“…The performance parameters of fermentation are temperature, pH, alcohol tolerance, growth rate, productivity, osmotic tolerance, specificity, yield, genetic stability, and inhibitor tolerance [46]. Thermophilic microorganisms can be categorized into several groups: moderate thermophiles, or simple thermophiles, are those that grow optimally between 50 and 64 1C, extreme thermophiles are those with optimal growth temperatures between 65 and 79 1C and the organisms that can grow optimally above 80 1C are called hyperthermophiles [47]. Contamination is a major factor in industrial fermentation which results in the reduction of the ethanol yield and formation of inhibitory compounds.…”

“…Another enzyme known to be involved in catalysis of acetaldehyde production from pyruvate is CoA-acetylating ALDH (encoded by mhpF and adhE). Pyruvate is oxidized into acetyl-CoA by either PFOR or PFL, and ALDH catalyzes the reduction of acetyl-CoA to acetaldehyde in thermophilic/thermotolerant bacteria (e.g., Geobacillus and Thermoanaerobacter) [47]. Finally, acetaldehyde is reduced to ethanol by ADH.…”