Biochemical characterization of a bifunctional acetaldehyde-alcohol dehydrogenase purified from a facultative anaerobic bacterium Citrobacter sp. S-77

Comparative Transcriptome Analysis Reveals the Molecular Mechanisms of Acetic Acid Reduction by Adding NaHSO3 in Actinobacillus succinogenes GXAS137

Acetic acid (AC) is a major by-product from fermentation processes for producing succinic acid (SA) using Actinobacillus succinogenes. Previous experiments have demonstrated that sodium bisulfate (NaHSO3) can significantly decrease AC production by A. succinogenes GXAS137 during SA fermentation. However, the mechanism of AC reduction is poorly understood. In this study, the transcriptional profiles of the strain were compared through Illumina RNA-seq to identify differentially expressed genes (DEGs). A total of 210 DEGs were identified by expression analysis: 83 and 127 genes up-regulated and down-regulated, respectively, in response to NaHSO3 treatment. The functional annotation analysis of DEGs showed that the genes were mainly involved in carbohydrates, inorganic ions, amino acid transport, metabolism, and energy production and conversion. The mechanisms of AC reduction might be related to two aspects: (i) the lipoic acid synthesis pathway (LipA, LipB) was significantly down-regulated, which blocked the pathway catalyzed by pyruvate dehydrogenase complex to synthesize acetyl-coenzyme A (acetyl-CoA) from pyruvate; (ii) the expression level of the gene encoding bifunctional acetaldehyde-alcohol dehydrogenase was significantly up-regulated, and this effect facilitated the synthesis of ethanol from acetyl-CoA. However, the reaction of NaHSO3 with the intermediate metabolite acetaldehyde blocked the production of ethanol and consumed acetyl-CoA, thereby decreasing AC production. Thus, our study provides new insights into the molecular mechanism of AC decreased underlying the treatment of NaHSO3 and will deepen the understanding of the complex regulatory mechanisms of A. succinogenes.

Hepatoprotective Effects of Morchella esculenta against Alcohol-Induced Acute Liver Injury in the C57BL/6 Mouse Related to Nrf-2 and NF-κB Signaling

This study investigated the hepatoprotective effects of Morchella esculenta fruit body (ME) and the underlying mechanisms in mice with alcohol-induced acute liver injury. Systematic analysis revealed that ME contained 21 types of fatty acid, 17 types of amino acid, and 12 types of mineral. Subsequently, a mouse model of acute alcohol-induced liver injury was established by oral administration of alcohol for 14 days. Fourteen-day administration of ME prevented alcohol-induced increases in alanine aminotransferase and aspartate aminotransferase levels and reduced the activity of acetaldehyde dehydrogenase in blood serum and liver tissue. ME appears to regulate lipid metabolism by suppressing triglycerides, total cholesterol, and high-density lipoprotein in the liver. ME inhibited the production of inflammatory factors including chitinase-3-like protein 1 (YKL 40), interleukin-7 (IL-7), plasminogen activator inhibitor type 1 (PAI-1), and retinol-binding protein 4 (RBP4) in blood serum and/or liver tissue. ME treatment relieved the alcohol-induced imbalance in prooxidative and antioxidative signaling via nuclear factor-erythroid 2-related factor 2 (Nrf-2), as indicated by upregulation of superoxide dismutase-1, superoxide dismutase-2, catalase, heme oxygenase-1, and heme oxygenase-2 expression and downregulation of kelch-like ECH-associated protein 1 (Keap-1) in the liver. Moreover, ME reduced the levels of phosphorylated nuclear factor kappa-B kinase α/β, inhibitor of nuclear factor kappa-B α and nuclear factor kappa-B p65 (NF-κB p65) in the liver. The hepatoprotective effects of ME against alcohol-induced acute liver injury were thus confirmed. The mechanism of action may be related to modulation of antioxidative and anti-inflammatory signaling pathways, partially via regulation of Nrf-2 and NF-κB signaling.

Association of Anoxybacillus sp. with acid off-flavor development in a spoiled, boiled, rice dish

Off-flavor is one of the most common food complaints. In this study, we demonstrated that acetic acid produced by Anoxybacillus sp. contamination of takikomi-gohan (boiled rice with sweet potato mixed in advance) was considered the causative agent of acid off-flavor development. First, we conducted whole genome sequencing of the bacterial strain (S1674) isolated from the remains of the contaminated takikomi-gohan, and phylogenetic analysis of k-mer diversity demonstrated that S1674 belongs to the Anoxybacillus genus. Gene expression analysis of S1674 RNA sequencing (RNA-seq) and quantitative reverse transcription polymerase chain reaction (qRT-PCR) indicated that the genes encoding enzymes responsible for acetic acid formation, namely ackA1, eutD, pflA, pflB, and pykA, were upregulated in high-temperature cultures in Thermus medium supplemented with soluble starch. Additionally, we succeeded in reproducing the acid off-flavor by adding S1674 to boiled rice stored at 37 °C, 45 °C, and 60 °C. The most strongly detected organic acid was acetic acid, at the odor threshold value or more in both the air and condensation samples. Our findings suggest that some Anoxybacillus sp. produce acetic acid as a byproduct of carbohydrate metabolism, potentially causing the complaint of acid off-flavor even under high-temperature conditions in which other bacteria cannot survive.

Glyceraldehyde-3-phosphate dehydrogenase from Citrobacter sp. S-77 is post-translationally modified by CoA (protein CoAlation) under oxidative stress

Protein CoAlation (S-thiolation by coenzyme A) has recently emerged as an alternative redox-regulated post-translational modification by which protein thiols are covalently modified with coenzyme A (CoA). However, little is known about the role and mechanism of this post-translational modification. In the present study, we investigated CoAlation of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) from a facultative anaerobic Gram-negative bacterium Citrobacter sp. S-77 (Cb GAPDH). GAPDH is a key glycolytic enzyme whose activity relies on the thiol-based redox-regulated post-translational modifications of active-site cysteine. LC-MS/MS analysis revealed that CoAlation of Cb GAPDH occurred in vivo under sodium hypochlorite (NaOCl) stress. The purified Cb GAPDH was highly sensitive to overoxidation by H₂O₂ and NaOCl, which resulted in irreversible enzyme inactivation. By contrast, treatment with coenzyme A disulphide (CoASSCoA) or H₂O₂/NaOCl in the presence of CoA led to CoAlation and inactivation of the enzyme; activity could be recovered after incubation with dithiothreitol, glutathione and CoA. CoAlation of the enzyme in vitro was confirmed by mass spectrometry. The presence of a substrate, glyceraldehyde-3-phosphate, fully protected Cb GAPDH from inactivation by CoAlation, suggesting that the inactivation is due to the formation of mixed disulphides between CoA and the active-site cysteine Cys149. A molecular docking study also supported the formation of mixed disulphide without steric constraints. These observations suggest that CoAlation is an alternative mechanism to protect the redox-sensitive thiol (Cys149) of Cb GAPDH against irreversible oxidation, thereby regulating enzyme activity under oxidative stress.