Pyruvate/lactate/ethanol metabolism

See also LDH Isoenzymes, Pyruvate/Lactate/Ethanol Metabolism, Anaerobic Process for Generating Metabolic Energy, Lactic Acid Fermentation, Ethanol Metabolism and Gluconeogenesis... 

Metabolic Fates of Pyruvate (Pyruvate/Lactate/Ethanol Metabolism)... 

Answer The first step in the synthesis of glucose from lactate in the liver is oxidation of the lactate to pyruvate like the oxidation of ethanol to acetaldehyde, this requires NAD+. Consumption of alcohol forces a competition for NAD+ between ethanol metabolism and gluconeogenesis, reducing the conversion of lactate to glucose and resulting in hypoglycemia. The problem is compounded by strenuous exercise and lack of food because at these times the level of blood glucose is already low. 

The study of Oglno et al. (30) on the metabolic regulations and pyruvate transport in anaerobic coli cells is of special interest because the [1- C] glucose metabolites were detected by proton correlation spectroscopy and only signals from extracellular metabolites which had diffused through the cell membrane and accumulated in the medium were observed (acetate, lactate, ethanol, succinate, and pyruvate). This allowed the evaluation of perturbations to the cell on the influx and egress of pyruvate. 

A theoretical yield of 2 moles of succinic acid per mole glucose and per mole of carbon dioxide captured can be obtained. Carbon feedstock, pH and carbon dioxide are critical for the production of succinic acid and any succinate derivative. A proper combination of these parameters must be selected for each microorganism, as they use different pathways for succinate production and tolerate different levels of CO, pH and H. CO is an electron acceptor that diverts metabolism to pyruvate and lactate/ethanol when present at low levels but to succinate when present at high levels (Sadhukhan et al. 2014). COj can be supplied from an external gas stream and carbonates added to the medium (e.g. MgCOj, NaC03, NaHCOj or CaC03) or from a combination of these sources. 

The glycolytic pathway, or glycolysis, is a metabolic sequence in which glucose is broken down to pyruvic acid. The subsequent fate of pyruvate then depends upon whether or not the organism is aerobic or anaerobic Under aerobic conditions, pyruvate is oxidized via oxidative phosphorylation under anaerobic conditions, pyruvate is converted further into compounds such as lactate or ethanol, depending upon the organism. 

Variations of the alcoholic and homolactic fermentations. The course of a fermentation is often affected drastically by changes in conditions. Many variations can be visualized by reference to Fig. 17-9, which shows a number of available metabolic sequences. We have already discussed the conversion of glucose to triose phosphate and via reaction pathway a to pyruvate, via reaction c to lactate, and via reaction d to ethanol. 

This, the final step in alcohol fermentation, is analogous to lactate fermentation. Both reactions regenerate NAD+ and produce low-molecular-weight, water-soluble, metabolic end products that diffuse out of the cells in which they were produced. In the case of alcoholic fermentation, the second reaction is reversible, so that if oxygen becomes available to previously anaerobic yeast cells, the ethanol is oxidized to acetaldehyde. Unlike lactate fermentation, in which the lactate is oxidized to pyruvate, alcoholic fermentation cannot form pyruvate from acetaldehyde. Instead, the acetaldehyde... 

The first metabolic pathway that we encounter is glycolysis, an ancient pathway employed by a host of organisms. Glycolysis is the sequence of reactions that metabolizes one molecule of glucose to two molecules ofpyruvate with the concomitant net production of two molecules of ATP. This process is anaerobic (i.e., it does not require O2) inasmuch as it evolved before the accumulation of substantial amounts of oxygen in the atmosphere. Pyruvate can be further processed anaerobically (fermented) to lactate (lactic acidfermentation) or ethanol (alcoholic fermentation). Under aerobic conditions, pyruvate can be completely oxidized to CO2, generating much more ATP, as will be discussed in Chapters 17 and 18. 

As seen in the above, NAD works as the coenzyme in glucose metabolism, therefore, NADH has to be re-oxidized to NAD to support continued glucose oxidation. Anaerobic bacteria such as Clostridia re-oxidizes NADH by producing organic acids like lactate, acetate and butyrate as well as alcohols like ethanol, butanol and butanediol, from pyruvate. Many bacteria have multiple metabolic pathways, and change the pathway in accordance with metabolite availability and/or pH in the culture liquid. Therefore, in general, it is very... 

In anaerobic conditions, cells can metabolize pyruvate to lactate or to ethanol plus CO2 (in the case of yeast), with the reoxidation of NADH. In aerobic conditions, pyruvate is transported into the mitochondrion, where pyruvate dehydrogenase converts it into acetyl CoA and CO2 (see Figure 8-5). 

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