**4. Metabolic pathways and hosts for ethanol production**

Ethanol is produced from glucose via fermentative consumption of pyruvate, the end product of glycolysis [18]. Glycolysis is a metabolic process that converts glucose to pyruvate while producing ATP. The pyruvate formed by glycolysis is further metabolized via one of the three catabolic routes. In aerobic organisms or tissues, under aerobic conditions, pyruvate is oxidized with the loss of its carboxyl group as CO2 to yield the acetyl-CoA; the acetyl-CoA is then completely oxidized to CO2 in the citric acid cycle (**Figure 4**). The second route for pyruvate is its reduction to lactate via lactic acid fermentation in vigorously contracting muscle. Under anaerobic condition, pyruvate is reduced to lactate. Another major route of pyruvate catabolism leads to ethanol. In some plant tissues and certain invertebrates, protists and microorganisms such as brewer's yeast, pyruvate under anaerobic conditions can be fermented to ethanol by sequential reactions of pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH) (**Figure 4**).

Microbial fermentation was introduced by Louis Pasteur in the late 1850s and was the first to recognize the relationship between the presence of yeast cells and the conversion of sugar to ethanol. Today ethanol producing *Saccharomyces cerevisiae* (yeast) have been exploited to produce a wide variety of alcoholic beverages and biofuels. Many microorganisms are being used for ethanol and biofuel production, but all have certain limitations such as industrial robustness, substrate utilization, productivity and yield. *Saccharomyces cerevisiae* (yeast) is a leading traditional industrial biocatalyst microorganism for ethanol production and with technological advancement in genetic engineering, bacteria such as *Escherichia coli*, *Zymomonas mobilis*, *Corynebacterium glutamicum* and *Bacillus subtilis* have been developed [19].

*Zymomonas mobilis* (*Z. mobilis*), a bacterium commonly found in plant saps and in honey has many desirable industrial biocatalyst characteristics and has been suggested as an alternative to the classical model, *Saccharomyces cerevisiae* (yeast) due to its advantage for ethanol yield. *Saccharomyces cerevisiae* uses the Embden-Meyerhof-Parnas (EMP) pathway for glycolysis while *Z. mobilis* uses the Entner-Doudoroff

#### **Figure 4.**

*Three possible catabolic fates of pyruvate formed in glycolysis (adapted from Lehninger principles of biochemistry).*

(ED) pathway. The ED pathway is found in strict aerobic microorganisms and conducts fermentation with 50% less ATP produced relative to the EMP pathway, which leads to improved ethanol yield. Moreover, *Z. mobilis* has a high-specific cell surface area and consumes glucose faster than *S. cerevisiae*, leading to higher ethanol productivity than *S. cerevisiae* [20] (**Figure 5**). Although the EMP pathway is a major glycolytic route in most eukaryotes and prokaryotes, glycolytic pathways are much more diverse in prokaryotes [21]. Among different glycolytic pathways, the ED and EMP pathways are the most abundant pathway for glycolysis [21]. Two molecules of ATP are produced from each molecule of glucose consumed using EMP pathway, while the ED pathway produces only one ATP molecule from one glucose molecule. Given that ATP is tightly coupled with anabolism and cell growth, ED pathway-utilizing *Z. mobilis* produces less energy than EMP pathway-dependent species such as *S. cerevisiae* and consequently, *Z. mobilis* has more available carbons for ethanol fermentation with 2.5-fold higher specific ethanol productivity than that of *S. cerevisiae* [22].

*Z. mobilis* is an obligately fermentative bacterium which lacks a functional system for oxidative phosphorylation. Like the *Saccharomyces cerevisiae*, *Z. mobilis* produces ethanol and carbon dioxide as principal fermentation products. Ethanol is produced by *Z. mobilis* using a short pathway which requires two enzymatic activities: pyruvate decarboxylase and alcohol dehydrogenase. Pyruvate decarboxylase is the key enzyme in this pathway which diverts the flow of pyruvate to ethanol. In this pathway, the non-oxidative decarboxylation of pyruvate to produce acetaldehyde and carbon dioxide is catalyzed by pyruvate decarboxylase. In *Z. mobilis*, two alcohol dehydrogenase isozymes are present which catalyzes the reduction of acetaldehyde to ethanol during fermentation.

**41**

**Figure 5.**

**5. Ethanol and malnutrition**

*G3P: glyceraldehyde 3-phosphate.*

Ethanol has a caloric value of 7.1 kcal/g.

Ethanol consumption has an effect on person's nutritional status. Many people who consume one to two glasses or less of alcoholic beverages per day consider those beverages a part of their normal diet and acquire a certain number of calories from them. When consumed in excess, ethanol can interfere with the nutritional status of the consumer. Ethanol can alter the intake, absorption and utilization of various nutrients [23, 24]. The primary constituents of alcoholic beverages are water, ethanol (alcohol), sugars (carbohydrates) and negligible amounts of other nutrients like proteins, vitamins and minerals. Because alcoholic beverages provide almost no nutrients, they are considered as "empty calories". Any calories provided by alcoholic beverages are derived from the carbohydrates and alcohol they contain.

*Ethanol fermentation pathways in S. cerevisiae and Z. mobilis (solid line: EMP glycolysis pathway; dashed line: ED glycolysis pathway). KDPG: 2-keto-3-deoxy-6-phosphogluconate; G6P: glucose 6-phosphate; F6P: fructose 6-phosphate; FBP: fructose 1,6-bisphosphate; DHAP: dihydroxyacetone phosphate;* 

*Ethanol*

*DOI: http://dx.doi.org/10.5772/intechopen.79861*

*Psychology of Health - Biopsychosocial Approach*

(ED) pathway. The ED pathway is found in strict aerobic microorganisms and conducts fermentation with 50% less ATP produced relative to the EMP pathway, which leads to improved ethanol yield. Moreover, *Z. mobilis* has a high-specific cell surface area and consumes glucose faster than *S. cerevisiae*, leading to higher ethanol productivity than *S. cerevisiae* [20] (**Figure 5**). Although the EMP pathway is a major glycolytic route in most eukaryotes and prokaryotes, glycolytic pathways are much more diverse in prokaryotes [21]. Among different glycolytic pathways, the ED and EMP pathways are the most abundant pathway for glycolysis [21]. Two molecules of ATP are produced from each molecule of glucose consumed using EMP pathway, while the ED pathway produces only one ATP molecule from one glucose molecule. Given that ATP is tightly coupled with anabolism and cell growth, ED pathway-utilizing *Z. mobilis* produces less energy than EMP pathway-dependent species such as *S. cerevisiae* and consequently, *Z. mobilis* has more available carbons for ethanol fermentation with 2.5-fold higher specific ethanol productivity than

*Three possible catabolic fates of pyruvate formed in glycolysis (adapted from Lehninger principles of* 

*Z. mobilis* is an obligately fermentative bacterium which lacks a functional system for oxidative phosphorylation. Like the *Saccharomyces cerevisiae*, *Z. mobilis* produces ethanol and carbon dioxide as principal fermentation products. Ethanol is produced by *Z. mobilis* using a short pathway which requires two enzymatic activities: pyruvate decarboxylase and alcohol dehydrogenase. Pyruvate decarboxylase is the key enzyme in this pathway which diverts the flow of pyruvate to ethanol. In this pathway, the non-oxidative decarboxylation of pyruvate to produce acetaldehyde and carbon dioxide is catalyzed by pyruvate decarboxylase. In *Z. mobilis*, two alcohol dehydrogenase isozymes are present which catalyzes the reduction of

**40**

that of *S. cerevisiae* [22].

**Figure 4.**

*biochemistry).*

acetaldehyde to ethanol during fermentation.

#### **Figure 5.**

*Ethanol fermentation pathways in S. cerevisiae and Z. mobilis (solid line: EMP glycolysis pathway; dashed line: ED glycolysis pathway). KDPG: 2-keto-3-deoxy-6-phosphogluconate; G6P: glucose 6-phosphate; F6P: fructose 6-phosphate; FBP: fructose 1,6-bisphosphate; DHAP: dihydroxyacetone phosphate; G3P: glyceraldehyde 3-phosphate.*
