. Zymomonas mobilis is one of the only organisms that can only metabolize glucos

Question

. Zymomonas mobilis is one of the only organisms that can only metabolize glucose through the Entner-Doudoroff

pathway and does not contain any type of electron transport chain. Instead, it utilizes ethanol fermentation. Explain the

purpose of fermentation for this bacteria?

b. Interestingly, the alcohol industry has considered using Zymomonas instead of yeast in alcohol production because it

uses glucose and produces a higher concentration of ethanol at a faster rate. Standard Saccharomyces cerevisiae yeast

use the Embden-Meyerhof pathway for its glucose metabolism. How does this helps explain the rate of glucose

utilization and ethanol production difference between the two organisms?

Explanation / Answer

a) Zymomonas mobilis is a gram negative aerotolerant anaerobe that is a natural ethanogen and can produce ethanol in a wide range of pH (3.5-7.5). It utilizes the EntnerDoudoroff (ED) pathway for ethanol production from glucose. This organism can only metabolize glucose, sucrose and fructose. It has the capacity to produce pyruvate from glucose and glyceraldehyde 3 phosphate. This pathway produces 50% less ATP as compared to the EMP pathway for fermentation. As a result, there is improved ethanol yield. Z mobilis has an incomplete EMP pathway and incomplete TCA and cannot carry out catabolism. All NADH is obtained only from ED pathway. Nearly 98% of the carbon sources are converted to ethanol, making it a very efficient ethanol producer. It uses fermentation mode in order to generate ATP at a faster rate under anaerobic conditions. Due to lack of functional TCA and EMP pathway, it will ferment glucose. The lack of electron transport chain leads to ethanol fermentation.

b) S. cervisiae and Z mobilis can convert glucose to ethanol by fermentation. Z. mobilis uses Entner-Doudoroff pathway while S. cerevisiae utilizes the Embden- Meyerhoff (EMP) pathway for ethanol production. Z.mobilis has a high specific surface area than S. cerevisiae. Hence, it metabolizes glucose faster than S. cerevisiae. It also does not require oxygen.

Zymomonas produces 94% ethanol while Saccharomyces produces 88% for Saccharomyces, under anaerobiosis. Z mobilis produces only one ATP per mol of glucose. Hence, it has 50% less ATP produced than s. ceerviisae. It generates one ATP molecule per glucose metabolized. To compensate for its low energy yield, it will metabolize glucose faster via the ED pathway. As it has an incomplete TCA cycle and lacks ETC, it cannot metabolize glucose efficiently. Hence, it uses ethanol fermentation for generation of ATP. In this way, the organism can efficiently metabolize glucose, generating energy faster. It maintains a high glycolytic flux to compensate for low ATP yield. Along with ethanol, sorbitol is also produced due to inactivation of phosphofructokinase by glucose. Sorbitol protect the organism from high osmotic stress. Its preference for low pH will also prevent contamination. It produces around 1.5-1.9 mol of ethanol per mol of glucose. It can also grow in high glucose and high ethanol levels. There is also less biomass produced than S. cerveisiae due to ATP levels being linked to anabolic pathways. Z. mobilis has carbons that are more available for ethanol fermentation. It has a 2.5-fold higher specific ethanol productivity than that of S. cerevisiae that results in up to 97% theoretical yield. S cerevisiae produces 2 ATP molecules per mol of glucose via EMP pathway under anaerobic conditions. Hence, it will ferment glucose slower than Z mobilis. It also has less alcohol tolerance. The pH of ethanol production was between 6-6.5.

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