Engineering Chemistry

Lesson 20. Enzymes in Alcohol and Acetic Acid Fermentation

20.2   Overview of Alcohol fermentation

Yeast and other microorganisms ferment glucose to ethanol and CO_2.The overall equation of alcoholic fermentation can be written as

Where, ATP stands for Adenosine triphosphate and ADP for Adenosine diphosphate. These serve as store house of energy in the cell.

The first 10reactions of this pathway is common to all organisms, and the pathway is known as Glycolysis or EMP pathway.

20.3   Glycolysis

In glycolysis, one Glucose molecules is degraded into 2 molecules of Pyruvate.

The 10 reactions of Glycolysis and brief explanation to the reactions are as follows

20.3.1 

The D-Glucose molecule is phosphorylated to yield Glucose-6-phosphate by transfer of phosphate group from ATP to glucose. This reaction is catalyzed by enzyme Hexokinase. Hexokinase requires Mg²⁺ for functioning.

20.3.2 

Glucose-6-phosphate is isomerized to Fructose-6-phosphate in reaction catalyzed by phosphoglucoisomerase. Phosphoglucoisomerase requires Mg²⁺ for functioning.

20.3.3

Fructose-6- phosphate is phosphorylated to fructose-1,6-biphosphate by transfer of a phosphate group from ATP molecule to fructose-6-phosphate. This reaction is catalyzed by Phosphofructokinase. This enzyme requires Mg²⁺ for functioning.

20.3.4 

Fructose-1,6-biphosphate is cleaved to yield Glyceraldehyde-3-phosphate and Dihydroxy acetone phosphate. This reaction is catalyzed by enzyme fructose diphosphate aldolase, which is abbreviated as aldolase. This enzyme requires Zn²⁺ for functioning.

20.3.5 

Dihydroxy acetone phosphate is converted into Glyceraldehyde-3-phosphate  in a reaction catalyzed by the enzyme triose phosphate isomerase.

20.3.6

Glyceraldehyde-3- phosphate is oxidized to 1,3-biphosphoglycerate catalyzed by glyceraldehydes phosphate dehydrogenase.

20.3.7

The enzyme phosphoglycerate kinase transfers phosphate group from 1,3-biphosphoglycerate  to ADP.

20.3.8

3-phosphoglyarete is converted to 2-phosphoglycerate by shift of phosphate group within the substrate molecule. This reaction is catalyzed by enzyme phosphoglycerate mutase. Mg ²⁺ is essential for the enzyme action.

20.3.9 

2-phosphoglycerate is dehydrated to phosphoenol pyruvate. The enzyme enolase promotes reversible removal of water molecules. Enolase requires Mg²⁺ which makes complex with enzyme before substrate molecule can attach.

20.3.10 

This reaction involves transfer of phosphate group from phosphoenol pyruvate to ADP catalyzed by pyruvate kinase. This enzyme requires K⁺ and either Mg²⁺ or Mn²⁺ for functioning.

20.4 Reactions of alcohol fermentation

20.4.1 

Pyruvate loses its carboxyl group by action of pyruvate decarboxylase. This enzyme requires Mg²⁺ and coenzyme thiamine pyrophosphate (Obtained from thiamine - vitamin B₁).

20.4.2 

In this reaction acetaldehyde is reduced to ethanol through the action of alcohol dehydrogenase.

20.5   Acetic acid fermentation

Acetic acid is produced by different pathways by different bacteria. The bacteria of the genus Acetobacter oxidize ethanol to acetic acid.

 

The anaerobic bacteria of the genus Clostridium or Acetobacterium convert glucose to acetic acid without producing ethanol. The pathway followed in propionobacteria is as follows

20.5.1

 

This reaction is catalyzed by the enzyme Pyruvateferredoxin oxidoreduuctase.

20.5.2

 

Acetyl coenzyme A is converted to acetyl phosphate by the action of enzyme Phosphoacetyl transferase.

20.5.3           

 

Acetyl phosphate is converted to Acetic acid by the action of enzyme Acetate kinase. A phosphate group is transferred to ADP and one molecule of ATP is generated.

References & Further Reading:

1. Lehninger A. (1987), “Principles of Biochemistry”. B S Publishers & Distributers, pp. 402-412, 426-428.

2. Satyanarayana U. & Chakrapani U. (2011), “Biochemistry”. Books and Allied (P) Ltd. pp. 246-248.

3. Albert G. Moat, John W. Foster and Michael P. Spector (2002), “Microbial Physiology”. Wiley-Liss, Inc., pp. 412-431.