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Glycolysis and Alcoholic Fermentation

Glycolysis (from glycose, an older term for glucose + -lysis degradation) is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+. The free energy released in this process is used to form the high-energy compounds ATP (adenosine triphosphate) and NADH (reduced nicotinamide adenine dinucleotide).Yeast cells obtain energy under anaerobic conditions using a very similar process called alcoholic fermentation,  also referred to as ethanol fermentation, is a biological process in which sugars such as glucose, fructose, and sucrose are converted into cellular energy and thereby produce ethanol and carbon dioxide as metabolic waste products.

Glycolysis requires 11 enzymes which degrade glucose to lactic acid (Fig. 2). Alcoholic fermentation follows the same enzymatic pathway for the first 10 steps. The last enzyme of glycolysis, lactate dehydrogenase, is replaced by two enzymes in alcoholic fermentation. These two enzymes, pyruvate decarboxylase and alcoholic dehydrogenase, convert pyruvic acid into carbon dioxide and ethanol in alcoholic fermentation.

The most commonly accepted evolutionary scenario states that organisms first arose in an atmosphere lacking oxygen.1,2 Anaerobic fermentation is supposed to have evolved first and is considered the most ancient pathway for obtaining energy. However, there are several scientific odds against that.

First of all, it takes ATP energy to start the process that will only later generate a net gain in ATP. Two ATPs are put into the glycolytic pathway for priming the reactions, the expenditure of energy by conversion of ATP to ADP being required in the first and third steps of the pathway (Fig. 2). A total of four ATPs are obtained only later in the sequence, making a net gain of two ATPs for each molecule of glucose degraded. The net gain of two ATPs is not realized until the tenth enzyme in the series catalyzes phosphoenolpyruvate to ATP and pyruvic acid (pyruvate). This means that neither glycolysis nor alcoholic fermentation realizes any gain in energy (ATP) until the tenth enzymatic breakdown.

Enzymes are proteins consisting of amino acids united in polypeptide chains. Their complexity may be illustrated by the enzyme glyceraldehyde 3-phosphate dehydrogenase, which is the enzyme that catalyzes the oxidation of phosphoglyceraldehyde in glycolysis and alcoholic fermentation. Glyceraldehyde phosphate dehydrogenase consists of four identical chains, each having 330 amino acid residues. The possible number of different combinations of these amino acid chains is infinite.

 

Glyceraldehyde-3-phosphate dehydrogenase

 

To illustrate, let us consider a simple protein containing only 100 aim acids. There are 20 different kinds of L-amino acids in proteins, and each can be used repeatedly in chains of 100. Therefore, they could be arranged in 20^100 or 10^130 different ways. Even if a hundred million billion of these (10^17) combinations could function for a given purpose, there is only one chance in 10^113 of getting one of these required amino acid sequences in a small protein consisting of 100 amino acids. By comparison, Sir Arthur Eddington has estimated there are no more than 10^80 (or 3,145 x 10^79) particles in the universe! Consider the 10 enzymes of the glycolytic pathway. If each of these were a small protein having 100 amino acid residues with some flexibility and a probability of 1 in 10^113 or 10^-113, the probability for arranging the amino acids for the 10 enzymes would be: P = 10^-1,130 or 1 in 10^1,130, and this result is only the odds against producing the 10 glycoytic enzymes by chance. It is estimated that the human body contains 25,000 enzymes. If each of these were only a small enzyme consisting of 100 amino acids with a probability of 1 in 10^-113, the probability of getting all 25,000 would be (10^-113)^25,000, which is 1 chance in 10^2,825,000…

Figure 2

 

 

There are still other problems with that theory. There are numerous complex regulatory mechanisms which control these chemical pathways. For example, phosphofructokinase is a regulatory enzyme which limits the rate of glycolysis. Glycogen phosphorylase is also a regulatory enzyme; it converts glycogen to glucose-1-phosphate and thus makes glycogen available for glycolytic breakdown. In complex organisms there are several hormones such as somatotropin, insulin, glucagon, glucocorticoids, adrenaline thyroxin and a host of others which control utilization of glucose.

In addition, complex cofactors are absolutely essential for glycolysis. One of the two key ATP energy harvesting steps in glycolysis requires a dehydrogenase enzyme acting in concert with the “hydrogen shuttle” redox reactant, nicotinamide adenine dinucleotide (NAD+). To keep the reaction sequence going, the reduced cofactor (NADH + H +) must be continuously regenerated by steps later in the sequence (Fig. 2), which requires one enzyme in glycolysis (lactic dehydrogenase) and another (alcohol dehydrogenase) in alcoholic fermentation.

Further, at one point, an intermediate in the glycolytic pathway is “stuck” with a phosphate group (needed to make ATP) in the low energy third carbon position. A remarkable enzyme, a “mutase” (Step 8), shifts the phosphate group to the second carbon position—but only in the presence of pre-existent primer amounts of an extraordinary molecule, 2,3-diphosphoglyceric acid. Actually, the shift of the phosphate from the third to the second position using the “mutase” and these “primer” molecules accomplishes nothing notable directly, but it “sets up” the ATP energy-harvesting reaction which occurs two steps later!

 

by Jean Sloat Morton, Ph.D.

 

References

1 A.I. Oparin, Origin of Life, New York: Dover Pub., lnc., 1965, pp. 225-26.
2 (Jark and Synge (eds.), The Origin of Life on the Earth, New York: Pergamon Press, 1959, p. 52.
3 Ernil Borel, Probabilities and Life, New York: Dover Pub., Inc., 1962, p. 28.

 

Cite this article: Morton, J. S. 1980. Glycolysis and Alcoholic Fermentation. Acts & Facts. 9 (12).

From: http://www.icr.org/article/glycolysis-alcoholic-fermentation/

 

2 responses to “Glycolysis and Alcoholic Fermentation

  1. Pingback: DCA As a Glycolitic Inhibitor - Dr. Dalal Akoury

  2. Pingback: Glycolysis Inhibitors Help in Fighting Cancer - Dr. Dalal Akoury

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