Alcoholic Fermentation / Second Edition, 1914

by Arthur Harden

2. The dihydroxyacetone then passes through the stages previously postulated (p. [106]).

(b) 4 C₃H₆O₃ + 4 R₂HPO₄ = 4 C₃H₅O₂PO₄R₂ + 4 H₂O.
(c) 4 C₃H₅O₂PO₄R₂ = 2 C₆H₁₀O₄(R₂PO₄)₂.
(d) 2 C₆H₁₀O₄(R₂PO₄)₂ + 4 H₂O = 2 C₆H₁₂O₆ + 4 R₂HPO₄.

After which the hexose, C₆H₁₂O₆ re-enters the cycle at (a).

3. The fermentation of the glyceraldehyde occurs according to the scheme developed by Kostytscheff (p. [109]), pyruvic acid being formed along with hydrogen and then decomposed into carbon dioxide and acetaldehyde, which is reduced by the hydrogen. Lebedeff, however, suggests [[1914, 1], [2]] that glyceric acid is first formed (1) and then converted by an enzyme, which he terms dehydratase into pyruvic acid (2):—

(1) CH₂(OH)·CH(OH)·CHO + H₂O → CH₂(OH)·CH(OH)·CH(OH)₂
→ CH₂(OH)·CH(OH)·COOH + 2 H
(2) CH₂(OH)·CH(OH)·COOH = CH₃·CO·COOH + H₂O.

The experimental basis for this idea is the fact that glyceric acid is fermented by dried yeast and maceration juice [compare Neuberg and Tir, [1911]].

This scheme has the merit of recognising the fact that the carbon dioxide does not wholly arise from the products of decomposition of hexosephosphate, nor from its direct fermentation. The function assigned to the phosphate is that of removing dihydroxyacetone and thus preventing it from inhibiting further conversion of hexose into triose, according to the reversible reaction

C₆H₁₂O₆ ⇌ 2 C₃H₆O₃.

This however appears to be quite inadequate, since, on the one hand, the fermentation of glucose proceeds quite freely in presence of as much as 5 grams per 100 c.c. of dihydroxyacetone [Harden and Young, [1912]], and on the other hand alcoholic fermentation appears not to proceed at all in the absence of phosphate (see p. [55]). This forms the chief objection to the theory in its present form. The slow rate at which [p109] glyceraldehyde is fermented also affords an argument against the validity of Lebedeff's view, but this may possibly be accounted for to some extent by the fact that glyceraldehyde is a strong inhibiting agent so that it might be more rapidly fermented if added in a more dilute condition.

The unfermented glyceraldehyde cannot be recovered from the solution and nothing is known as to its fate except that it readily gives rise both to lactic acid and glycerol [Oppenheimer, [1914, 1], [2]]. Evidently the reaction between glyceraldehyde and yeast-juice is by no means a simple one.