88 OXIDATION-REDUCTION POTENTIALS 



BACTERIAL GLUCOSE FERMENTATION 



In 1901, Harden found that the fermentation of glucose by Bacillus coli could be 

 represented by the equation : — 



2 Glucose -> 2 lactic acid + acetic acid + ethyl alcohol + 2CO2 + 2H2 



Variations are observed under different experimental conditions and it would seem 

 that pyruvic acid is formed as in the glycolytic and alcoholic fermentation chains 

 discussed previously, and that the subsequent breakdown of pyruvic acid depends upon 

 the experimental conditions, a pH below 7 favouring lactic acid formation when 

 phosphate is present in appreciable amount. 



With haemolytic streptococci and pneumococci it was found (Hewitt, 1932) that 

 about 70 per cent, of the glucose was transformed into lactic acid under a variety of 

 experimental conditions, but in the absence of inorganic phosphate glucose breakdown 

 did not occur. 



It is evident that the very different enzyme systems possessed by different 

 micro-organisms will be reflected in different products of carbohydrate metabolism. 



NEUBERG'S FERMENTATION SYSTEMS 



Other products besides quantitative yields of ethyl alcohol and carbon dioxide 

 are found as the result of fermentation by yeast, small amounts of glycerol, acetic 

 acid, etc., being formed. Neuberg's first form of fermentation was a series of reactions 

 formulated to explain alcoholic fermentation as already described. The " Second 

 Form " of fermentation occurs when sodium bisulphite is added to the fermentation 

 system. The acetaldehyde formed instead of being reduced to alcohol (with the 

 intervention of reduced Coenzyme I) is combined with the bisulphite as an end product 

 of the fermentation. The reduced coenzyme which normally reduces acetaldehyde is 

 now available for another purpose and, in fact, phosphoglyceraldehyde acts as 

 hydrogen acceptor, being reduced to phosphoglycerol, which in the presence of yeast 

 phosphatase is broken down to glycerol. So in this case acetaldehyde and glycerol 

 are the fermentation products. 



Neuberg's " Third Form " of fermentation occurs when the fermentation 

 liquors are kept alkaline. In these circumstances the acetaldehyde is not quantita- 

 tively reduced to alcohol by reduced Coenzyme I, but again the coenzyme produces 

 glycerol. The acetaldehyde is regarded as undergoing " dismutation," one molecule 

 being reduced to alcohol and another being oxidised to acetic acid : — 



2CH3.CHO -> CH3CH2OH + CH3.COOH 



It will be recalled that this type of Cannizzaro reaction in animal tissues has been found 

 to be due to two enzyme reactions catalysed by alcohol and aldehyde dehydrogenase 

 respectively, with diaphorase and diphosphopyridine nucleotide as coenzymes. 



AEROBIC OXIDATION AND CITRIC ACID CYCLE 



When, in place of anaerobic glycolysis, glucose is broken down aerobically the 

 initial stages are not very different from those of anaerobic glycolysis. Glucose is 

 not oxidised directly to carbon dioxide, but the chain proceeds as in anaerobiosis until 



