]\IECHANIS:M of anaerobic respiration 543 



increase in the concentration of the respiratory substrate results in a transient 

 acceleration of the rate of respiration. 



6. None of the oxidizing-reducing enzymes of plants are able to catalyze 

 the oxidation of sugar directly, but apparently certain of them can bring about 

 the oxidation of the intermediate products of anaerobic respiration to carbon 

 dioxide and water. 



The Theoretical Mechanism of Anaerobic Respiration. — The respira- 

 tory substrate in the higher green plants is usually a hexose sugar. The two 

 common hexose sugars in plants are glucose and fructose. As discussed in 

 Chap. XXII there is good evidence that both of these sugars can exist in highly 

 reactive forms known as gamma-sugars. Sugars of this type are so highly 

 labile that they have never been isolated in the pure state. 



Recent extensive investigations of Neuberg and his co-workers (1922, 

 1924, etc.) have thrown considerable light on the chemical nature of the 

 intermediate products in alcoholic fermentation. The same or similar prod- 

 ucts are believed to be formed in the anaerobic respiration of higher plants. 

 The supposed stages of the process are as follows: 



1. "Activation" of the stable hexose molecules, or, in other words, con- 

 version of molecules of this type into the labile gamma-sugars. Formerly it 

 w^as believed that gamma-glucose was the sugar usually oxidized in the respira- 

 tion process, but recent investigations indicate that gamma-fructose may often 

 be cast in this role. 



2. The "activated" hexose molecule {i.e. gamma-hexose) is split by the 

 enzyme glycolase (a component of the zymase complex) into t\vo molecules 

 of methyl glyoxal, with the elimination of two molecules of water as follows : 



CcHiaOe ^^^^^ 2 CH3 • CO- CHO -f 1 H2O 



Methyl glyoxal 



3. The next step is the conversion of two molecules of methyl glyoxal 

 into one molecule of glycerol and one of pyruvic acid, by a reaction with two 

 molecules of water. The formation of glycerol from methyl glyoxal is a 

 reduction process, the formation of pyruvic acid an oxidation process. A 

 reaction of this type is known as a Cannizzaro reaction: 



CH3 • CO • CHO CH2OH • CHOH • CH2OH 



Methyl glyoxal f^^ _j_ J^^Q Glycerol 



+ + r " -> + 

 CH3COCH0 ^ CH3COC00H 



Methyl glyoxal Pyruvic acid 



This reaction may be catalyzed by an enzyme of the dehydrogenase type, 

 but there is no convincing evidence on this point. 



