206 BACTERIOLOGICAL CHEMISTRY 



the hydrogen is finally handed over to atmospheric 

 oxygen. 



The flavin adenine dinucleotide and its carrier 

 protein (sometimes called diaphorase) are present in 

 animal tissues and micro-organisms and constitute the 

 enzyme necessary to oxidise reduced co -enzyme I and 

 co-enzyme II. Warburg's " yellow respiratory enzyme," 

 consisting of riboflavin phosphate and protein, serves the 

 same purpose but is not found in animal tissues. 



The aerobic oxidation of a substrate such as lactate 

 can be summarised by the following equations : — 



(1) Substrate + co-enzyme I > 



Oxidised substrate + dihydroco -enzyme I 



(2) Dihydroco-enzyme I + flavoprotein > 



Co-enzyme I -f- leucoflavoprotein. 



(3) Leucoflavoprotein + oxidised cytochrome ^ 



Flavoprotein + reduced cytochrome. 



(4) Reduced cytochrome + oxygen > 



Oxiclit^ed cytochrome + water. 



The necessity for the long series of steps between the 

 initial substrate and oxygen arises because the reactions 

 must all take place under conditions of pH. and tempera- 

 ture compatible with living cells, but must provide a 

 considerable amount of energy. Obviously if the change 

 took place in one step the reversal necessary to keep up 

 a supply of the enzyme would involve somewhat drastic 

 conditions. As a somewhat crude analogy the process 

 of enzymatic oxidation of a substrate might be likened 

 to transferring sacks full of some cargo from the deck of 

 a ship to the hold and returning the sacks to be refilled. 

 The sacks might be dropped straight into the hold, but 

 the distance might well be too great to throw the empty 

 sacks back again. If, however, the cargo were emptied 

 from the sack at deck level into one at a slightly lower 

 level, the empty sack could be handed back and refilled 

 easily, and the process carried on from level to level until 



