Oxidative Pathways of Carbohydrate Metabolism 157 



purified glucose dehydrogenase preparations has given two 

 different values of the equilibrium constant (at 21°): 



(Gluconolactone) (DPNH) (H+) ^ ^ _ , _ 

 (Glucose) (DPN+) 



respectively (Brink, 1953; Strecker and Korkes, 1952, re- 

 calculated. Brink, 1953). These correspond with AG' values 

 (pH constant = 7) of this reaction of —0-7 or —2 kg. cal., 

 respectively. From the values cited by Krebs and Kornberg 

 (1957), AG' for the hydrolysis and neutralization of the 

 lactone is about —5 to —6-3. It is assumed in Fig. 2 that the 

 lower values are the more reliable, and that they may be 

 applied to the phosphorylated substrates and to the use of 

 TPN instead of DPN, with either of which coenzymes Brink 

 (1953) reports equal reaction rates (at pH 7-6). On this 

 evidence, the neutralization of the acid would represent the 

 difficultly reversible stage in the HMP oxidative pathway. 

 For the second TPN oxidative stage : 



6-PG3- + TPN+ -> Ru5P2- + CO2 H- TPNH 



the equilibrium constant, K, has been estimated by Horecker 

 and Smyrniotis (1952) to be quite close to that of the similar 

 i^ocitric dehydrogenase reaction, namely 



(6-PG) (TPN+) ^ ^ , 1 n N 1 



K = ^^ — =1-9 (moles/l.)-i. 



(Ru5P) (CO2) (TPNH) ^ ' ^ 



From this the AG' at pH 7 is approximately —0-4 kg. 

 cal. /mole. These values of AG' are used in Fig. 2, which also 

 gives for comparison the approximate free energy changes of 

 related reactions. Those for the glycolytic reactions are 

 taken from the excellent review by Krebs and Kornberg 

 (1957), except that values for phosphatase hydrolysis have 

 been added. Despite some uncertainty about these values 

 (Burton and Krebs, 1953), they are probably not very 

 seriously in error. 



