262 



Essays in Biochemistry 



torn" three carbon atoms of F-6-P, yielding II. In a steady state the 

 proportion of I and II in the pool of F-6-P (III) will depend on the 

 rates of supply of F-6-P (I) and triose phosphate and on the rate of 

 exchange between them. 



(1) 



(i) 

 1C 



I 

 2C 



I 

 3C 



4C 



(2) 



3 = 4C 



I I 



5C + 2 = 5C 



I I 



6 COP 1 = 6 COP 



1 = 



I -> FDP -> 2 = 5 C 



I 

 3 = 4 COP 



(ii) 

 1C 



I 

 2C 



I 



TA 3 C 



I 



3 = 4C 4C 



2 = 5 C + 5 C 



1 = 6 COP 6 COP 

 6C 



14-11 = 



FDP 



(IV) 



1 = 6C 



2 = 5C 



I 



3 = 4 C 



3 = 4C 



2 = 5 C 



I 

 1 = 6 COP 



I 4- IV = 



(V) 



1 > 6C 



I 



2 > 5C 



I 



3 > 4C 



I 



4 > 3C 



I 



5 >2C 



6 > 1 COP 



Fig. 3. Possible effects of transaldolase (TA) and fructose diphosphatase (FDP) on 

 the distribution of carbon in the fructose-6-phosphate (F-6-P) pool. 



Alternatively, in reaction 2 (Fig. 3), part of the F-6-P may arise 

 by hydrolysis of fructose diphosphate (FDP), in a fraction of which 

 G-l, 2, 3 and G-6, 5, 4 have been equilibrated through the action 

 of aldolase and triose isomerase. The resulting F-6-P is given by IV 

 and the F-6-P pool by V. If both reactions 1 and 2 occur, a different 

 pool will be formed.* 



* The enzymes involved in reaction 1 are known to be present in E. coli, but 

 the exchange reaction indicated for transaldolase in this sequence, though expected, 



