REGENERATIVE SYSTEMS 365 



single step but a monolinear chain involving three enzymes. Inhibition 

 of Eo (or any enzyme in a chain taking A to B) will not reduce the rate of 

 formation of BH2 as long as (A) can rise to maintain the initial rate. How- 

 ever, inhibition of E3 will depress the rate to some degree since the rise 

 in (CoH.,) will decrease (Co), unless Co is initially at a concentration more 

 than required to saturate E^. As in scheme 7-39, the effect will depend on 

 the relative constants of Ej and E3, so that inhibition of E3 may reduce the 

 formation of BH.j to a lesser degree than expected on the basis of the in- 

 hibition of the isolated enzyme. In the example cited above, the rate of 

 lactate formation will be reduced to different degrees by inhibition of the 

 glyceraldehyde-3-phosphate dehydrogenase, enolase, or lactic dehydro- 

 genase when the inhibitors are present at concentrations depressing the 

 isolated enzymes identically. 



Certain multienzyme systems involving phosphate transfer can also be 

 represented as regenerative sequences. If the substrate must be phosphor- 

 ylated for further reaction, regeneration of the phosphate donor may 

 occur: 



El 

 A + ATP -> AP -4- ADP 



E2 

 AP -> BP (7-47) 



E3 

 BP + ADP ^ C + ATP 



A -> C 



This system will behave kinetically and in response to inhibition in an 

 identical manner to the previous sequence 7-46. On the other hand, such 

 a system may serve to form ATP if reaction 3 is an oxidation coupled with 

 the synthesis of more than one high energy phosphate bond: 



El 

 A + ATP -> AP + ADP 



E2 

 AP -^ B + P 



E3 

 B + wADP + nF ^ C + «ATP 



A + in - 1)ADP + (n - 1)P ^ C + {n - 1)ATP 



(7-48) 



During transformation of A -> C there is the over-all production of ATP, 

 such as occurs, for example, during glycolysis. An interesting feature of 

 this system is that adequate concentrations of both ADP and ATP must 

 be present for the rate to be appreciable. Since one may assume (ADP) + 

 (ATP) to be constant, any condition which makes the ratio (ADP) /(ATP) 

 very high or very low will inhibit the over-all rate. Such regulation may 

 operate in glycolysis, which may be depres.sed by either low (ADP) or 

 low (ATP) (Lynen, 1958), although there is some evidence that the gly- 



