322 7. INHIBITION IN MULTIENZYME SYSTEMS 



VI. Regenerative systeyns 



(The substance X is regenerated during the reactions.) A + X-^ B + Y 



C + Y^D + X 



V 



VII. Self-regulatory system (feedback) 

 (D affects the rate of A -> B.) A^ B-» C^ D 



VIII. Network systems M - M - M 



(Electrons flow from A to B through j j^ J^ 



., „ ^,. , A^M-M-M^B 



a grid 01 metal ions.) , , , 



^ M - M - M ^ 



Any or all of the reactions indicated may, of course, be reversible. Many 

 reactions in multienzyme systems are truly reversible but for practical 

 purposes may be considered as irreversible if the back-reaction under the 

 experimental circumstances is negligible. 



Each enzyme of a multienzyme system catalyzes a reaction which may 

 be represented in a steady state, as described in Chapter 2, since for the 

 simplest enzyme reaction there are two steps, the formation of the ES 

 complex and its breakdown. Thus each single reaction in the schemes above 

 is in reality usually complex kinetically and the steady state of a multi- 

 enzyme system implies the steady state of each component reaction. 



IRREVERSIBLE MONOLINEAR CHAINS 



The simplest multienzyme system may be represented by: 



El Ej 



A^B^C (7-1) 



where each reaction is essentially irreversible. The kinetics and inhibi 

 tion of this system can be readily extended to chains of greater length. 

 Actually there are probably relatively few simple monolinear chains in the 

 cell because most systems depend in some manner on other reactions or 

 sequences, or are parts of more complex metabolic patterns. However, 

 in isolated preparations or reconstructed systems, monolinear chains are 

 more common. Under the appropriate conditions the following may behave 

 as monolinear chains: ethanol -> acetaldehyde -^ acetate; epinephrine -^ 

 o-quinone form -> adrenochrome; ATP -> ADP -> AMP; phenylalanine -> 

 hydroxyphenylalanine -> hydroxyphenylpyruvate -^ homogentisate; and 

 succinate -> fumarate -^ malate. If reaction 7-1 is in a steady state and 

 some mechanism is provided whereby the concentrations of A and C 

 do not change, the rate of each step is the same, i.e., i\ = t',^ = v^^ where 



