CELLULAR REGULATION 283 



source, which surely cannot be considered as the slowest process present. 

 A series of different factors can influence the resuhant of a number of 

 reactions catalysed by a system of enzymes, both their velocity and the 

 direction at alternative pathways and intersections will be affected. 



A. Enzyme Concentration 



In a chain of biochemical reactions, the concentration of an enzyme 

 affects the velocity of the metabolic reaction it catalyses. The relative 

 importance of two divergent paths at a fork in the metabolic chain can 

 therefore be influenced by the concentrations of the participating enzymes. 

 However, it is difficult to know with certainty what is the active concen- 

 tration of a given enzyme and there are only a very few measurements of 

 this type. 



B. Kinetic Characteristics 



The velocity of an enzyme reaction is defined as the change in substrate 

 concentration per minute. The velocity of the reaction depends not only 

 on the concentration of the enzyme, but also on the turnover number. 



v=W.Ce 

 V = velocity of the W = turnover number Ce = enzyme 



reaction (in moles of (in moles of substrate/ concentration 



substrate/1, per min). min per mole of enzyme). (moles/1.) 



Under conditions where the enzyme is saturated with substrate, the 

 turnover number is defined as the number of substrate molecules acted upon 

 per minute by one mole of enzyme. 



The turnover number and the enzyme concentration (see A, above) are 

 therefore to be considered separately as regulating factors rather than their 

 product which is the reaction velocity. However, conditions where the 

 enzyme is saturated with substrate are rarely realized in cells and under 

 actual conditions the Michaelis constant K^i is more useful since it 

 enables us to calculate the reaction velocity for each substrate concentration. 



C. Relation Between Thermodynamic Equilibrium 

 AND THE Stationary State 



In the case of a reversible reaction situated in the middle of a chain of 

 reactions, when the stationary state is set up the establishment of 

 equilibrium is only possible if the velocities of the reactions situated on 

 either side of the reversible reaction are sufficiently low. If they are faster 

 than the reaction in the middle, equilibrium will not be attained (see 

 p. 148). There are cases where thermodynamic equilibrium is attained and 

 where the concentrations of certain products (which, under these 

 conditions, depend on equilibrium constants) decide the subsequent course 



