78 



CONTROL MECHANISMS IN CELLULAR PROCESSES 



by catalyzing a pair of reversible reactions ( Ichihara and Greenberg, 

 1957). 



The immediate reaction of the physiologist in contemplating the 

 delicacy of feedback control is to emphasize only its beneficial as- 

 pects. However, closer scrutiny reveals a somewhat complicating 

 consequence owing to the fact that the formation and action of a given 

 enzyme are so rigidly geared to a specific function. This conse- 

 quence is that such an enzyme lacks the versatility expected of the 

 reaction it catalyzes. 



TABLE 3-3 

 The Functions of Threonine and Serine Deaminotion in E. co// 



1. Biosynthesis of isoleucine 

 CHXH— CH— COO- 



OH NH..+ 



Threonine 



CHo— CH— coo- 



OH NH3+ 



Serine 



o 



// 



CH3CH0— c— coo 



a-Ketobutyrate 



o 



// 



CH3C— coo- 



An obligatory intermediate in 

 _ isoleucine biosynthesis 



Questionable value to the cell 



Pyruvate 



2. Anaerobic amino acid degradation 



CHXH— CH— COO 



OH NH3 + 



O 



Threonine 



CH.— CH— COO 



OH NH3 + 



-^ ^ A hydrogen acceptor 



CH3CH0— c— coo- 



a-Ketcbutyrate 



o 



// 



CH,— C— COO- 



A fermentable energy source 



Serine 



Pyruvate 



For example, in £. coli the L-threonine deaminase is also an 

 L-serine deaminase. As shown in Table 3-3, these reactions can 

 have two distinct functions. The one is in isoleucine biosynthesis. 

 The enzyme catalyzing this reaction also deaminates L-serine in vitro, 

 but it seems unlikely that it does so in the cell to endogenously 

 formed serine. A second, distinct function is in furnishing a ferment- 

 able energy source (pyruvate) and a hydrogen acceptor (a-keto- 

 butyrate) during anaerobic growth on a mixture of amino acids (e.g., 

 nutrient broth) in the absence of any fermentable sugar. Clearly, 

 the enzyme which is inhibited by isoleucine could not fulfill such a 



