ENZYMES 159 



C. Transferring Enzymes (Transferases) 



Assembled in this group are the transferring biocatalysts having acceptors 

 other than water or phosphoric acid. They catalyse the transfer of a group 

 from one compound to another, and, in certain cases, they also transfer 

 the energy of the bond to which the group was attached. This point is 

 particularly important in biochemical energetics. 



D— X + E ?^ D— X— E ^ D + E— X 

 E— X -f A ^ A— E— X ^ A— X + E 



As the equations show, when the transferred portion X combines with the 

 enzyme E before passing to the acceptor A, the energy of the D — X bond 

 is not dissipated but is also transferred with little energy loss. In such a case 

 the system is a reversible one and there is only a small change in AF. 



(a) Transphosphorylases 



( Transphosphatases, Phosphokinases) 



These important enzymes are numerous and universally present in the 

 biosphere. The Lohmann enzyme or creatine-phosphokinase has been 

 known for a long time. It catalyses the transfer of a phosphoric acid 

 residue from adenosine triphosphate to creatine (Lohmann reaction). 



NH 



il 

 ATP + H2N— C— N— CH2— COOH ^ 



CH3 



Creatine 



O NH 



II !i 



ADP -^ HO— P— NH— C-N— CH— COOH 



OH CH3 



Hexokinase transfers a phosphate group from ATP to glucose with the 

 formation of glucose-6-phosphate, and fructohexokinase performs the 

 same transfer to fructose forming fructose- 1 -phosphate. The trans- 

 phosphorylases can be divided into several types according to the 

 magnitude of AF for the transfer reaction. 



Two examples will illustrate this. 



Let us take the case of hexokinase which transfers a phosphate residue 

 from ATP to glucose with the formation of ADP and glucose-6-phosphate. 

 The hydrolysis of the pyrophosphate linkage of ATP gives 12,000 calories 

 and that of glucose-6-phosphate only yields 2,000-4,000 cal. During the 



