160 UNITY AND DIVERSITY IN BIOCHEMISTRY 



course of the transfer there will be a AF of the order of 9000 cal. The 

 position of equilibrium is such that there is almost complete conversion of 

 glucose into its phosphate and consequently glucose-6-phosphate is not 

 readily able to give up its phosphate to ADP. The situation is quite different 

 when the transfer takes place with a small value of — AF as is the case in 

 the Lohmann reaction. ATP and phosphocreatine are molecules containing 

 an energy-rich bond and equilibrium between them will be established 

 when their concentrations are of about the same order. 



Among the transphosphorylase systems in which energy-rich bonds are 

 involved and the —AF is small (so that the reaction is readily reversible) 

 we may list the following : 



Creatine phosphokinase ATP+creatine ^ ADP+phosphocreatine 



Arginine phosphokinase ATP + arginine;=±ADP + phosphoarginine 



Myokinase ATP+AMP ^ 2 ADP 



Phosphoglyceric phosphokinase ATP + 3-phosphoglyceric acid :^ 



ADP + 1, 3-diphosphogIyceric acid 



Conversely, the following systems whose —AF values are high will be 

 practically irreversible : 



Hexokinase ATP+gIucose^ADP+G-6-P 



Fructohexokinase ATP-f-f ructose— >ADP+F— i — P 



Phosphohexokinase ATP+F— 6— P-^ADP+F— l,6-PP 



Galactohexokinase ATP+galactose-»ADP+Gal.— l — P 



Glucose- 1 -phosphokinase aTP-|-G— i— P-^ADP+G— 1,6— PP 



(b) Transaminases 



These enzymes were discovered in 1930 by D. M. Needham, they 

 catalyse transfers of the following general type : 



RCH— COOH + R'CO— COOH - RCO— COOH + R'CHCOOH 



I I 



I I 



NHo NH. 



An example is the glutamate-aspartate transaminase which catalyses the 

 reaction 



glutamate + oxaloacetate ^ a-ketoglutarate + aspartate, and also 

 glutamate-alanine transaminase 



glutamate -|- pyruvate r^ a-ketoglutarate + alanine. 



There also exist transaminases which catalyse the transfer of an amino 

 group from amino-purines to a-ketoglutaric acid. Pyridoxal phosphate is 

 the coenzyme for transaminations between amino acids and keto-acids and 

 also between amino-purines and keto-acids. 



