152 GENERAL BIOCHEMISTRY 



Succinic dehydrogenase occurs in insoluble form in an enzymatically 

 active particle. It appears to be a flavoprotein complexed with cyto- 

 chrome b. The dehydrogenases are essential to the functioning of all 

 metabolizing cells. 



Transphosphorylases 



This group of enzymes fits into the important and universal class 

 of transferring enzymes. Transphosphorylases catalyze exchanges of 

 phosphate groups from one substrate or coenzyme to another. Most 

 are rather specific with definite requirements for substrate, coenzyme, 

 and activator. The reactions are fundamental to the energy transfers 

 of cells and participate in the conversion of the chemical energy of 

 substrates into energy for growth, repair, body heat, and mechanical 

 activity. Additional information on these processes appears in Chap- 

 ter 7. 



Hexokinase, also called glucokinase, has already been mentioned. 

 It transforms glucose and ATP into glucose-6-phosphate and ADP 

 when activated by magnesium ions, thus beginning the utilization of 

 glucose. Corresponding enzymes phosphorylate fructose, galactose, 

 pyruvate, arginine, and other related compounds and their derivatives. 



Transglycosidases 



These enzymes are important in carbohydrate syntheses and transfer 

 monosaccharide units from one substrate to another. These reactions 

 are probably all reversible, serving either the formation or the break- 

 down of carbohydrates, depending on the needs of the cell. For 

 example, phosphorylase catalyzes the formation of reserve carbohy- 

 drates when activated by magnesium. 



Glucose-1 -phosphate + (glucose) „ ^ (glucose) „+! -f HPO4'" 



The invertases fall in this group, converting sucrose into glucose and 

 fructose by way of larger carbohydrates. Enzymes of this group also 

 form sucrose and maltose. 



sucrose 



Glucose-1 -phosphate + fructose ' sucrose -j- HP04^ 



phosphorylase 



Still others transfer ribosc to pyrimidines, purines, and nicotinamide 

 to form the nucleosides. 



Transpeptidases 



Transpeptidases transfer or exchange amide, hydroxylamine, or 

 amino acid groups. Although these activities are fairly well known, 

 the extent of their biological occurrence is still in doubt. 



