II ENERGY-RICH PHOSPHATE BONDS 2"] 



E. Generation of Energy- Rich Phosphate Bonds 



I. Substrate phosphorylation 



Part of the energy liberated as a result of catabolic processes is conserved in the 

 form of nucleotide compounds containing energy-rich phosphate bonds. These 

 compounds differ from most of the esterified phosphorus compoimds in that 

 approximately 10-16 kcal of free energy rather than 2-5 kcal are liberated as a 

 result of the hydrolysis of the phosphate linkage. 



Mechanisms for the generation of high-energy phosphate bonds may be classi- 

 fied into three categories: i) substrate phosphorylation, 2) oxidative phosphory- 

 lation, and 3) photosynthetic phosphorylation. Substrate phosphorylation and 

 oxidative phosphorylation occur in most organisms; photosynthetic phosphory- 

 lation takes place in the chloroplasts of plants. In the case of oxidative phosphory- 

 lation, the formation of the energy-rich bond is coupled with the oxidation of 

 DPNH or reduced flavin through the mitochondrial electron transport system. 

 We shall discuss oxidative phosphorylation and photosynthetic phosphorylation 

 in the next section. Let us first direct attention to some examples of substrate 

 phosphorylation. 



(a) 77?^ oxidation of glyceraldehyde phosphate to j-phosphoglyceric acid 



There are two substrate phosphorylation reactions in the glycolytic sequence. 

 a) The first of these occurs during the oxidation of glyceraldehyde phosphate to 



1,3-diphosphoglycerate: (Fig. 11; Krimsky and Racker, 1955). In the proposed 



CONH 



CONH, 



CONH, + H 



OPCH, 



Fig. 1 1. Postulated mechanism for the glyceraldehyde phosphate dehydrogenase reaction. 



sequence of events, a DPN^-enzyme compound interacts with the aldehyde sub- 

 strate to yield reduced DPN"^ and an acyl enzyme, in which the acyl moiety of the 

 oxidized substrate is attached to a sulfur atom of the protein ; this thiol ester is 

 split by inorganic phosphate to yield the regenerated SH-enzyme and 1,3-di- 

 phosphoglycerate, a compound containing the energy-rich acyl phosphate bond. 

 Glutathione is a cofactor of glyceraldehyde phosphate dehydrogenase. One mole 



Literalure p. 124 



