2 NITROGEN METABOLISM 



hydrogen peroxide. Organisms which by chance or by 

 necessity are Hving in an anaerobic environment must use 

 a substance other than oxygen for this purpose. Such a sub- 

 stance may be derived from the environment (e.g. CO 2 , 

 nitrate or acetate) or may be a product of the organism's 

 cataboHsm (e.g. in the lactic acid bacteria, pyruvate is 

 reduced to lactate). 



The esterification of inorganic orthophosphate is an 

 integral part of the mechanism whereby endergonic reac- 

 tions are able to utilize the energy made available by oxido- 

 reduction reactions. Our conception of this mechanism is 

 mainly due to Lipmann [10], who pointed out that phos- 

 phorylated compounds can be divided into two groups 

 according to the amount of energy released by their hydro- 

 lysis: some yield about 3,000 cal. per mole whilst others 

 liberate 10,000 to 16,000 cal. per mole. Lipmann proposed 

 that the latter should be known as high-energy (or energy- 

 rich) phosphate compounds and that they contain what he 

 termed high-energy (or energy-rich) phosphate bonds, the 

 hydrolysis of which yields 10,000 or more calories of free 

 energy per mole of inorganic orthophosphate liberated. The 

 significance of certain biological oxido-reduction reactions 

 lies in the fact that they are associated with the formation of 

 energy-rich phosphate bonds: these arise either during the 

 actual oxidation of the organic substrate or else during 

 the transfer of hydrogen (or electrons) to a H-acceptor. In 

 the former case oxidation of the organic substrate is accom- 

 panied by its esterification with inorganic orthophosphate 

 and in consequence most of the energy made available by 

 the oxidation reaction is not liberated as heat but is retained 

 in the oxidized substrate in association with the newly in- 

 corporated phosphate group. The only known example of an 

 energy-rich phosphate group arising by a non-oxidative re- 

 action is found in e«o/-2-phosphopyruvic acid, a substance 

 formed by the dehydration of 2-phosphoglyceric acid under 

 the influence of enolase. The phosphate groups and their 

 associated energy can be transferred, in the presence of the 

 appropriate enzyme (a phosphokinase), to adenosine diphos- 

 phate (ADP), or sometimes to adenosine monophosphate 



