282 LIBERATION OF OXYGEN CHAP. 11 



or (more commonly) dismuted to water and oxygen: 



catalase 



(1 1.3) 2 H2O2 > 2 H2O + O2 



Instead of hydrogen peroxide, organic 'peroxides may be formed and 

 decomposed by reactions analogous to (11.1), (11.2) and (11.3). 



In many metabolic oxidations, the formation of free peroxides is 

 avoided by the action of enzymes (oxidases), which bind oxygen and 

 release water: 



(11.4) 4RH + O2 — >4R + 2H20 



Oxygen evolution in photosynthesis may proceed by the reversal 

 of any of these mechanisms. We shall thus consider the following 

 possibilities: 



(a) the intermediary formation of hydrogen peroxide, e. g., by a 

 photochemical reversal of reaction (11.2), followed either by the reduction 

 of this peroxide by a reversal of reaction (11.1), or (more probably) by 

 its dismutation according to equation (11.3); 



(6) similar processes involving orgastic peroxides; and 



(c) oxygen evolution without the intermediate formation of free 

 peroxides, i. e., a reversal of reaction (11.4). 



2. The Hydrogen Peroxide Hypothesis 



If the primary photochemical oxidation product is an oxidized 

 intermediate, Z (c/. Eq. 7.10a), the formation of hydrogen peroxide by 

 the next step in photosynthesis can be formulated as follows: 



(11.5) 2 Z + 2 H2O > 2 HZ + H2O2 



If, on the other hand, water participates directly in the photochemical 

 process, in the form of a complex, {H2O}, the formation of hydrogen 

 peroxide can be interpreted as dimerization of the primary radicals: 



(11.6) 2 {OH) >H202 



Whether hydrogen peroxide occurs as an intermediate in respiration 

 is of considerable importance for the utilization of energy released in 

 this process, because the dismutation of one mole of hydrogen peroxide 

 according to reaction (11.3) liberates as much as 23 kcal and this energy 

 must be considered as lost for the organism. 



In discussing the thermochemical background of photosynthesis in chapter 9, we 

 stated that, as a rule, the energies of dismutations are small. However, in the case of 

 hydrogen peroxide, the difference between the strength of two single O — O bonds 

 (72 kcal) and that of a double 0^0 bond (118 kcal) is so large that the dismutation 

 of this compound into water and oxygen is strongly exothermal (c/. Table 11. 1). 



