236 REDUCTION OF CARBON DIOXIDE CHAP. 9 



hydrogen or sulfur can create local conditions under which a spontaneous 

 reaction between carbon dioxide and water becomes possible. Therefore, 

 a true chemical coupling between autoxidation (e. g., 2 H2 + O2) and 

 oxidoreduction (e. g., 2B.2 + CO2) must exist, that is, at a certain stage 

 of the autoxidation intermediate products of high energy must be formed 

 which can re&,ct with carbon dioxide and thus cause a "branching" of 

 the reaction chain — one part of hydrogen being accepted by oxygen and 

 another by carbon dioxide. Our suggestion is that this branching occurs 

 in the oxidation of the intermediate H2X. 



The highest ratio between the rates of autotrophic reduction of 

 carbon dioxide and of autoxidation is found when hydrogen serves as a 

 reductant. According to Ruhland (1924), one molecule of hydrogen is 

 utilized by Bacillus pycnoticus for the reduction of carbon dioxide for 

 every 2.5 hydrogen molecules oxidized to water (c/. page 120). Gaffron 

 (1944) concluded, from experiments on adapted algae, that the limiting 

 value of this ratio is 2 (c/. page 140). In other words, out of twelve 

 hydrogen atoms which enter into the enzymatic apparatus, four are 

 "promoted" and react with carbon dioxide, while eight are "degraded" 

 by union with oxygen. Similarly, in photosynthesis, according to re- 

 actions (7.14) and (9.10), eight reduction products HX are formed, and 

 four of them are reoxidized by the oxidation products, Z, while the other 

 four are "promoted" so as to be able to react with carbon dioxide. 

 We may thus try to use scheme 9. Ill for the interpretation of chemo- 

 synthesis with hydrogen as a reductant by substituting molecular hydro- 

 gen for H2Y as a hydrogen donor, and molecular oxygen for Z as the 

 "promoting" oxidant. This leads to the following mechanism: 



hydrogenase 



(9.11a) 4 H2 + 4 X :f M H2X 



oxidase 



(9.11b) 4 H2X + 4 O2 > 4 HO2 + 4 HX 



(9.11c) 4 HX + 4 {CO2} > {CH2OI + 3 CO2 + H2O + 4 X 



(9.11d) 4 HO2 > 2 H2O + 3 O2 



(9.11) 4H2 + 02+1C02} )> {CH2OI +3H2O 



Reaction scheme (9.11) implies, in analogy to (9.10), a "half and 

 half" split of hydrogen between the two oxidants (CO2 and O2) corre- 

 sponding to a ratio of 4 for AH2 : AO2, while the largest experimeutal 

 value of this ratio is 3. This difference can be explained in several ways, 

 for instance, by the assumption that the radical HO2 does not undergo a 

 "double dismutation" into water and oxygen, as postulated in (9. lid), 

 but is first reduced by hydrogen (through the intermediary of H2Y) to 

 a peroxide and then undergoes a single dismutation: 



