222 REDUCTION OF CARBON DIOXIDE CHAP. 9 



The "aromatic" stabilization of the reductant (hydroquinone) tends to make qui- 

 nones strong oxidants, but the efficient resonance stabilization of the oxidant (quinone) 

 counteracts this tendency and allows the potential to increase, from — 0.36 volt for 

 o-benzoquinone to as much as + 0.39 volt for rosindone sulfonate. 



Biocatalysts of the dye-leuco dye type have potentials up to +0.18 volt (ribo- 

 flavin). However, the potential of the latter compound decreases considerably when 

 it is associated with a protein, as in the "yellow ferment" {Eq = + 0.06 volt). 



The action of oxidation-reduction biocatalysts derived from 'pyridine ("pyridinium 

 nucleotides") is based on the transition: 



RNH2+ . RHNH2 -I- H+ 



-2H 

 They have potentials in the region of + 0.25 to + 0.30 volt (at pH 7). Similar systems 



+ 2H 



based on the oxidation of the sulfhydryl group (RS — SR v ^ 2 RSH) may have 



-2H 



potentials up to + 0.35 and + 0.40 volt (e. g. cysteine-cystine). 



The normal potentials of systems based on the conversion of ferrous iron into 

 ferric iron depend on the relative stabilization of the oxidant and reductant by complex 

 formation. They are as low as — 0.77 for nonassociated ions, and as high as 4- 0.24 

 volt for the system ferriheme-ferroheme, with hemoglobin and cytochrome c midway 

 between these two extremes (^0' = — 0.21 and — 0.26 volt, respectively). No com- 

 plex iron compounds are known which are thermodynamically capable of reducing 

 carbon dioxide or carboxyl group in a neutral medium. 



In chapter 6, mention was made of hydrogenase, an enzyme capable of reacting 

 reversibly with molecular hydrogen. The potential of this enzyme must be close to 

 that of the hydrogen electrode at pH 7, that is, + 0.42 volt. Its chemical nature is 

 as yet unknown. Its potential, although considerably higher than that of all known 

 respiratory catalysts, is still hardly sufficient to reduce directly either carbon dioxide 

 or a carboxyl group. 



To sum up: in looking for a reductant which could serve for a non- 

 photochemical reduction of carbon dioxide or of a carboxylic acid in vivo, 

 we find ourselves facing the same difficulty as we did in chapter 8, when 

 searching for an appropriate acceptor for carbon dioxide: All compounds 

 which are likely to occur in the cells appear thermodynamically incapable 

 of performing the desired function. 



4. Formation of Carboxyl Groups in Respiration. 

 The Role of Phosphorylation 



In the case of decarboxylation, we found that the least difficult to 

 reverse are the biological decarboxylations which form a part of the 

 respiration mechanism, particularly that of oxalacetic acid. It may be 

 of interest to look again at the mechanism of respiration and to inquire 

 whether in this process the formation of carboxyl groups by the oxidation 

 of carbonyl groups also approaches the ideal of reversibility. We may 

 do this by considering schemes 9.1 and 9. II, which represent one of the 

 several mechanisms of aerobic glucose metabolism in the muscle. 



