224 



REDUCTION OP CARBON DIOXIDE 



CHAP. 9 



^Pyruvic acid 

 (regenerated) 



Pyruvic acid 

 (from triose) 



Fumaric acid 



HOOC-CH^CH-COOH 



Malic acid 



HOOC-CHOH-CH2- COOH 



J 



2{h} 



r 



Oxalacctic acid 

 O- 



JL 



1(1} 



{CO,} 



Scheme 9.II.— The C4-dibasic acid cycle (net reaction 9.6). 



However, nature has found a way of conducting these two oxida- 

 tions without a dissipation of energy — by couphng them with endergonic 

 phosphorylations {i. e., transfers of phosphoric acid from orthophosphate 

 to an "energy rich" organic phosphate), or transphosphorylations (i. e., 

 transfers of phosphate from a low energy organic ester to a "high- 

 energy" ester). 



The effect of phosphorylation is to destroy the resonance which 

 stabilizes the carboxyl group and thus to make its hydrogenation easier. 

 Carhoxyl phosphates belong to the "high-energy organic phosphates" 

 which play an important role in the energy balance of many biochemical 

 processes (c/. the reviews by Kalckar 1941, and Lipmann 1941). The 

 hydrolysis of these esters liberates about 10 kcal per mole (while the 

 hydrolysis of "low-energy" phosphates has a heat effect close to zero). 

 Carbonyl phosphates, on the other hand, must be "normal" (since no 

 resonance energy is gained by their hydrolysis) ; therefore, the reduction 

 of a carboxyl phosphate to a carbonyl phosphate: 



(9.7) 



O 



R— C— OH2PO3 + 2 {H) 



OH 



I 

 -^R— CH 



OH2PO3 



should require 10 kcal less total energy than the hydrogenation of a 



