FORMATION OF CARBOXYL GROUPS IN RESPIRATION 



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A - 



Glucose (CgHiaOg) 



^2 Triose (C3H8O3 

 ( Glyccraldehydc) 

 -4H 



B < 



♦ dehydrogenase — ►to O2 



2 Glyceric acid (C3H5O4) 



-2H^0 



Oxalacotic 'COg 

 decarboxylose 



2 Pyruvic acid(cnol) (CH2 = C0HC00H) 

 2.Pyruvic acid (CH3COCOOH) 



^^Og Pyruvic 



decarboxylase 



Dehydrogenases 



i 



toO, 



Scheme 9.1. — Respiration mechanism. 

 Net reaction: CsHizOe + 3 H2O > 3 CO2 + 14 H + C3H4O3 (pyruvic acid). 



In stage A of scheme 9.1, a hexose molecule is split into two molecules 

 of a triose, which, in stage B, are oxidized to two molecules of pyruvic 

 acid (via glyceric acid and enol-pyruvic acid). In stage C, a transfor- 

 mation of two molecules of pyruvic acid leads to the regeneration of one 

 of them, and the disintegration of the other one into three carbon dioxide 

 molecules (liberated through the intermediary of specific "decarboxyl- 

 ases") and ten hydrogen atoms, which are transferred through specific 

 "dehydrogenases" to oxygen as the final acceptor. 



(9.6) 



2 CH3COCOOH 



-> CH3COCOOH + 3 CO2 + 10 {H} 



Not all details of cycle C have been worked out, and they may 

 possibly vary from case to case; but scheme 9. II gives a simplified form 

 of this cycle (based on suggestions by Thunberg). 



According to schemes 9.1 and 9. II, carboxyl groups are created in 

 respiration by the oxidation of glyceraldehyde to glyceric acid, and of 

 acetaldehyde to acetic acid. The potentials of these carbonyl-carboxyl 

 systems in neutral solution lie, according to table 9. IV, between + 0.5 

 and + 0.6 volt. The "dehydrogenases," which accept the hydrogen 

 atoms from glyceraldehyde and acetaldehyde, are pyridinium nucleotides, 

 whose potentials Ue between + 0.2 and + 0.3 volt. Thus, the hydrogen 

 transfer from the aldehydes to the dehydrogenases should liberate a 

 considerable amount of free energy, and thus be practically irreversible. 



