102 PHOTOSYNTHESIS 



reduction takes place in a second step. For example, it has 

 been found more easy to reduce a carboxylic ester than the 

 free acid: the formation of the ester eliminates the ionization 

 of the carboxyl group. This is the principle of a well-known 

 reversible biological system, where the oxidation and reduc- 

 tion of an aldehyde/acid system is essentially a two-stage 

 process. 



The triose phosphate dehydrogenase. This enzyme catalyses 

 the first of two steps in the oxidation of an aldehyde to a 

 carboxylic acid; the reaction being as follows: 



Triose phosphatc + DPN+ + phosphate -^ 



diphosphoglyceric acid+DPNH 



Diphosphoglyceric+ADP -^ phosphoglyceric+ATP 



In the second step the free phosphoglyceric acid is formed 

 by the action of a phosphokinase which transfers the phos- 

 phate group attached to the carboxyl group to adenylphos- 

 phate or to adenylic acid. Both reactions are freely reversible. 

 They are concerned with the breakdown of carbohydrate in 

 both fermentation and in respiration of carbohydrate via 

 pyruvate. The triose phosphate dehydrogenase is inhibited 

 by iodoacetate, and it is considered that the activity of the 

 enzyme depends upon the presence of an SH group. Racker 

 and others have considered that the specific sulphur atom 

 belongs to glutathione which is combined with the enzyme. 

 The SH group is regarded as being in combination with 

 the coenzyme, DPN+, in the form of a neutral DPN-S- 

 compound. The glyceraldehyde phosphate, represented by 

 R.CHO reacts by displacing DPNH and forming a R.CO- 

 S-compound (which may be compared with acetyl co- 

 enzyme A). The acyl thio-link is then broken by a simul- 

 taneous reaction with more DPN+ and inorganic phosphate, 

 HOPO3H2, giving R.COOPO3H2, the original DPN-S- 

 compound and one H+. This is in accord with the fact that 

 the H of the aldehyde group is actually transferred to the 

 bulk concentration of DPN+ forming DPNH. This mech- 

 anism shows how an oxidation-reduction step involving two 

 equivalents of hydrogen may virtually take place in stages. 

 A triose phosphate system has been found in the green 



