40 • PHOTOSYNTHESIS 



Rimington (1953). Porphyrins are also rapidly formed with 

 cell extracts in presence of ^-aminolaevulinic acid, the sub- 

 stance concluded to be the specific precursor of haem by 

 Shemin and Russell (1953) and by Neuberger and Scott 

 (1953). Porphobilinogen results by the condensation of two 

 molecules of ^-aminolaevulinic acid. The chemical relation- 

 ships are shown in Fig. 3.7; the four-carbon compound is 

 here represented as an 'active' succinyl group corresponding 

 with the general type of group transfer reactions associated 

 with coenzyme A (p. loi). Porphyrin of the uro-type is very 

 easily formed by the condensation of four molecules of por- 

 phobilinogen, in the absence of any biological catalyst. The 

 formation of the porphyrins corresponding to the biological 

 haem compounds depends on decarboxylation and oxidation 

 involving the side chains together with a rearrangement by 

 transfer of a CHgNHg group, prior to the completion of the 

 porphyrin ring structure. 



Granick (1950) discovered porphyrin derivatives in mu- 

 tants from a Chlorella strain and has suggested that the bio- 

 synthesis of chlorophyll and haem compounds follows a 

 common path until either iron or magnesium enters the 

 tetrapyrrolic pigment nucleus. For example: one mutant was 

 found to accumulate metal-free protoporphyrin, while in 

 another mutant the magnesium derivative of protoporphyrin 

 was found. Oxidation of this last compound could lead to the 

 magnesium-porphyrin corresponding to protochlorophyll; 

 while the final reduction yielding chlorophyll « is a process 

 known to occur in the living plant under illumination. This 

 final step in the biosynthesis of chlorophyll a has been 

 studied recently in detail by J. H. C. Smith. The reduction 

 of protochlorophyll in the plant can be initiated at relatively 

 low temperatures and depends on the absorption of light by 

 protochlorophyll itself. 



The synthesis of chlorophyll in an etiolated leaf is a rela- 

 tively rapid process and even in a normal mature leaf there 

 is evidence that chlorophyll is being constantly renewed, 

 although the rate of destruction of chlorophyll must be 

 extremely small compared with the rate of photosynthesis. 



