Biological Assimilation and Dissimilation of Nitrogen 533 



QH 



4-HOOC-CH2-CH2-CO~R 



(2)NH2-CH2-COOH + 0=CH 

 glycine 



HxCH-COOH 



N °^ 

 pyridoxal enzyme u I J 



Ik /7\^ 



HC J-/ ^N 



( active succinate) 



OH 



->- RH 4-CO,+ HOOC •CHyCHj-CO-CHj-NHj^ 0=CH- 

 (T-aminolaevulinic acid (ALA) 



'/ \ 



{2a)HOOC-CHp-CH,-CO~R + H-^H-COOH- 



N »-M^ 



"active Succinate" 



HC- 



-^ 



-RH f HOOC • CHj- CHj- CO • CHj- NHj + CO2 +^ 

 (î-aminoiaevulinic acid . 



-+OCH'COOH + M""* 



COOH 



COOH 



(3) 



COOH 



CH2 

 CH2 -I- 

 CO 



CH2 



I 



NH2 



CHg 

 CHj 

 CO 

 CHo 



COOH CHo 

 CHp CH2 



■2H,0 



CH2 NH 



I 



NH2 



2 ALA 



pyridoxal-dependent enzymic reaction. If such were the case, there would 

 inevitably follow spontaneous formation of porphyrins. 



This speculation suggests a solution of the perplexing and very important 

 problem of early abiogenic formation of biocatalytic porphyrin pigments. 



I would like to add one comment which may be of interest with respect to the 

 problem of biopoesis. Spontaneous formation of the highly intricate structure 

 of the porphyrin molecule from small organic molecules Hke glycine and succi- 

 nate would, at the first glance, appear as an extremely improbable event. Yet 

 this molecule has a considerably lower free energy and enthalpy than the sum 

 of its precursors. This is due to the marked resonance stabilization of the 

 porphyrin ring with its extensive system of conjugated double bonds. In fact, 

 the formation of 'active succinate' is the only energy-requiring, 'uphül' step in 

 the mentioned reaction chain of porphyrin biosynthesis, and even the activation 

 energies are rather low for most of the following steps, since they can proceed 



