BIOCHEMICALLY IMPORTANT COMPOUNDS 201 



of hydrophobic compounds of high molecular weight which 

 are rich in hydrocarbon groups. 



The process of the formation of petroleum, which is going 

 on at present at considerable depths, and therefore under 

 anaerobic conditions, to some extent confirms this idea. 

 Direct experiments on the synthesis of individual lipids 

 analogous to those of Miller with amino acids have, un- 

 fortunately, not yet been carried out under conditions which 

 reproduce the state of the primaeval surface of the Earth. 

 Our knowledge of the primary formation of lipids is there- 

 fore still very scanty and unreliable. It is considerably more 

 meagre than what we have in respect of carbohydrates. 



Most contemporary authors dealing with the problem of 

 the origin of life affirm Tvith complete conviction that at 

 some stage in organic-chemical evolution in the waters of 

 the primaeval ocean there must have occurred the primary 

 development of those biologically important heterocyclic 

 compounds, the porphyrins. These assertions are, however, 

 usually of a very general nature and have but little experi- 

 mental corroboration. 



Only recently, and mainly thanks to the work of D. 

 Shemin"^ and others, has there been a great increase in our 

 knoAvledge of the biosynthesis of porphyrins in living organ- 

 isms. Shemin showed that the starting substances in this 

 synthesis were fairly simple compounds, glycine and succinic 

 acids, i.e. substances which could undoubtedly have arisen 

 from the simpler hydrocarbons, ammonia and water. How- 

 ever, the actual process of biosynthesis takes place in many 

 stages and requires for its accomplishment the presence of 

 a very highly organised living system containing numerous 

 enzymes and intact protoplasmic structures. 



In this synthesis the succinic acid must first be activated. 

 In the living cell this is brought about by taking it into 

 the succinic acid-glycine metabolic cycle. In this, succinyl- 

 coenzyme A is formed and condenses with the a carbon atom 

 of glycine and in this way a-amino-^-oxoadipic acid is formed. 

 It must be noted that the condensation of succinate with 



