236 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



2 CH3— COOH -> CH3— CO— CH2— COOH 



• o«o •©• o 



CH3— CO— CH2— COOH -> CH3— CO— CH3 + COj 



H.C 



H.C 



sO 



C=0 + CH3 



-COOH 



H,C 



H3C 



C^ 



CH- 



-COOH 



jS-methylcrotonic acid 



H,C 



\o e o 



-> C— CH = CH2 



/ 

 H2C Isoprene 



Fig. 60 (Fukushima and Rosenfeld) — The synthesis of isoprene 

 O = carboxyl carbon • = methyl carbon 



(/) Biosynthesis of the Steroid Ring System 



The use of isotopes and mutant strains has demonstrated that acetate is 

 the essential starting material for the synthesis of the principal plant sterol, 

 ergosterol. Studies of cholesterol synthesis in animals have also provided 

 evidence of the importance of acetate. Neither formate nor labile methyl 

 groups (of methionine for example) can take part in the reaction, as 

 has been shown by the use of labelled carbon. It is most unlikely that 

 cholesterol obtains any of its carbons from a source other than the acetate 

 molecule. As shown in Fig. 61, by means of a series of chemical degrada- 

 tions it has been possible to demonstrate the origin of most of the atoms in 

 the skeleton either in the methyl or in the carboxyl of acetic acid. 



HO 



Fig 



61 (Fukushima and Rosenfeld) — Origin of the carbon atoms of cholesterol. 

 # = derived from the methyl of acetate. 

 O = derived from the carboxyl group. 



The bile acids, the sex hormones, the hormones of the adrenal cortex, 

 in animals, can all be obtained by modifying cholesterol and also directly from 



