268 UNITY AND DIVERSITY IN BIOCHEMISTRY 



bases in one of the chains, copied on a RNA chain, is read in terms of 

 amino acid succession, remains to be explained. Crick, Griffith and Orgel 

 have devised a code in which the names of the twenty amino acids are read 

 in terms of twenty triplets of bases, but this ingenious suggestion has not 

 yet been experimentally confirmed. 



We have said that we know at present a number of mutants of micro- 

 organisms, characterized by the lack of a definite enzyme. Other mutants 

 are characterized by the addition of a new enzyme. In each case we are 

 dealing with changes in structure of protein molecules resulting in the 

 loss or in the acquisition of the enzymatic activity. 



This leads to the notion that, in certain cases at least, a mutation is 

 primarily a change in the structure of a protein molecule, i.e. an aspect of 

 the molecular evolution (heteromorphic evolution, p. 336) of proteins. 

 Abnormal haemoglobins show that the alteration of a single residue may 

 deeply change the properties of a protein macromolecule. In this case, a 

 single mistake in copying the genetic code during its duplication may lead 

 to a permanent change in the properties of haemoglobin. Each half molecule 

 of haemoglobin is formed by about 300 amino acid residues. Figure 80 

 shows, in a definite small segment of this unit, the sequence of amino acid 

 residues in normal haemoglobin (HbA), in sickle cell haemoglobin (HbS), 

 in haemoglobin C disease (HbC) and in haemoglobin G disease (HbG). 

 Figure 80 makes it quite clear that, in each of these diseases, only one 

 residue, printed in italics, is altered. 



D. BlOSYNTHETIC INTERRELATIONS 



The same remarks which were made about the pathways of complete 

 degradation apply also to the pathways of biosynthesis. 



The biosynthetic pathways described here are the longest paths it has 

 been possible to trace. These paths are not always traversed from end to 

 end and they can be entered at numerous junctions. The existence of 

 transferring enzymes and the reversibility of many sections of the metabolic 

 routes, both degradative and synthetic, establish multiple interrelations 

 between the routes for biosynthesis and those for degradations. 



REFERENCES 



Anfinsen, C. B. (1959) The Molecular Basis of Evolution, John Wiley and Sons, 



New York. 

 Block, K. (1954). Biological synthesis of cholesterol. Harvey Lectures, 48, 68-88. 

 Davis, B. D. (1955). Intermediates in amino acid biosynthesis. Advanc. EnzymoL, 



16, 247-312. 

 Greenberg, D. M. (1954). Chemical Pathways of Metabolism, 2 vol. Academic 



Press, New York. 



