Biosynthesis of Branelied-Chaiii 

 Compounds 



KONRAD BLOCH 



The branching of carbon chains is a structural feature that is typical 

 of a wide array of naturally occurring compounds. Transforma- 

 tion of a straight carbon skeleton to one which is branched, i.e., sub- 

 stitution of a carbon-bound hydrogen by the bulkier alkyl group, 

 may be expected to cause profound changes of physical properties, 

 but the biochemical significance of such structural modifications is yet 

 to be defined. For the moment, it is the biological origin of branched 

 molecules which is of special interest, not only as a problem of bio- 

 genesis but also as a problem of comparative biochemistry. All living 

 forms carry out syntheses leading to branched chains, but there exist 

 notable restrictions in some organisms with regard to the type of 

 branched-chain compounds that can be formed. Thus the animal cell 

 is dependent on an outside supply of the amino acids valine, leucine, 

 and isoleucine, though it can readily form the closely related acids 

 which are precursors of the terpenes and steroids. Similarly one may 

 point to the fact that the triterpenoid hydrocarbon squalene is syn- 

 thesized by animals, although other isoprene derivatives such as the 

 carotenes are not. An inability to produce branched chains per se is 

 therefore not the cause for the exacting nutritional requirements which 

 the animal organism has developed. 



It is the main object of this essay to consider the branching of carbon 

 chains as it relates to the biogenesis of terpenes and steroids. A 

 critical discussion of this area is particularly tempting because the 

 information at hand, though considerable, has not clarified the central 

 issues. It is interesting to note and it may be stated at the outset that 

 the carbon chains of the branched amino acids on the one hand, and 

 of the terpenes and isoprene derivatives on the other, arise by path- 

 ways that are entirely distinct. For isoprene derivatives, acetic acid 

 is the sole carbon source, but this precursor does not enter directly 



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