BIOSYNTHESIS 263 



containing eight glucose units, then a new enzyme comes into play: this is 

 the branching enzyme. It is a transglucosidase, amylo-1, 4 -> 1,6-trans- 

 glucosidase, which converts a 1-4 linkage into a 1-6 linkage thus producing 

 a fork in the chain. 



We see that whereas the concentration of glycogen in a cell can be 

 influenced by the requisite enzymes, by hormones and by other possible 

 regulators, the structure of the glycogen molecule depends on the relation 

 between the respective activities of phosphorylase, the branching enzyme 

 and the unbranching enzyme. 



2. Synthesis of amylopectin 



The so-called "Q-enzyme" is a transglucosidase which converts one 

 in every twenty 1-4 linkages of amy lose into 1-6 linkages to form a branched 

 chain. The enzyme only acts when at least forty-two glucose residues have 

 been united to form amylose. The formation reaction is the following : 



a-D-glucose-1 -phosphate ^ amylose — ^ amylopectin 



Like the branching enzyme, the "Q-enzyme" acts like a transglucosidase 

 capable of forming 1-6 bonds. 



3. Synthesis of ^ linkages of polysaccharides 



Fitting and Doudoroff (1952) have described an enzyme which in the 

 course of the synthesis of a glycoside linkage causes inversion of the type 

 of bond. 



a-D-glucosyl-D-glucose -f P ^ ^-D-glucose-1-P + D-glucose 



This demonstrates the possibility of the formation of complex poly- 

 saccharides containing /S-linkages by the action of enz^Tnes causing trans- 

 glucosidation. 



(6) Proteins 



By analogy with what is observed when amide bonds are synthesized in 

 peptides or amides, Borsook has suggested that during the synthesis of 

 proteins from free amino acids, it is the carboxyl of one of the amino acids 

 which is activated by utilization of a pyrophosphate linkage, the amino 

 group of the other amino acid not needing to be activated before formation 

 of the peptide bond. There are many arguments in favour of the idea that 

 proteins are synthesized from free amino acids. The activated amino acids 

 are transported on to a "template" where they arrange themselves in a 

 definite order. The peptide linkages are established for the price of the 

 energy of the pyrophosphate bonds and the molecule thus formed is 

 detached from the template. 



