Glycogen 29 



giant molecule is built up and broken down according to the de- 

 mands placed upon it in health and disease. 



Glycogen, like amylopectin, the branched fraction of starch, 

 is composed of the monomer anhydro-glucose, existing as units 

 which are joined together by a - 1,4 glucosidic linkages to form 

 chains, with branches at a - 1, 6 glucosidic linkages. 



Three models have been proposed for glycogen : 



1. The Staudinger model consists of a long chain of glucoses 

 bound by a - 1, 4 links with unbranched side chains attached 

 to the main chain at branch points by a - 1, 6 links. 



2. The Haworth model consists of a number of chains attached 

 one to the next one at branch points by a - 1, 6 links, so that each 

 chain has but one branch point. 



3. The Meyer model consists of a combination of the two so that 

 each chain may branch further whether its origin be a 1,4 link 

 or a 1,6 link at the branching glucose. 



Rabbit liver glycogen, which has been most studied, appears to 

 possess a structure half way between the Haworth and ideal Meyer 



models. 



Glucose is built up into glycogen in the following manner (Cori, 



1952-3) : 



1. The C6 atom of glucose is phosphorylated in all normal cells 

 with adenosine triphosphate (ATP) as donor of phosphorus and 

 hexokinase as enzyme common to all tissues. In this way glucose-6- 

 phosphate is produced through the expenditure of energy and so 

 the energy level of glucose is raised and it is prepared for its many 

 pathways in metabolism. 



2. Glucose-6-phosphate is catalysed by the enzyme phospho- 

 glucomutase and Mg ion to glucose- 1 -phosphate. 



3. The enzymes phosphorylase and amylo (1, 4—1, 6) trans- 

 glucosidase ("brancher enzyme") catalyse glucose- 1 -phosphate to 

 glycogen. The build up of glycogen thus depends on the activities 

 of the phosphorylase, which makes or breaks the 1,4- linkage, and 

 the brancher enzyme amylo (1, 4 — 1, 6) transglucosidase. Phos- 

 phorylase lengthens the outer polysaccharide chains until they 

 reach the critical length of at least 8 residues at which they become 



