176 A SYMPOSIUM ON RESPIRATORY ENZYMES 



second dissociation constant of these two acids is different, the posi- 

 tion of the equihbrium changes with pH (1, 2). 



The enzymatic phosphorylation of glycogen and starch is of in- 

 terest from the standpoint of the configuration of these polysac- 

 charide molecules. There is considerable evidence, based on the 

 hydrolysis of methylated starch and glycogen, that these polysac- 

 charides contain other than the prevalent 1—4 glucosidic linkages. 

 At points at which a branching of chains occurs, a 1—6 glucosidic 

 linkage has been postulated. Starch appears to be made up of 

 relatively straight chains consisting of 24 to 30 glucose units; this 

 explains its ability to assume a crystalline structure and to exhibit 

 well-defined x-ray diffraction patterns. Glycogen seems to be made 

 up of relatively short chains (consisting of 12 to 18 glucose units), 

 with many branchings which give it the properties of greater solu- 

 bility and lack of crystallizability. Judging from the properties of 

 various plant starches, transitions exist between these two extremes. 

 These various forms of polysaccharide raise the problem of enzyme 

 specificity. The questions are whether one and the same enzyme 

 splits (or builds up) both the 1—4 and the 1—6 glucosidic linkages 

 and what determines the special configuration of the polysaccharide 

 synthesized. It is to be noted here that phosphorylases cannot poly- 

 merize glucose-1-phosphate unless a small amount of polysaccharide 

 is added to prime the reaction (1). However, the nature of the poly- 

 saccharide synthesized seems to be solely determined by the type of 

 phosphorylase used and not by the nature of the activating poly- 

 saccharide. For example, muscle phosphorylase, when primed with 

 hver glycogen, synthesizes a typical starch in vitro, and liver phos- 

 phorylase, when primed with plant starch, synthesizes glycogen (3). 

 Important also in this connection is the fact that the same enzyme, 

 muscle phosphorylase, can synthesize both starch and glycogen, the 

 former in vitro and the latter in the intact cell. The difference in 

 activity of the muscle phosphorylase in the two situations has not 

 been explained, but it suggests that unknown environmental factors 

 and perhaps the physical state of the enzyme have something to do 

 with the nature of the polysaccharide which is formed. 



As has been stated, the phosphorylation of glycogen is an enzy- 

 matic reaction which provides for the entrance of inorganic phos- 

 phate into the phosphate cycle. All other reactions leading to the 

 uptake of inorganic phosphate are linked with oxidations. The reac- 

 tion between orthophosphate and phosphoglyceraldehyde during 

 oxidation of the latter to phosphoglyceric acid has been elucidated 



