STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 163 



Mention may be made of a group of reversible phosphorylytic reactions 

 of this type in which ATP is in equilibrium with ADP and P. It may be 

 supposed that kinases and phosphorylases act in sequence as follows: 



kinase 



X + ATP ^^ X . P + ADP 

 X.P+Y^X.Y + P 



For example: 



carbamate (NHg + CO.,) + ATP ^ carbamyl phosphate + ADP 



carbamj'l phosphate + aspartate ^ carbamyl aspartate (xireidosucciiiate) + P- 



Intact mitochondria have an apparent adenosine triphosphatase which 

 catalyzes an exchange of the terminal phosphate of ATP and inorganic 

 phosphate, possibly via a phosphorylytic mechanism. A similar soluble 

 enzyme has been isolated from these structures (Plant, 1957). It is suspected 

 that such an enzyme may be involved in the phenomenon of oxidative 

 phosphorylation. 



5. On PhospJioproteins 



A number of protems have been found to contain phosphorus bound 

 directly to amino acids. For example, for many years casein has 

 been known to contain P bound to the primary alcohol of serine, i.e., 

 HOOCCHNHgCHoOPOgHg. Most recently, serine has been implicated in 

 the active site of several esterases and proteases. ]\Iost significantly, serine 

 phosphate has also been isolated from hydrolyzates of yeast hexokinase and 

 from phosphoglucomutase, as summarized by Kennedy and Kosliland (1957). 

 In the case of hexokinase, it is supposed that the phosphorylated protein is 

 in fact the active form of a phosphotransferase (Agren and Engstrom, 1956a) 

 as the phosphorylated form of phosphoglucomutase has been shown to be 

 (Najjar and Pullman, 1954; Sidbury and Najjar, 1957). Engstrom and Agren 

 (1956) have similarly isolated radioactive phosphoserine from phosphorylase 

 incubated with glycogen and radioactive P^^. Agren (1956) and Agren and 

 Engstrom (1956b) have also found major fractions of the phosphoproteins in 

 the cell walls of E. coli and erythrocytes. It seems possible that these cell 

 wall proteins, embodying phosphoserine, are the active metabohc centers of 

 phosphate transfer for many enzymatic systems. 



B. Polysaccharide Biosynthesis 



1. Phosphorylases and Other Enzymes 



The reversible phosphorylase reaction of glycogen biosynthesis has been 

 noted in a previous section. The enzyme occurs in many animal tissues, 

 higher plants, and some bacteria, and does not act unless a small amount of 



