38 INTERMEDIARY METABOLISM AND GROWTH I 



products of the reactions catalyzed by choline (ethanolamine) kinase are inter- 

 mediates in phospholipid synthesis. 



The free energy of hydrolysis of the phosphate esters which are the products of 

 the Group I reactions is approximately 2-4 kcal/mole. Consequently, the reactions 

 of Group I are essentially irreversible. 



B. Nucleotide Transfer Reactions 



The reactions of Group II, are reversible reactions. Reactions 35, 36, 39, and 53, 

 when written from right to left, will be recognized as the substrate phosphoryl- 

 ation reactions discussed previously. Reactions 41 to 52 and 54 to 56 are nucleoside 

 monophosphate and nucleoside diphosphate kinase reactions. As a result of these 

 reactions, the entire series of nucleotide mono-, di-, or tri-phosphates may readily 

 be generated. Reactions 33, 34, 38, and 40 will be discussed later. 



C. Uridyl Transferase Reactions 



Reaction 66 is catalyzed by the enzyme, uridyl transferase, which has been 

 observed in yeast, spinach leaves, and various animal tissues (Leloir and Cardini, 

 1956; Mills et al., 1954). In liver tissue, the enzyme is apparently localized in the 

 nuclei of the cells. UDP-acetyl glucosamine may substitute for UDPG as a sub- 

 strate for this enzyme (Reaction 67). A similar yeast enzyme catalyzes the for- 

 mation of UDP-galactose from galactose- 1 -phosphate (Reaction 73) (Kalckar 

 et al., 1953) while a guanylic transferase functions in the synthesis of GDP-mannose 

 from mannose- 1 -phosphate (Reaction 65) (Munch-Peterson, 1955a, 1955b). A 

 viridyl transferase enzyme of mung bean seeds catalyzes the formation of UDP- 

 xylose from a-D-xylose-i -phosphate (Reaction 60) (Ginsberg et al., 1956). 



Purified uridyl transferase of yeast catalyzes an exchange of 32p_32p ^ith 

 UTP in the absence ofglucose-i -phosphate (Munch-Peterson, 1955a). The enzyme 

 also catalyzes an exchange of the phosphate of glucose- 1 -phosphate -^^P- or ^"^C- 

 glucose- 1 -phosphate with UDPG in the absence of pyrophosphate. On the other 

 hand, no exchange of ^'*G of UMP-^''C takes place in the presence of UTP or 

 UDPG. The following mechanism is consistent with the experimental observa- 

 tions: 



a) UDPG + enzyme ■•■ — >■ UMP ~ Enzyme + glucose- i-^^P 



1 32p_32p 



UTP + enzyme 



The same mechanism is involved in the 32p_32p exchange reactions with GTP 

 and ATP which are catalyzed by crude yeast enzyme extracts. 



The reactions mentioned above are of significance in connection with the syn- 

 thesis of hyaluronic acid and other structural polysaccharides. Considerable 

 progress has been made recently in the elucidation of the biosynthetic pathways 

 of hyaluronic acid synthesis. Let us digress for a moment to summarize the results 

 of this research. 



