17417
The crystal structure of this compound in complex with human HGPRT shows that it fills or partially fills three critical locations in the active site : the binding sites of the purine base , the 5 ' - phosphate group , and pyrophosphate .
 Chemical: purine; Chemical: 5 ' - phosphate; Chemical: pyrophosphate
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: purine; Chemical: 5 ' - phosphate; Chemical: pyrophosphate