24594
X - ray crystallographic analysis of the yeast proteasome in complex with 3e revealed that the ligand accommodates predominantly to the primed substrate binding channel and covalently binds to the active site threonine residue via its beta - lactone ring - opening .
 threonine, beta - lactone
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 threonine, beta - lactone