o
El—SH
lJbiquitin —C—O +
ATI*
AMP + PP,
O
Ubiquitin —C—S—El
- E2—SH
El —SH
Isopeptide
bond
Figurę 32-76 Kcactions involvcd in the attachment of ubiquitin to a protein. In the first part of the process. ubiquitin's terminal carboxyl group is joined. via a thioester linkage. to El in a reaction driven by ATP hydrolysis.The activated ubiquitin is subsequenUy transferred to a sulfhydryl group of an E2 and then. in a reaction catalyzed by an E3. to a Lys e-amino group on a condemned protein, thereby flagging the protein for proteolytic degradation by the 26S proteasome.