This SUMO (green) is linked through its C-terminal glycine (yellow) to a lysine (magenta) in a ubiquitin-conjugating enzyme (blue).

Pichler A, Knipscheer P, Oberhofer E, van Dijk WJ, Korner R, Olsen JV, Jentsch S, Melchior F, Sixma TK.
SUMO modification of the ubiquitin-conjugating enzyme E2-25K.
Nat Struct Mol Biol. 2005 Mar;12(3):264-9(PubMed)

2bf8 (PDB)