Fig. 6. The Nζ-acetyl lysine binding site, showing electron density from the final 2mF_o – DF_c map. Nitrogen atoms are black and oxygen atoms are grey. The acetyl group is surrounded by a ring of water molecules (grey balls) at the base of the slot, and the carbonyl oxygen of the acetyl forms a hydrogen bond to the side chain of Asn407. An unacetylated lysine could not form these hydrogen bonds, and would introduce an unpaired charge into the slot.