Superoxide dismutase of Mn (SOD) I have a little problem with my paper about Mn superoxide dismutase. On this picture we have "B" behind the 3 OH ions, I really don't understand what is this. Where is this "B" from? Please, help me, my lecture will not accept my work today.
That ain't MnSOD, it's arginase. 

B = boron

The molecule in red is 2(S)-amino-6-boronohexanoic acid. 

See figure 2 of Arginase−boronic acid complex highlights a physiological role in erectile function Nature Structural Biology 6, 1043 - 1047 (1999)