Superoxide dismutase of Mn (SOD)
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)