Hi, Without looking at your model/maps it is difficult to make a really educated guess. By looking at the single screen shot you attach, I would say it could be a couple of different things, in order of likelihood based on a single screenshot (at whatever level of confidence this assures): 1-Zn coordinate to His, and some waters around it. 2-Phosphorylation of the His. Is this expected or known for this enzyme? Although I dont really see nice tetrahedral geometry, and I dont know that you really would at your resolution. 3-Alternate His side chain conformation. I cant really say if this fits your density with just that one screen shot. It doesnt really look like it to me. I would inspect distances and angles for each of the scenarios presented above and then build a model that makes good chemical and physical sense.
Hope his helps. yuri
