I have to prepare the pdb structure 1K4T to my study. When I do the dock prep process, it gives net charges as,

PTR+DT -2

TGP +0

I added charges for standard resides by AMBER ff12SB. For other residues, I added charges by Gasteiger method.

My problem is why PTR+DT gives -2 charge? I am not familiar with this one. 

Can anyone help me?

Thank you.