Lasso domain with structural Zn2+. The coordination of Zn2+ is shown as present in hTK1. In Uu-TK, the coordinating amino acids are C153, C156, C191, and H194. The lasso is shown in orange for hTK1. In Uu-TK, this loop is slightly longer (shown in green). Additionally, the hydrogen bonding from the conserved Arg–Tyr couple is shown.