Figure 1 - full size

 

Figure 1.
Figure 1. View along the 2-fold axis. One potein molecule is shown in white with its symmetry-related molecule in yellow. On top, His87 forms a hydrogen bond accross the 2-fold axis to the very same histidine of the symmetry-related molecule. The ammonium ion (middle) and the sulphate ion (bottom) are involved in a hydrogen bonding network between the two symmetry-related molecules.

The above figure is reprinted by permission from Elsevier: J Mol Biol (2000, 295, 953-962) copyright 2000.