FIGURE 4.

Crystal structure of 3-[iodo-Tyr^B26,Nle^B29]Insulin. A, R₆ hexamer with A and B chains (black and green ribbons, respectively). Iodine atoms (green spheres) and the two axial zinc ions (red spheres) are aligned at center, each coordinated by 3-fold-related His^B10 side chains (light gray). B, superposition of WT protomer (light gray) and 3-I-Tyr^B26 analog (dark gray). Side chains of Tyr^B26 and 3-I-Tyr^B26 are shown as sticks. For clarity, the iodine atom is shown as a transparent sphere; Nle^B29 is not shown.