Figure 82.

A. The precatalytic complex of PHM showing the relative position of bound substrate, N-acetyl-diiodotyrosyl-D-threonine (IYT), and dioxygen at Cu_M site. The 2F_o-F_c electron density is shown for dioxygen and the IYT peptide. Substrate and protein atoms are colored by atom type; iodine atoms are purple. The water molecule is represented by a red sphere and molecular oxygen by a red rod. Dotted lines indicate hydrogen bonds and bonds to the copper atoms (green spheres). B. The structure of the dioxygen binding site showing the precatalytic complex. Dioxygen (the red rod) is shown bound to Cu_M (the green sphere) in an end-on manner. Amino acid ligands to Cu_M are shown colored by atom type. Simulated annealing difference omit maps that leave out either both oxygen atoms (red mesh) or the distal oxygen atom of dioxygen (blue mesh) are shown contoured at 8σ. In this crystal structure, Cu_M-O-O angle is 110.2°, Cu_M-O_proximal is 2.106 Å, the O-O distance is 1.232 Å and the distance between the α-C in the substrate and the Cu_B-O-O is about 4-5 Å. C. The backbone of IYT and the anchor residuals are shown: Arg240 forming a salt bridge, the hydroxyl of Tyr318 and the side chain amide of Asn316 both in hydrogen bonds to the D-threonine main-chain amide. The hydrogen bonds are shown in dotted lines. D. A structure-based model of substrate dioxygen interaction. The structure of the PHM active site is oriented looking down the copper-oxygen bond. The modeled position of the substrate hydrogen atom that is abstracted during the PHM reaction is shown in turquoise. Rotating the copper-oxygen bond by 110° brings the terminal oxygen of dioxygen within 2.2 Å of the substrate hydrogen, a position consistent with hydrogen abstraction by an activated oxygen species. (From Ref. ¹²⁵. Reprinted with permission from AAAS.)