Ribocation Transition State Capture and Rebound in Human Purine Nucleoside Phosphorylase  Mahmoud Ghanem, Andrew S. Murkin, Vern L. Schramm  Chemistry & Biology  Volume 16, Issue 9, Pages 971-979 (September 2009) DOI: 10.1016/j.chembiol.2009.07.012 Copyright © 2009 Elsevier Ltd Terms and Conditions

Figure 1 Reactions Catalyzed by Purine Nucleoside Phosphorylase Depending on reaction conditions, the ribo-oxacarbenium-ion transition state can partition between (a) return to inosine [1], (b) reaction with phosphate to yield hypoxanthine [2] and α-D-ribose 1-phosphate [4], (c) reaction with water to yield hypoxanthine [2] and D-ribose [3], or (d) reaction with N3 of hypoxanthine to yield N3-isoinosine [5]. Chemistry & Biology 2009 16, 971-979DOI: (10.1016/j.chembiol.2009.07.012) Copyright © 2009 Elsevier Ltd Terms and Conditions

Figure 2 Views of the Subunit-Subunit Interface of Human PNP in Complex with a TS Analog and Phosphate (PO4) (A) PNP catalytic-site contact residues surrounding Immucillin-H (ImmH) (PDB 1RR6). Adjacent subunits in the trimeric protein have been colored yellow and blue. (B) Contact residues map, with distances in Å units. The two water molecules (W) closest to ImmH have been included. Chemistry & Biology 2009 16, 971-979DOI: (10.1016/j.chembiol.2009.07.012) Copyright © 2009 Elsevier Ltd Terms and Conditions

Figure 3 13C NMR (I) and 1H NMR (II) Spectra of [1′-13C]inosine, [1-13C]Ribose, and Hypoxanthine Spectra are of (A) 5.3 mM free [1′-13C]Inosine and (B) [1-13C]ribose and hypoxanthine obtained through the irreversible arsenolysis of 5.3 mM [1′-13C]Inosine in the presence of human PNP and 50 mM Na2HAsO4 (pH 7.4). Upfield signals have been omitted to highlight the downfield regions of interest. All spectra were acquired at 25°C in 10% D2O. [1] Ino, inosine; [2] Hx, hypoxanthine. Chemistry & Biology 2009 16, 971-979DOI: (10.1016/j.chembiol.2009.07.012) Copyright © 2009 Elsevier Ltd Terms and Conditions

Figure 4 13C and 1H NMR Spectra of [1′-13C]Inosine with Human PNP in the Absence of Phosphate (Panel I) 13C NMR spectra of [1′-13C]inosine with native human PNP after (A) 30 min and (B) 24 hr. (Panel II) 1H NMR spectra of [1′-13C]inosine with native human PNP after (A) 30 min, (B) 3 hr, (C) 6 hr, (D) 12 hr, and (E) 24 hr. (Panel III) 1H NMR spectra of [1′-13C]inosine with F200G after (A) 40 min and (B) 24 hr. All 13C and 1H spectra were acquired with 5.3 mM [1′-13C]inosine and 6 μM PNP in the absence of phosphate at 25°C in 10% D2O. [1] Ino, inosine; [5] N3-Ino, N3-inosine. Chemistry & Biology 2009 16, 971-979DOI: (10.1016/j.chembiol.2009.07.012) Copyright © 2009 Elsevier Ltd Terms and Conditions

Figure 5 13C NMR (I) and 1H NMR (II) Spectra of [1′-13C]Inosine with Human PNP in the Presence of Phosphate [1′-13C]Inosine (5.3 mM) was incubated (A) alone or with (B) 6 μM native human PNP, (C) 14 μM Y88G, (D) 33 μM F159G, (E) 151 μM H257G, or (F) 264 μM F200G. All spectra were acquired in the presence of 50 mM KH2PO4, pH 7.4, at 25°C in 10% D2O and after ∼2 × 107 enzymatic turnovers (20–24 h). [1] Ino, inosine; [2] Hx, hypoxanthine; [4] R-1-P, α-D-ribose 1-phosphate; [5] N3-Ino, N3-isoinosine. Chemistry & Biology 2009 16, 971-979DOI: (10.1016/j.chembiol.2009.07.012) Copyright © 2009 Elsevier Ltd Terms and Conditions

Figure 6 Hydrolysis Reactions under High Enzyme Concentrations (A) Single-turnover hydrolysis reaction kinetics, plotting the concentration of hypoxanthine (isolated on a C18 analytical column by reverse-phase HPLC) formed during the hydrolysis reaction of native (•), Y88G (•), F159G (•), H257G (•), and F200G human PNP (•) as a function of mixing time prior to acid quenching. Data were fit to Equation (1). (B) 13C NMR spectra of the PNP hydrolysis reactions. [1′-13C]inosine (900 μM) with (A) native bovine PNP (BtPNP), (B) native human PNP (HsPNP), (C) Y88G, (D) F159G, (E) H257G, and (F) F200G. All spectra were acquired after 20 hr at 25°C with 200 μM PNP in 10% D2O. [1] Ino, inosine; [3], D-ribose; [5] N3-Ino, N3-isoinosine. Chemistry & Biology 2009 16, 971-979DOI: (10.1016/j.chembiol.2009.07.012) Copyright © 2009 Elsevier Ltd Terms and Conditions