Plasmodium falciparum purine nucleoside phosphorylase is critical for viability of malaria parasites

Human malaria infections resulting from Plasmodium falciparum have become increasingly difficult to treat due to the emergence of drug-resistant parasites. The P. falciparum purine salvage enzyme purine nucleoside phosphorylase (PfPNP) is a potential drug target. Previous studies, in which PfPNP was targeted by transition state analogue inhibitors, found that those inhibiting human PNP and PfPNPs killed P. falciparum in vitro. However, many drugs have off-target interactions, and genetic evidence is required to demonstrate single target action for this class of potential drugs. We used targeted gene disruption in P. falciparum strain 3D7 to ablate PNP expression, yielding transgenic 3D7 parasites (Deltapfpnp). Lysates of the Deltapfpnp parasites showed no PNP activity, but activity of another purine salvage enzyme, adenosine deaminase (PfADA), was normal. When compared with wild-type 3D7, the Deltapfpnp parasites showed a greater requirement for exogenous purines and a severe growth defect at physiological concentrations of hypoxanthine. Drug assays using immucillins, specific transition state inhibitors of PNP, were performed on wild-type and Deltapfpnp parasites. The Deltapfpnp parasites were more sensitive to PNP inhibitors that bound hPNP tighter and less sensitive to MT-ImmH, an inhibitor with 100-fold preference for PfPNP over hPNP. The results demonstrate the importance of purine salvage in P. falciparum and validate PfPNP as the target of immucillins.     

Remote mutations and active site dynamics correlate with catalytic properties of purine nucleoside phosphorylase

It has been found that with mutation of two surface residues (Lys²² → Glu and His¹⁰⁴ → Arg) in human purine nucleoside phosphorylase (hPNP), there is an enhancement of catalytic activity in the chemical step. This is true although the mutations are quite remote from the active site, and there are no significant changes in crystallographic structure between the wild-type and mutant active sites. We propose that dynamic coupling from the remote residues to the catalytic site may play a role in catalysis, and it is this alteration in dynamics that causes an increase in the chemical step rate. Computational results indicate that the mutant exhibits stronger coupling between promotion of vibrations and the reaction coordinate than that found in native hPNP. Power spectra comparing native and mutant proteins show a correlation between the vibrations of Immucillin-G (ImmG):O5′…ImmG:N4′ and H257:Nδ…ImmG:O5′ consistent with a coupling of these motions. These modes are linked to the protein promoting vibrations. Stronger coupling of motions to the reaction coordinate increases the probability of reaching the transition state and thus lowers the activation free energy. This motion has been shown to contribute to catalysis. Coincident with the approach to the transition state, the sum of the distances of ImmG:O4′…ImmG:O5′…H257:Nδ became smaller, stabilizing the oxacarbenium ion formed at the transition state. Combined results from crystallography, mutational analysis, chemical kinetics, and computational analysis are consistent with dynamic compression playing a significant role in forming the transition state. Stronger coupling of these pairs is observed in the catalytically enhanced mutant enzyme. That motion and catalysis are enhanced by mutations remote from the catalytic site implicates dynamic coupling through the protein architecture as a component of catalysis in hPNP.     

Second-sphere amino acids contribute to transition-state structure in bovine purine nucleoside phosphorylase

Transition-state structures of human and bovine of purine nucleoside phosphorylases differ, despite 87% homologous amino acid sequences. Human PNP (HsPNP) has a fully dissociated transition state, while that for bovine PNP (BtPNP) has early SN1 character. Crystal structures and sequence alignment indicate that the active sites of these enzymes are the same within crystallographic analysis, but residues in the second-sphere from the active sites differ significantly. Residues in BtPNP have been mutated toward HsPNP, resulting in double (Asn123Lys; Arg210Gln) and triple mutant PNPs (Val39Thr; Asn123Lys; Arg210Gln). Steady-state kinetic studies indicated unchanged catalytic activity, while pre-steady-state studies indicate that the chemical step is slower in the triple mutant. The mutant enzymes have higher affinity for inhibitors that are mimics of a late dissociative transition state. Kinetic isotope effects (KIEs) and computational chemistry were used to identify the transition-state structure of the triple mutant. Intrinsic KIEs from [1'-3H], [1'-14C], [2'-3H], [5'-3H], and [9-15N] inosines were 1.221, 1.035, 1.073, 1.062 and 1.025, respectively. The primary intrinsic [1'-14C] and [9-15N] KIEs indicate a highly dissociative SN1 transition state with low bond order to the leaving group, a transition state different from the native enzyme. The [1'-14C] KIE suggests significant nucleophilic participation at the transition state. The transition-state structure of triple mutant PNP is altered as a consequence of the amino acids in the second sphere from the catalytic site. These residues are implicated in linking the dynamic motion of the protein to formation of the transition state.     

Tryptophan-free human PNP reveals catalytic site interactions

Human purine nucleoside phosphorylase (PNP) is a homotrimer, containing three nonconserved tryptophan residues at positions 16, 94, and 178, all remote from the catalytic site. The Trp residues were replaced with Tyr to produce Trp-free PNP (Leuko-PNP). Leuko-PNP showed near-normal kinetic properties. It was used (1) to determine the tautomeric form of guanine that produces strong fluorescence when bound to PNP, (2) for thermodynamic binding analysis of binary and ternary complexes with substrates, (3) in temperature-jump perturbation of complexes for evidence of multiple conformational complexes, and (4) to establish the ionization state of a catalytic site tyrosine involved in phosphate nucleophile activation. The (13)C NMR spectrum of guanine bound to Leuko-PNP, its fluorescent properties, and molecular orbital electronic transition analysis establish that its fluorescence originates from the lowest singlet excited state of the N1H, 6-keto, N7H guanine tautomer. Binding of guanine and phosphate to PNP and Leuko-PNP are random, with decreased affinity for formation of ternary complexes. Pre-steady-state kinetics and temperature-jump studies indicate that the ternary complex (enzyme-substrate-phosphate) forms in single binding steps without kinetically significant protein conformational changes as monitored by guanine fluorescence. Spectral changes of Leuko-PNP upon phosphate binding establish that the hydroxyl of Tyr88 is not ionized to the phenolate anion when phosphate is bound. A loop region (residues 243-266) near the purine base becomes highly ordered upon substrate/inhibitor binding. A single Trp residue was introduced into the catalytic loop of Leuko-PNP (Y249W-Leuko-PNP) to determine effects on catalysis and to introduce a fluorescence catalytic site probe. Although Y249W-Leuko-PNP is highly fluorescent and catalytically active, substrate binding did not perturb the fluorescence. Thermodynamic boxes, constructed to characterize the binding of phosphate, guanine, and hypoxanthine to native, Leuko-, and Y249W-Leuko-PNPs, establish that Leuko-PNP provides a versatile protein scaffold for introduction of specific Trp catalytic site probes.     

Mechanisms of protein kinase C regulation of airway contractility

To elucidate the role of protein kinase C (PK-C) in regulating airway contractility, the effects of PK-C activation with phorbol esters, 12-deoxyphorbol 13-isobutyrate (DPB), and phorbol 12-myristate 13-acetate (PMA), and with the diacylglycerol analogue 1-oleoyl-2-acetate-rac-glycerol (OAG) were separately evaluated in isolated rabbit tracheal smooth muscle (TSM) segments. The latter agents produced dual and opposing contractile effects, with DPB being the most potent. Lower doses of DPB (less than or equal to 10(-6) M) elicited significant increases in isometric tension in both untreated TSM, as well as in TSM half-maximally precontracted with methacholine. These potentiated TSM contractions were inhibited by the Ca2+ channel blockers, nifedipine (10(-4) M) and diltiazem (10(-5) M). In contrast, higher doses of DPB (greater than or equal to 10(-6) M) induced airway relaxation, which was ablated by preinhibition of the electrogenic Na+-K+ pump with ouabain (5 x 10(-6) M) or K+-free buffer. Indeed, in separate experiments DPB (10(-7) M) was found to significantly potentiate the functional activity of the Na+-K+ pump, an effect occurring independent of inhibition of Na+-H+ exchange with amiloride (10(-4) M) or extracellular Ca2+ influx with nifedipine (10(-4) M).(ABSTRACT TRUNCATED AT 250 WORDS)     

Extrinsic signals for monitoring the association reaction of proteins as introduced by fluorescent and non-fluorescent labels.

Two known dansyl labels (I, II) and 5-[2-(iodoacetamido)ethylamino]-1-naphthalene-sulfonic acid (III) and three new azo-dyes (IV - VI) were covalently attached to alpha-chymotrypsin and to basic pancreatic trypsin inhibitor by four different reactive groups. In order to protect the contact region of the proteins the complex of the two proteins was labeled. Advantage was taken of the fact that a group which is buried in the complex reacts about (see article) times slower than a group which is always exposed (K = dissociation equilibrium constant, [C] = concentration of the complex). The complex was dissociated at pH 3 and the labeled proteins were isolated by column chromatography. They were fully active. The dansyl label was immobilized when introduced by dansyl chloride but highly mobile when attached via the longer imidoester group (II). Changes of absorption and of fluorescence which occur when differently labeled reaction partners recombine were studied. Changes in absorption (up to 18%) were mainly due to interactions of the label of one protein with the other protein. Fluorescence changes of up to 480% could be obtained. They were interpreted in terms of a F?rster type energy transfer between donor and acceptor labels and changes of absorption and quantum yield due to interactions of the labels with the proteins. The kinetic constants of complex formation are not seriously altered by the labels (B?sterling, B & Engel, J. (1976) this J. 357, 1297-1307, succeeding). It is concluded that the labeling technique may be of general value for kinetic and equilibrium studies of protein associations.