A massive rock and ice avalanche caused the 2021 disaster at Chamoli, Indian Himalaya.

On 7 February 2021, a catastrophic mass flow descended the Ronti Gad, Rishiganga, and Dhauliganga valleys in Chamoli, Uttarakhand, India, causing widespread devastation and severely damaging two hydropower projects. More than 200 people were killed or are missing. Our analysis of satellite imagery, seismic records, numerical model results, and eyewitness videos reveals that ~27 × 106 cubic meters of rock and glacier ice collapsed from the steep north face of Ronti Peak. The rock and ice avalanche rapidly transformed into an extraordinarily large and mobile debris flow that transported boulders greater than 20 meters in diameter and scoured the valley walls up to 220 meters above the valley floor. The intersection of the hazard cascade with downvalley infrastructure resulted in a disaster, which highlights key questions about adequate monitoring and sustainable development in the Himalaya as well as other remote, high-mountain environments.

Geomorphic and geologic controls of geohazards induced by Nepal's 2015 Gorkha earthquake.

The Gorkha earthquake (magnitude 7.8) on 25 April 2015 and later aftershocks struck South Asia, killing ~9000 people and damaging a large region. Supported by a large campaign of responsive satellite data acquisitions over the earthquake disaster zone, our team undertook a satellite image survey of the earthquakes' induced geohazards in Nepal and China and an assessment of the geomorphic, tectonic, and lithologic controls on quake-induced landslides. Timely analysis and communication aided response and recovery and informed decision-makers. We mapped 4312 coseismic and postseismic landslides. We also surveyed 491 glacier lakes for earthquake damage but found only nine landslide-impacted lakes and no visible satellite evidence of outbursts. Landslide densities correlate with slope, peak ground acceleration, surface downdrop, and specific metamorphic lithologies and large plutonic intrusions.

Interaction of thymidylate synthase with the 5'-thiophosphates, 5'-dithiophosphates, 5'-H-phosphonates and 5'-S-thiosulfates of 2'-deoxyuridine, thymidine and 5-fluoro-2'-deoxyuridine.

New analogs of dUMP, dTMP and 5-fluoro-dUMP, including the corresponding 5'-thiophosphates (dUMPS, dTMPS and FdUMPS), 5'-dithiophosphates (dUMPS2, dTMPS2 and FdUMPS2), 5'-H-phosphonates (dUMP-H, dTMP-H and FdUMP-H) and 5'-S-thiosulfates (dUSSO3, dTSSO3 and FdUSSO3), have been synthesized and their interactions studied with highly purified mammalian thymidylate synthase. dUMPS and dUMPS2 proved to be good substrates, and dTMPS and dTMPS2 classic competitive inhibitors, only slightly weaker than dTMP. Their 5-fluoro congeners behaved as potent, slow-binding inhibitors. By contrast, the corresponding 5'-H-phosphonates and 5'-S-thiosulfates displayed weak activities, only FdUMP-H and FdUSSO3 exhibiting significant interactions with the enzyme, as weak competitive slow-binding inhibitors versus dUMR The pH-dependence of enzyme time-independent inhibition by FdUMP and FdUMPS was found to correlate with the difference in pKa values of the phosphate and thiophosphate groups, the profile of FdUMPS being shifted (approximately 1 pH unit) toward lower pH values, so that binding of dUMP and its analogs is limited by the phosphate secondary hydroxyl ionization. Hence, together with the effects of 5'-H-phosphonate and 5'-S-thiosulfate substituents, the much weaker interactions of the nucleotide analogs (3-5 orders of magnitude lower than for the parent 5'-phosphates) with the enzyme is further evidence that the enzyme's active center prefers the dianionic phosphate group for optimum binding.

Nucleoside phosphate analogues of biological interest, and their synthesis via aryl nucleoside H-phosphonates as intermediates.

This review presents a brief account of the chemistry and mechanistic aspects of aryl H-phosphonates, and selected applications of this class of compounds as intermediates in the synthesis of a wide range of biologically important analogues of nucleoside phosphates, and oligonucleotides, in which the phosphate moieties are replaced by other structurally related groups. The aryl nucleoside H-phosphonates, compounds of controlled reactivity, have proven to be more versatile and superior to various mixed anhydrides as synthetic intermediates, particularly for preparation of nucleotide analogues bearing P-N or P-S bonds in various configurational arrangements at the phosphate moiety.

Interaction of Escherichia coli purine nucleoside phosphorylase (PNP) with the cationic and zwitterionic forms of the fluorescent substrate N(7)-methylguanosine.

Steady-state and time-resolved fluorescence spectroscopy, and enzyme kinetics, were applied to study the reaction of purine nucleoside phosphorylase (PNP) from Escherichia coli with its substrate N(7)-methylguanosine (m7Guo), which consists of an equilibrium mixture of cationic and zwitterionic forms (pK(a)=7.0), each with characteristic absorption and fluorescence spectra, over the pH range 6-9, where absorption and intrinsic fluorescence of the enzyme are virtually unchanged. The pH-dependence of kinetic constants for phosphorolysis of m7Guo were studied under condition where the population of the zwitterion varied from 10% to 100%. This demonstrated that, whereas the zwitterion is a 3- to 6-fold poorer substrate, if at all, than the cation for the mammalian enzymes, both ionic species are almost equally good substrates for E. coli PNP. The imidazole-ring-opened form of m7Guo is neither a substrate nor an inhibitor of phosphorolysis. Enzyme fluorescence quenching, and concomitant changes in absorption and fluorescence spectra of the two ionic species of m7Guo on binding, showed that both forms are bound by the enzyme, the affinity of the zwitterion being 3-fold lower than that of the cation. Binding of m7Guo is bimodal, i.e., an increase in ligand concentration leads to a decrease in the association constant of the enzyme-ligand complex, typical for negative cooperativity of enzyme-ligand binding, with a Hill constant <1. This is in striking contrast to interaction of the enzyme with the parent Guo, for which the association constant is independent of concentration. The weakly fluorescent N(7)-methylguanine (m7Gua), the product of phosphorolysis of m7Guo, is a competitive non-substrate inhibitor of phosphorolysis (K(i)=8+/-2 microM) and exhibits negative cooperativity on binding to the enzyme at pH 6.9. Quenching of enzyme emission by the ligands is a static process, inasmuch as the mean excited-state lifetime, =2.7 ns, is unchanged in the presence of the ligands, and the constants K(SV) may therefore be considered as the association constants for the enzyme-ligand complexes. In the pH range 9.5-11 there is an instantaneous reversible decrease in PNP emission of approximately 15%, corresponding to one of the six tyrosine residues per subunit readily accessible to solvent, and OH- ions. Relevance of the overall results to the mechanism of phosphorolysis, and binding of substrates/inhibitors is discussed.

Selectivity of 4,5,6,7-tetrabromobenzotriazole, an ATP site-directed inhibitor of protein kinase CK2 ('casein kinase-2').

The specificity of 4,5,6,7-tetrabromo-2-azabenzimidazole (TBB), an ATP/GTP competitive inhibitor of protein kinase casein kinase-2 (CK2), has been examined against a panel of 33 protein kinases, either Ser/Thr- or Tyr-specific. In the presence of 10 microM TBB (and 100 microM ATP) only CK2 was drastically inhibited (>85%) whereas three kinases (phosphorylase kinase, glycogen synthase kinase 3 beta and cyclin-dependent kinase 2/cyclin A) underwent moderate inhibition, with IC(50) values one--two orders of magnitude higher than CK2 (IC(50)=0.9 microM). TBB also inhibits endogenous CK2 in cultured Jurkat cells. A CK2 mutant in which Val66 has been replaced by alanine is much less susceptible to inhibition by TBB as well as by another ATP competitive inhibitor, emodin. These data show that TBB is a quite selective inhibitor of CK2, that can be used in cell-based assays.

Calf spleen purine nucleoside phosphorylase: structure of its ternary complex with an N(7)-acycloguanosine inhibitor and a phosphate anion.

The calf spleen purine nucleoside phosphorylase (PNP) ternary complex with an N(7)-acycloguanosine inhibitor and a phosphate ion has been crystallized in the cubic space group P2(1)3, with unit-cell parameter a = 94.11 A and one monomer per asymmetric unit. X-ray diffraction data were collected using synchrotron radiation (Station X31, EMBL Outstation, DESY, Hamburg). The crystal structure was refined to a resolution of 2.2 A and R and R(free) values of 17.5 and 24.5%, respectively. The acyclonucleoside inhibitor is bound in the active site in an inverted ('upside-down') orientation of the purine base compared with natural substrates. The side chain of Asp243 forms two hydrogen bonds with the base ring: N(delta) donates a hydrogen to N(3) and O(delta) accepts a hydrogen from the guanine N(2)-amino group. N(1)--H of the base is hydrogen bonded to O(epsilon) of Glu201, while N(9) accepts a hydrogen bond from Thr242 O(gamma). In addition, a water molecule (W417) bridges the N(2)-amino group of the base and O(epsilon) of Glu201. In the phosphate-binding site, a phosphate ion is bound to Ser33, His64, Arg84, His86, Ala116 and Ser220. The acyclic chain of the N(7)-acycloguanosine inhibitor is in a folded conformation and together with a water molecule (W388) occupies the pentose-binding site, with possible hydrogen bonds to Tyr88 O(eta) and His257 N(delta 1). This new binding mode fully accounts for the previously observed substrate properties of 7-beta-D-ribofuranosides of hypoxanthine and guanine. It also provides a new starting point for the design of inhibitors of PNP for therapeutic and other applications.

Purine nucleoside phosphorylases: properties, functions, and clinical aspects.

The ubiquitous purine nucleoside phosphorylases (PNPs) play a key role in the purine salvage pathway, and PNP deficiency in humans leads to an impairment of T-cell function, usually with no apparent effects on B-cell function. This review updates the properties of the enzymes from eukaryotes and a wide range of prokaryotes, including a tentative classification of the enzymes from various sources, based on three-dimensional structures in the solid state, subunit composition, amino acid sequences, and substrate specificities. Attention is drawn to the compelling need of quantitative experimental data on subunit composition in solution, binding constants, and stoichiometry of binding; order of ligand binding and release; and its possible relevance to the complex kinetics exhibited with some substrates. Mutations responsible for PNP deficiency are described, as well as clinical methods, including gene therapy, for corrections of this usually fatal disease. Substrate discrimination between enzymes from different sources is also being profited from for development of tumour-directed gene therapy. Detailed accounts are presented of design of potent inhibitors, largely nucleosides and acyclonucleosides, their phosphates and phosphonates, particularly of the human erythrocyte enzyme, some with Ki values in nanomolar and picomolar range, intended for induction of the immunodeficient state for clinical applications, such as prevention of host-versus-graft response in organ transplantations. Methods of assay of PNP activity are reviewed. Also described are applications of PNP from various sources as tools for the enzymatic synthesis of otherwise inaccessible therapeutic nucleoside analogues, as coupling enzymes for assays of orthophosphate in biological systems in the micromolar and submicromolar ranges, and for coupled assays of other enzyme systems.

Formycin A and its N-methyl analogues, specific inhibitors of E. coli purine nucleoside phosphorylase (PNP): induced tautomeric shifts on binding to enzyme, and enzyme-->ligand fluorescence resonance energy transfer.

Steady-state and time-resolved emission spectroscopy were used to study the interaction of Escherichia coli purine nucleoside phosphorylase (PNP) with its specific inhibitors, viz. formycin B (FB), and formycin A (FA) and its N-methylated analogues, N(1)-methylformycin A (m(1)FA), N(2)-methylformycin A (m(2)FA) and N(6)-methylformycin A (m(6)FA), in the absence and presence of phosphate (P(i)). Complex formation led to marked quenching of enzyme tyrosine intrinsic fluorescence, with concomitant increases in fluorescence of FA and m(6)FA, independently of the presence of P(i). Fluorescence of m(1)FA in the complex increased only in the presence of P(i), while the weak fluorescence of FB appeared unaffected, independently of P(i). Analysis of the emission, excitation and absorption spectra of enzyme-ligand mixtures pointed to fluorescence resonance energy transfer (FRET) from protein tyrosine residue(s) to FA and m(6)FA base moieties, as a major mechanism of protein fluorescence quenching. With the non-inhibitor m(2)FA, fluorescence emission and excitation spectra were purely additive. Effects of enzyme-FA, or enzyme-m(6)FA, interactions on nucleoside excitation and emission spectra revealed shifts in tautomeric equilibria of the bound ligands. With FA, which exists predominantly as the N(1)-H tautomer in solution, the proton N(1)-H is shifted to N(2), independently of the presence of P(i). Complex formation with m(6)FA in the absence of P(i) led to a shift of the amino-imino equilibrium in favor of the imino species, and increased fluorescence at 350 nm; by contrast, in the presence of P(i), the equilibrium was shifted in favor of the amino species, accompanied by higher fluorescence at 430 nm, and a higher affinity for the enzyme, with a dissociation constant K(d)=0.5+/-0.1 microM, two orders of magnitude lower than that for m(6)FA in the absence of P(i) (K(d)=46+/-5 microM). The latter was confirmed by analysis of quenching of enzyme fluorescence according to a modified Stern-Volmer model. Fractional accessibility values (f(a)) varied from 0.31 for m(1)FA to 0.70 for FA, with negative cooperative binding of m(1)FA and FB, and non-cooperative binding of FA and m(6)FA. For all nucleoside ligands, the best model describing binding stoichiometry was one ligand per native enzyme hexamer. Fluorescence decays of PNP, FA and their mixtures were best fitted to a sum of two exponential terms, with average lifetimes () affected by their interactions. Complex formation resulted in a 2-fold increase in of FA, and a 2-fold decrease in of enzyme fluorescence. The amplitude of the long-lifetime component also increased, confirming the shift of the tautomeric equilibrium in favor of the N(2)-H species. The findings have been examined in relation to enzyme-nucleoside binding deduced from structural studies.

Crystal structure of the purine nucleoside phosphorylase (PNP) from Cellulomonas sp. and its implication for the mechanism of trimeric PNPs.

The three-dimensional structure of the trimeric purine nucleoside phosphorylase (PNP) from Cellulomonas sp. has been determined by X-ray crystallography. The binary complex of the enzyme with orthophosphate was crystallized in the orthorhombic space group P212121 with unit cell dimensions a=64.1 A, b=108.9 A, c=119.3 A and an enzymatically active trimer in the asymmetric unit. X-ray data were collected at 4 degrees C using synchrotron radiation (EMBL/DESY, Hamburg). The structure was solved by molecular replacement, with the calf spleen PNP structure as a model, and refined at 2.2 A resolution. The ternary "dead-end" complex of the enzyme with orthophosphate and 8-iodoguanine was obtained by soaking crystals of the binary orthophosphate complex with the very weak substrate 8-iodoguanosine. Data were collected at 100 K with CuKalpha radiation, and the three-dimensional structure refined at 2.4 A resolution. Although the sequence of the Cellulomonas PNP shares only 33 % identity with the calf spleen enzyme, and almost no identity with the hexameric Escherichia coli PNP, all three enzymes have many common structural features, viz. the nine-stranded central beta-sheet, the positions of the active centres, and the geometrical arrangement of the ligands in the active centres. Some similarities of the surrounding helices also prevail. In Cellulomonas PNP, each of the three active centres per trimer is occupied by orthophosphate, and by orthophosphate and base, respectively, and small structural differences between monomers A, B and C are observed. This supports cooperativity between subunits (non-identity of binding sites) rather than existence of more than one binding site per monomer, as previously suggested for binding of phosphate by mammalian PNPs. The phosphate binding site is located between two conserved beta- and gamma-turns and consists of Ser46, Arg103, His105, Gly135 and Ser223, and one or two water molecules. The guanine base is recognized by a zig-zag pattern of possible hydrogen bonds, as follows: guanine N-1...Glu204 O(epsilon1)...guanine NH2...Glu204 O(epsilon2). The exocyclic O6 of the base is bridged via a water molecule to Asn246 N(delta), which accounts for the inhibitory, but lack of substrate, activity of adenosine. An alternative molecular mechanism for catalysis by trimeric PNPs is proposed, in which the key catalytic role is played by Glu204 (Glu201 in the calf and human enzymes), while Asn246 (Asn243 in the mammalian enzymes) supports binding of 6-oxopurines rather than catalysis. This mechanism, in contrast to that previously suggested, is consistent with the excellent substrate properties of N-7 substituted nucleosides, the specificity of trimeric PNPs versus 6-oxopurine nucleosides and the reported kinetic properties of Glu201/Ala and Asn243/Ala point variants of human PNP.

Substrate/inhibitor properties of human deoxycytidine kinase (dCK) and thymidine kinases (TK1 and TK2) towards the sugar moiety of nucleosides, including O'-alkyl analogues.

Nucleoside analogues with modified sugar moieties have been examined for their substrate/inhibitor specificities towards highly purified deoxycytidine kinase (dCK) and thymidine kinases (tetrameric high-affinity form of TK1, and TK2) from human leukemic spleen. In particular, the analogues included the mono- and di-O'-methyl derivatives of dC, dU and dA, syntheses of which are described. In general, purine nucleosides with modified sugar rings were feebler substrates than the corresponding cytosine analogues. Sugar-modified analogues of dU were also relatively poor substrates of TK1 and TK2, but were reasonably good inhibitors, with generally lower Ki values vs TK2 than TK1. An excellent discriminator between TK1 and TK2 was 3'-hexanoylamino-2',3'-dideoxythymidine, with a Ki of approximately 600 microM for TK1 and approximately 0.1 microM for TK2. 3'-OMe-dC was a superior inhibitor of dCK to its 5'-O-methyl congener, consistent with possible participation of the oxygen of the (3')-OH or (3')-OMe as proton acceptor in hydrogen bonding with the enzyme. Surprisingly alpha-dT was a good substrate of both TK1 and TK2, with Ki values of 120 and 30 microM for TK1 and TK2, respectively; and a 3'-branched alpha-L-deoxycytidine analogue proved to be as good a substrate as its alpha-D-counterpart. Several 5'-substituted analogues of dC were good non-substrate inhibitors of dCK and, to a lesser extent, of TK2. Finally, some ribonucleosides are substrates of the foregoing enzymes; in particular C is a good substrate of dCK, and 2'-OMe-C is an even better substrate than dC.

Viral and host-cell protein kinases: enticing antiviral targets and relevance of nucleoside, and viral thymidine, kinases.

Numerous targets are known for development of antiviral agents, and some significant successes have been achieved with nucleoside analogues. These are "activated" by phosphorylation by viral and/or host-cell nucleoside kinases, the final target being principally the viral polymerase. With latency of herpes viruses, the viral thymidine kinase may be the ultimate target. Less attention has been devoted to viral protein kinases as antiviral targets, largely because 5 years ago, these the study of such enzymes was considered "still in its infancy." In the interim, identification of viral and host-cell protein kinases involved in viral gene expression, and viral replication, has made impressive advances. In conjunction with current progress in development of specific inhibitors of cellular protein kinases, and the differences in sequence motifs between these and the viral enzymes, the latter are indeed attractive targets, as are also some host-cell protein kinases. Examples include, amongst others, the essential protein kinases of vaccinia virus; the nonsegmented negative-strand RNA viruses, all essentially dependent on host-cell kinases, e.g., protein kinase CK-II (casein kinase-II), for which good inhibitors, such as halogenated benzimidazoles and benzotriazoles, are known; herpes viruses, with emphasis on human cytomegalovirus, the UL97 gene of which codes for a protein kinase that, like viral thymidine kinases, "activates," by phosphorylation, a nonpeptide antiviral acyclonucleoside ganciclovir, an analogue of the antiherpes aciclovir. The latter, in turn, is active against animal cytomegaloviruses following phosphorylation by the products of their UL97 gene homologues. Attention is also directed to the antiviral activity of the cyclic phosphate of ganciclovir, a structural analogue of the second messenger cyclic GMP.

Interactions of calf spleen purine nucleoside phosphorylase with antiviral acyclic nucleoside phosphonate inhibitors: kinetics and emission studies.

Association between calf spleen purine nucleoside phosphorylase and a series of phosphonylalkoxyalkyl derivatives of purine bases was studied by inhibition kinetics and fluorimetric titrations. Dissociation constants, determined by fluorimetric titration in phosphate-free conditions, were lower than inhibition constants in 1 mM phosphate, and inhibition was still weaker in 50 mM phosphate, in accord with the postulated bisubstrate analogue character of this class of inhibitors.

To(o) Queer or Not?

SUMMARY While much has been written on the implications of queer theory for lesbian sexuality, little attention has been paid to its impact on lesbian community and subsequent formulations of lesbian identity. The essay argues that theories of identity emerge in part from their interactions with lesbian sexual practice situated within the context of lesbian community. Because lesbian community serves as an important base for socio-political activism, queer theory's destabilization of identity and sexuality has a direct impact on our ability to effect social change. The essay concludes with suggestions for more positive relationships between queer and lesbian communities than have been explored in the past.

Crystallization and preliminary X-ray studies of purine nucleoside phosphorylase from Cellulomonas sp.

The commercially available enzyme purine nucleoside phosphorylase (PNP) from Cellulomonas sp. was purified by ion--exchange chromatography, partially sequenced and crystallized in two different crystal forms using the hanging-drop vapour-diffusion technique. Crystal form A grows as polyeders and/or cubes in the cubic space group P4232 with unit-cell dimension a = 162.5 A. Crystal form B appears as thick plates in the space group P212121 with unit-cell dimensions a = 63.2, b = 108.3 and c = 117.4 A. Both crystal forms contain three monomers (one trimer) in the asymmetric unit.

Acyclic analogues of 5-fluoro-dUMP and 5-fluoro-2'-deoxyuridine: synthesis and inhibition of thymidylate synthase and tumour cell growth.

1-[(2-Hydroxyethoxy)methyl]-5-fluorouracil (HEMFU) and 1-[(1,3-dihydroxy-2-propoxy)methyl]-5-fluorouracil (DHPFU) were prepared by alkylation of the di-O-TMS derivative of 5-fluorouracil and phosphorylated with the use of the wheat shoot phosphotransferase system to their monophosphates, HEMFUMP and DHPFUMP. 1-(2-Phosphonylmethoxyethyl)-5-fluorouracil (PMEFU) was obtained by condensation of diethyl-2-chloroethoxymethanephosphonate with 5-fluorouracil and cleavage of the alkylphosphoester with trimethylbromosilane. Inhibition of highly purified thymidylate synthase from mouse tumour Ehrlich carcinoma and leukemia L1210 cells by each of the nucleotide analogues, DHPFUMP, PMEFU and HEMFUMP, and of L5178Y mouse leukemia cell growth by the nucleoside (HEMFU) analogue, were studied. DHPFUMP proved to be the strongest inhibitor, non-competitive vs dUMP, with K(i)app 2.8 microM for time-independent interaction with the enzyme and N5,N10-methylenetetrahydrofolate (CH2H4PteGlu). In the presence of CH2H4PteGlu, DHPFUMP exhibited time-dependent inactivation of the enzyme, the inactivation rate plots being biphasic and pointing to Ki values in the microM range (10(3)-fold higher than for 5-fluoro-dUMP). HEMFUMP and PMEFU were much weaker inhibitors of the enzyme, with K(i)app values of 0.26 mM (non-competitive vs dUMP) and 30 mM (non-competitive vs dUMP), respectively. HEMFU, despite the weak interaction of its nucleotide analogue with the enzyme, proved to be a strong cell (L5178Y) growth inhibitor, with IC50 in the range 10(-5) M.

Unusual nucleoside triphosphate donors for nucleoside kinases: 3'-deoxyadenosine-2'-triphosphate and 2'-deoxyadenosine-3'-triphosphate.

Two non-conventional analogues of ATP, 3'-deoxyadenosine-2'-triphosphate (3'-d-2'-ATP) and 2'-deoxyadenosine-3'-triphosphate (2'-d-3'-ATP), the syntheses of which are described, were examined as potential phosphate donors for the nucleoside kinases: 2'-deoxycytidine kinase (dCK), cytosolic thymidine kinase (TK1) and mitochondrial thymidine kinase (TK2). The reactions were monitored by means of a mixture of [gamma-32P]ATP and cold analogue, and/or with the use of 3H-labelled acceptors and cold donor. With dCK, using equimolar mixtures of ATP with each analogue, and dC as acceptor, phosphate transfer from 3'-d-2'-ATP and 2'-d-3'-ATP amounted to 34% and 14%, respectively. With each analogue used alone (each at concentration of 100 microM), phosphate transfer from 3'-d-2'-ATP was 55% that from ATP, and from 2'-d-3'-ATP 16%. With human TK2, and equimolar mixtures of [gamma-32P]ATP with each of the analogues, and 1 microM dT as acceptor, there was no detectable transfer from either analogue. But, when each analogue was used alone, phosphate transfer attained 11% and 5%, respectively, that for ATP alone. With the low affinity form of human TK1, and dT as acceptor, only low phosphate transfer occurred with either analogue used alone. Both compounds exhibited Michaelis-Menten kinetics (with significantly lower Vmax than ATP), while ATP exhibited cooperative kinetics with all three kinases.

Crystal structure of the ternary complex of E. coli purine nucleoside phosphorylase with formycin B, a structural analogue of the substrate inosine, and phosphate (Sulphate) at 2.1 A resolution.

The ternary complex of purine nucleoside phosphorylase from E. coli with formycin B and a sulphate or phosphate ion crystallized in the hexagonal space group P6122 with unit cell dimensions a=123.11, c=241.22 A and three monomers per asymmetric unit. The biologically active hexamer is formed through 2-fold crystallographic symmetry, constituting a trimer of dimers. High-resolution X-ray diffraction data were collected using synchrotron radiation (Daresbury, England). The crystal structure was determined by molecular replacement and refined at 2.1 A resolution to an R-value of 0.196. There is one active centre per monomer, composed of residues belonging to two subunits of one dimer. The phosphate binding site is strongly positively charged and consists of three arginine residues (Arg24, Arg87 and Arg43 from a neighbouring subunit), Ser90 and Gly20. It is occupied by a sulphate or phosphate anion, each oxygen atom of which accepts at least two hydrogen bonds or salt-bridges. The sulphate or phosphate anion is also in direct contact with the ribose moiety of formycin B. The ribose binding site is composed of Ser90, Met180, Glu181 and His4, the latter belonging to the neighbouring subunit. The base binding site is exposed to solvent, and the base is unspecifically bound through a chain of water molecules and aromatic-aromatic interactions. In all monomers the nucleosides are in the high syn conformation about the glycosidic bonds with chi in the range 100 to 130 degrees. The architecture of the active centre is in line with the known broad specificity and the kinetic properties of E. coli PNP.