Upregulation of ß-cell genes and improved function in rodent islets following chronic glucokinase activation.

Glucokinase (GK) plays a critical role in controlling blood glucose; GK activators have been shown to stimulate insulin secretion acutely both in vitro and in vivo. Sustained stimulation of insulin secretion could potentially lead to ß-cell exhaustion; this study examines the effect of chronic GK activation on ß-cells. Gene expression and insulin secretion were measured in rodent islets treated in vitro with GKA71 for 72 h. Key ß-cell gene expression was measured in rat, mouse and global GK heterozygous knockout mouse islets (gk(del/wt)). Insulin secretion, after chronic exposure to GKA71, was measured in perifused rat islets. GKA71 acutely increased insulin secretion in rat islets in a glucose-dependent manner. Chronic culture of mouse islets with GKA71 in 5 mmol/l glucose significantly increased the expression of insulin, IAPP, GLUT2, PDX1 and PC1 and decreased the expression of C/EBPß compared with 5 mmol/l glucose alone. Similar increases were shown for insulin, GLUT2, IAPP and PC1 in chronically treated rat islets. Insulin mRNA was also increased in GKA71-treated gk(del/wt) islets. No changes in GK mRNA were observed. Glucose-stimulated insulin secretion was improved in perifused rat islets following chronic treatment with GKA71. This was associated with a greater insulin content and GK protein level. Chronic treatment of rodent islets with GKA71 showed an upregulation of key ß-cell genes including insulin and an increase in insulin content and GK protein compared with glucose alone.

Characterisation of CadR from Pseudomonas aeruginosa: a Cd(II)-responsive MerR homologue.

cadR from Pseudomonas aeruginosa encodes a transcriptional regulatory protein which responds to Cd(II)>>Zn(II)>Hg(II) at its cognate promoter PcadA. CadR will also act to induce transcription at the Escherichia coli ZntR cognate promoter, PzntA, however, the induction profile is altered to Hg(II)>Cd(II)>Zn(II). Two separate single base pair deletions within PzntA result in further alteration of relative specificity in metal-ion induction profile for CadR. This demonstrates that the operator/promoter sequence can play a role in defining optimal ligand response and that for these regulators specificity is not solely a function of the regulatory protein.

Variation in aspects of cysteine proteinase catalytic mechanism deduced by spectroscopic observation of dithioester intermediates, kinetic analysis and molecular dynamics simulations.

The possibility of a slow post-acylation conformational change during catalysis by cysteine proteinases was investigated by using a new chromogenic substrate, N-acetyl-Phe-Gly methyl thionoester, four natural variants (papain, caricain, actinidin and ficin), and stopped-flow spectral analysis to monitor the pre-steady state formation of the dithioacylenzyme intermediates and their steady state hydrolysis. The predicted reversibility of acylation was demonstrated kinetically for actinidin and ficin, but not for papain or caricain. This difference between actinidin and papain was investigated by modelling using QUANTA and CHARMM. The weaker binding of hydrophobic substrates, including the new thionoester, by actinidin than by papain may not be due to the well-known difference in their S2-subsites, whereby that of actinidin in the free enzyme is shorter due to the presence of Met211. Molecular dynamics simulation suggests that during substrate binding the sidechain of Met211 moves to allow full access of a Phe sidechain to the S2-subsite. The highly anionic surface of actinidin may contribute to the specificity difference between papain and actinidin. During subsequent molecular dynamics simulations the P1 product, methanol, diffuses rapidly (over<8 ps) out of papain and caricain but 'lingers' around the active centre of actinidin. Uniquely in actinidin, an Asp142-Lys145 salt bridge allows formation of a cavity which appears to constrain diffusion of the methanol away from the catalytic site. The cavity then undergoes large scale movements (over 4.8 A) in a highly correlated manner, thus controlling the motions of the methanol molecule. The changes in this cavity that release the methanol might be those deduced kinetically.

Kinetic and titration methods for determination of active site contents of enzyme and catalytic antibody preparations.

Kinetic characterization of enzymes and analogous catalysts such as catalytic antibodies requires knowledge of the molarity of functional sites. Various stoichiometric titration methods are available for the determination of active-site concentrations of some enzymes and these are exemplified in the second part of this article. Most of these are not general in that they require the existence of certain types of either intermediate or active-site residues that are susceptible to specific covalent modification. Thus they are not readily applicable to many enzymes and they are rarely available currently for titration of catalytic antibody active sites. In the first part of the article we discuss a general kinetic method for the investigation of active-site availability in preparations of macromolecular catalysts. The method involves steady-state kinetics to provide Vmax and Km and single-turnover first-order kinetics using excess of catalyst over substrate to provide the analogous parameters k(obs)lim and K(m)app. The active-site contents of preparations that contain only active catalyst (Ea) and inert material (Ei) may be calculated as [Ea](T) = Vmax)/k(obs)lim. This is true even if nonproductive binding to E(a) occurs. For polyclonal catalytic antibody preparations, which may contain binding but noncatalytic material (Eb) in addition to Ea and Ei, the significance of Vmax/k(obs)lim is more complex but provides an upper limit to E(a). This can be refined by consideration of the relative values of Km and the equilibrium dissociation constant of EbS. Analysis of the Ea, Eb, Ei system requires the separate determination of Ei. For catalytic antibodies this may be achieved by analytical affinity chromatography using an immobilized hapten or hapten analog and an ELISA procedure to ensure the clean separation of Ei from the Ea + Eb mixture.

Biochemical and X-ray crystallographic studies on shikimate kinase: the important structural role of the P-loop lysine.

Shikimate kinase, despite low sequence identity, has been shown to be structurally a member of the nucleoside monophosphate (NMP) kinase family, which includes adenylate kinase. In this paper we have explored the roles of residues in the P-loop of shikimate kinase, which forms the binding site for nucleotides and is one of the most conserved structural features in proteins. In common with many members of the P-loop family, shikimate kinase contains a cysteine residue 2 amino acids upstream of the essential lysine residue; the side chains of these residues are shown to form an ion pair. The C13S mutant of shikimate kinase was found to be enzymatically active, whereas the K15M mutant was inactive. However, the latter mutant had both increased thermostability and affinity for ATP when compared to the wild-type enzyme. The structure of the K15M mutant protein has been determined at 1.8 A, and shows that the organization of the P-loop and flanking regions is heavily disturbed. This indicates that, besides its role in catalysis, the P-loop lysine also has an important structural role. The structure of the K15M mutant also reveals that the formation of an additional arginine/aspartate ion pair is the most likely reason for its increased thermostability. From studies of ligand binding it appears that, like adenylate kinase, shikimate kinase binds substrates randomly and in a synergistic fashion, indicating that the two enzymes have similar catalytic mechanisms.

Ionization characteristics and chemical influences of aspartic acid residue 158 of papain and caricain determined by structure-related kinetic and computational techniques: multiple electrostatic modulators of active-centre chemistry.

The pK(a) of (Asp(158))-CO(2)H of papain (EC 3.4.22.2) was determined as 2.8 by using 4-chloro-7-nitrobenzofurazan (Nbf-Cl) as a reactivity probe targeted on the thiolate anion component of the Cys(25)/His(159) nucleophilic-acid/base motif of the catalytic site. The possibility of using Nbf-Cl for this purpose was established by modelling the papain-Nbf-Cl Meisenheimer intermediate by using QUANTA/CHARMM and performing molecular orbital calculations with MOPAC interfaced with Cerius 2. A pH-dependent stopped-flow kinetic study of the reaction of papain with Nbf-Cl established that the striking rate maximum at pH 3 results from reaction in a minor ionization state comprising (Cys(25))-S(-)/(His(159))-Im(+)H (in which Im represents imidazole) produced by protonic dissociation of (Cys(25))-SH/(His(159))-Im(+)H with pK(a) 3.3 and (Asp(158))-CO(2)H. Although the analogous intermediate in the reaction of caricain (EC 3.4.22.30) with Nbf-Cl has similar geometry, the pH-k profile (k being the second-order rate constant) lacks a rate maximum under acidic conditions. This precludes the experimental determination of the pK(a) value of (Asp(158))-CO(2)H of caricain, which was calculated to be 2.0 by solving the linearized Poisson-Boltzmann equation with the program UHBD ('University of Houston Brownian dynamics'). A value lower than 2.8 had been predicted by consideration of the hydrogen-bonded networks involving Asp(158) and its microenvironments in both enzymes. The difference between these pK(a) values (values not previously detected in reactions of either enzyme) accounts for the lack of the rate maximum in the caricain reaction and for the differences in the electronic absorption spectra of the two S-Nbf-enzymes under acidic conditions. The concept of control of cysteine proteinase activity by multiple electrostatic modulators, including (Asp(158))-CO(2)(-), which modifies traditional mechanistic views, is discussed.

The kinetic basis of a general method for the investigation of active site content of enzymes and catalytic antibodies: first-order behaviour under single-turnover and cycling conditions.

The theoretical foundation has been laid for the investigation of catalytic systems using first-order kinetics and for a general kinetic method of investigation of the active site content, E(a), of enzymes, catalytic antibodies, and other enzyme-like catalysts. The method involves a combination of steady-state and single-turnover kinetics to provide Vmax and Km and k(lim)(obs) and K(app)(m), respectively. The validity of the method is shown to remain valid for two extensions of the simple two-step enzyme catalysis model (a) when the catalyst preparation contains molecules (Eb) that bind substrate but fail to catalyse product formation and (b) when the catalyst itself binds substrate non-productively as well as productively. The former is a particularly serious complication for polyclonal catalytic antibodies and the latter a potential complication for all catalysts. For the simple model and for (b) Vmax/k(lim)(obs) provides the value of [Ea]T and for (a) its upper limit. This can be refined by consideration of the relative values of Km and the equilibrium dissociation constant of EbS. For the polyclonal catalytic antibody preparation investigated, the fact that K(app/m) > Km demonstrates for the first time the presence of a substrate-binding but non-catalytic component in a polyclonal preparation. First-order behaviour in catalytic systems occurs not only with a large excess of catalyst over substrate but also with lower catalyst/substrate ratios, including the equimolar condition, when K(app)(m) >> [S]0, a phenomenon that is not widely appreciated.

A general kinetic approach to investigation of active-site availability in macromolecular catalysts.

A potentially general kinetic method for the investigation of active-site availability in preparations of macromolecular catalysts was developed. Three kinetic models were considered: (a) the conventional two-step model of enzyme catalysis, where the preparation contains only active catalyst (E(a)) and inert (i.e. non-binding, non-catalytic) material (E(i)); (b) an extension of the conventional model (a) involving only E(a) and E(i), but with non-productive binding to E(a) (in addition to productive binding); (c) a model in which the preparation contains also binding but non-catalytic material (E(b)), predicted to be present in polyclonal catalytic antibody preparations. The method involves comparing the parameters V(max) and K(m) obtained under catalytic conditions where substrate concentrations greatly exceed catalyst concentration with those (klim/obs, the limiting value of the first-order rate constant, k(obs), at saturating concentrations of catalyst; and Kapp/m) for single-turnover kinetics, in which the reverse situation obtains. The active-site contents of systems that adhere to model (a) or extensions that also lack E(b), such as the non-productive binding model (b), may be calculated using [E(a)](T)=V(max)/klim/obs. This was validated by showing that, for alpha-chymotrypsin, identical values of [E(a)](T) were obtained by the kinetic method using Suc-Ala-Ala-Pro-Phe-4-nitroanilide as substrate and the well-known 'all-or-none' spectroscopic assay using N-trans-cinnamoylimidazole as titrant. For systems that contain E(b), such as polyclonal catalytic antibody preparations, V(max)/klim/obs is more complex, but provides an upper limit to [E(a)](T). Use of the kinetic method to investigate PCA 271-22, a polyclonal catalytic antibody preparation obtained from the antiserum of sheep 271 in week 22 of the immunization protocol, established that [E(a)](T) is less than approx. 8% of [IgG], and probably less than approx. 1% of [IgG].

Synthesis of a transition state analogue for the hydrolysis of cocaine: assistance to phosphonylation of a 3beta-hydroxytropane by a neighbouring amide group.

A simple synthesis of phenylphosphonate monoester analogues of the transition state for hydrolysis of the benzoyl ester group in cocaine is provided by the reaction of 2beta-amido-3beta-tropanols with phenylphosphonyl dichloride. Steric hindrance to phosphonylation of the hydroxyl is overcome because the neighbouring 2beta-amido group participates in the reaction. The intramolecular assistance by the amide to formation of the phosphonate ester is influenced by the electronic environment of the amide group.

ZntR is a Zn(II)-responsive MerR-like transcriptional regulator of zntA in Escherichia coli.

We have identified the promoter/operator region of the zntA gene of Escherichia coli and shown that Zn(II) is the primary inducer of expression of this Zn(II)/Cd(II) export gene. The promoter PzntA shows sequence similarities to the promoters of mercury resistance (mer) operons, including a long spacer region containing an inverted repeat sequence. The gene encoding the transcriptional regulator of PzntA, designated zntR, has been identified from genome sequence data, by expression of the gene product and by insertional inactivation/complementation. The ZntR product is a member of the MerR family of transcriptional regulators and appears to act as a hypersensitive transcriptional switch. A hybrid MerR/ZntR protein has been constructed and indicates that the C-terminal region of ZntR recognizes Zn(II).

Kinetic analysis of the mechanism of interaction of full-length TIMP-2 and gelatinase A: evidence for the existence of a low-affinity intermediate.

We have undertaken a detailed analysis of the mechanism of inhibition of matrix metalloproteinase-2 (gelatinase A) by tissue inhibitor of metalloproteinase-2 (TIMP-2). Quenched fluorescent substrates have been used to analyze the rate of inhibition of gelatinase A by TIMP-2 over a wide range of TIMP-2 concentrations. When the values of the observed rate constant for the inhibition are plotted against TIMP-2 concentration, saturation is observed at high concentrations, providing evidence for formation of an intermediate in the pathway. Rate constants for the formation and dissociation of the intermediate are 5.9 x 10(6) M-1 s-1 and 6.3 s-1 respectively, giving a Ki for the initial step of approximately 1 microM. The rate constant for the association of the final complex is 33 s-1. By studying the dissociation of 125I-labeled TIMP-2 from a gelatinase A-TIMP-2 complex using ligand exchange experiments, we obtained a rate constant for the dissociation of the final stable complex of 2 x 10(-)8 s-1. This gives a value for the overall dissociation constant of approximately 0.6 fM.

Enzymatically active papain preferentially induces an allergic response in mice.

Human exposure to papain, a cysteine proteinase, is associated with hypersensitivity reactions. We demonstrate in mice that enzymatically active papain preferentially induces an IgG1 response and results in mast cell degranulation, both features typical of an allergic reaction. Inactive papain, blocked with E-64, appears to desensitize mice to subsequent challenge by active enzyme. These results suggest that the enzymatic activity of papain is critical in inducing an allergic response and that the use of inactive allergens may be a possible strategy for desensitizing allergic individuals.

A classical enzyme active center motif lacks catalytic competence until modulated electrostatically.

The cysteine proteinase superfamily is a source of natural structural variants of value in the investigation of mechanism. It has long been considered axiomatic that catalytic competence of these enzymes mirrors the generation of the ubiquitous catalytic site imidazolium-thiolate ion pair. We here report definitive evidence from kinetic studies supported by electrostatic potential calculations, however, that at least for some of these enzymes the ion pair state which provides the nucleophilic and acid-base chemistry is essentially fully developed at low pH where the enzymes are inactive. Catalytic competence requires an additional protonic dissociation with a common pKa value close to 4 possibly from the Glu50 cluster to control ion pair geometry. The pH dependence of the second-order rate constant (k) for the reactions of the catalytic site thiol groups with 4,4'-dipyrimidyl disulfide is shown to provide the pKa values for the formation and deprotonation of the (Cys)-S-/(His)-Im+H ion pair state. Analogous study of the reactions with 2,2'-dipyridyl disulfide reveals other kinetically influential ionizations, and all of these pKa values are compared with those observed in the pH dependence of kcat/Km for the catalyzed hydrolysis of N-acetylphenylalanylglycine 4-nitroanilide. The discrepancy between the pKa value for ion pair formation and the common pKa value close to 4 related to generation of catalytic activity is particularly marked for ficin (pKa 2.49 +/- 0.02) and caricain (pKa 2.88 +/- 0.02) but exists also for papain (pKa 3.32 +/- 0.01).

Characterization of the hydrolytic activity of a polyclonal catalytic antibody preparation by pH-dependence and chemical modification studies: evidence for the involvement of Tyr and Arg side chains as hydrogen-bond donors.

The hydrolyses of 4-nitrophenyl 4'-(3-aza-2-oxoheptyl)phenyl carbonate and of a new, more soluble, substrate, 4-nitrophenyl 4'-(3-aza-7-hydroxy-2-oxoheptyl)phenyl carbonate, each catalysed by a polyclonal antibody preparation elicited in a sheep by use of an analogous phosphate immunogen, were shown to adhere closely to the Michaelis-Menten equation, in accordance with the growing awareness that polyclonal catalytic antibodies may be much less heterogeneous than had been supposed. The particular value of studies on polyclonal catalytic antibodies is discussed briefly. Both the kcat and kcat/K(m) values were shown to increase with increase in pH across a pKa of approx. 9. Group-selective chemical modification studies established that the side chains of tyrosine and arginine residues are essential for catalytic activity, and provided no evidence for the involvement of side chains of lysine, histidine or cysteine residues. The combination of evidence from the kinetic and chemical modification studies and from studies on the pH-dependence of binding suggests that catalysis involves assistance to the reaction of the substrate with hydroxide ions by hydrogen-bond donation at the reaction centre by tyrosine and arginine side chains. This combination of hydrogen-bond donors appears to be a feature common to a number of other hydrolytic catalytic antibodies. High-pKa acidic side chains may be essential for the effectiveness of catalytic antibodies that utilize hydroxide ions.