No abstract

The thermodynamics and kinetics of the interaction of dihydrofolate reductase (DHFR) with methotrexate have been studied by using fluorescence, stopped-flow, and single-molecule methods. DHFR was modified to permit the covalent addition of a fluorescent molecule, Alexa 488, and a biotin at the N terminus of the molecule. The fluorescent molecule was placed on a protein loop that closes over methotrexate when binding occurs, thus causing a quenching of the fluorescence. The biotin was used to attach the enzyme in an active form to a glass surface for single-molecule studies. The equilibrium dissociation constant for the binding of methotrexate to the enzyme is 9.5 nM. The stopped-flow studies revealed that methotrexate binds to two different conformations of the enzyme, and the association and dissociation rate constants were determined. The single-molecule investigation revealed a conformational change in the enzyme-methotrexate complex that was not observed in the stopped-flow studies. The ensemble averaged rate constants for this conformation change in both directions is about 2-4 s ؊1 and is attributed to the opening and closing of the enzyme loop over the bound methotrexate. Thus the mechanism of methotrexate binding to DHFR involves multiple steps and protein conformational changes.enzyme mechanisms ͉ protein dynamics ͉ fluorescence microscopy

We report a facile approach to prepare an artificial enzyme system for tandem catalysis. NiPd hollow nanoparticles and glucose oxidase (GOx) were simultaneously immobilized on the zeolitic imidazolate framework 8 (ZIF-8) via a co-precipitation method. The as-prepared GOx@ZIF-8(NiPd) nanoflower not only exhibited the peroxidase-like activity of NiPd hollow nanoparticles but also maintained the enzymatic activity of GOx. A colorimetric sensor for rapid detection of glucose was realized through the GOx@ZIF-8(NiPd) based multi-enzyme system. Moreover, the GOx@ZIF-8(NiPd) modified electrode showed good bioactivity of GOx and high electrocatalytic activity for the oxygen reduction reaction (ORR), which could also be used for electrochemical detection of glucose.

Recent studies have shown that hyperglycemia is a principal cause of cellular damage in patients with diabetes mellitus. A major consequence of hyperglycemia is increased oxidative stress. Glucose-6-phosphate dehydrogenase (G6PD) plays an essential role in the regulation of oxidative stress by primarily regulating NADPH, the main intracellular reductant. In this paper we show that increased glucose (10 -25 mM) caused inhibition of G6PD resulting in decreased NADPH levels in bovine aortic endothelial cells (BAEC). Inhibition was seen within 15 min. High glucose-induced inhibition of G6PD predisposed cells to cell death. High glucose via increased activity of adenylate cyclase also stimulated an increase in cAMP levels in BAEC. Agents that increased cAMP caused a decrease in G6PD activity. Inhibition of cAMP-dependent protein kinase A ameliorated the high glucose-induced inhibition of G6PD. Finally, high glucose stimulated phosphorylation of G6PD. These results suggest that, in BAEC, high glucose stimulated increased cAMP, which led to increased protein kinase A activity, phosphorylation of G6PD, and inhibition of G6PD activity. We conclude that these changes in G6PD activity play an important role in high glucose-induced cell damage/death.

Annexin A1 (ANXA1), a mediator of the anti-inflammatory action of glucocorticoids, is important in cancer development and progression, whereas NF-κB regulates multiple cellular phenomena, some of them associated with inflammation and cancer. We showed that glucocorticoids and chemopreventive modified nonsteroidal anti-inflammatory drugs, such as nitric oxide-donating aspirin (NO-ASA) and phospho-aspirin, induced ANXA1 in cultured human colon and pancreatic cancer cells. ANXA1 associated with NF-κB and suppressed its transcriptional activity by preventing NF-κB binding to DNA. The induction of ANXA1 by glucocorticoids was proportional to their anti-inflammatory potency, as was the suppression of NF-κB activity, which was accompanied by enhanced apoptosis and inhibition of cell growth mediated by changes in NF-κB-dependent cell signaling. The proposed novel mechanism was operational in the intestinal mucosa of mice treated with dexamethasone or NO-ASA. ANXA1-based oligopeptides displayed the same effects as ANXA1 on NF-κB. One such tripeptide (Gln-Ala-Trp) administered to nude mice inhibited the growth of SW480 human colon cancer xenografts by 58% compared with control (P < 0.01). Our findings reveal that ANXA1 is an inducible endogenous inhibitor of NF-κB in human cancer cells and mice, provide a novel molecular mechanism for the action of anti-inflammatory agents, and suggest the possibility of mechanism-driven drug development.

Intracerebroventricular treatment with redox-regulating Mn(III) Nhexylpyridylporphyrin (MnPorphyrin) is remarkably efficacious in experimental central nervous system (CNS) injury. Clinical development has been arrested because of poor blood-brain barrier penetration. Mn(III) meso-tetrakis (N-hexylpyridinium-2-yl) porphyrin (MnTnHex-2-PyP) was synthesized to include four six-carbon (hexyl) side chains on the core MnPorphyrin structure. This has been shown to increase in vitro lipophilicity 13,500-fold relative to the hydrophilic ethyl analog Mn(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin (MnTE-2-PyP). In normal mice, we found brain MnTnHex-2-PyP accumulation to be ϳ9-fold greater than MnTE-2-PyP 24 h after a single intraperitoneal dose. We then evaluated MnTnHex-2-PyP efficacy in outcome-oriented models of focal cerebral ischemia and subarachnoid hemorrhage. For focal ischemia, rats underwent 90-min middle cerebral artery occlusion. Parenteral MnTnHex-2-PyP treatment began 5 min or 6 h after reperfusion onset and continued for 7 days. Neurologic function was improved with both early (P ϭ 0.002) and delayed (P ϭ 0.002) treatment onset. Total infarct size was decreased with both early (P ϭ 0.03) and delayed (P ϭ 0.01) treatment. MnTnHex-2-PyP attenuated nuclear factor B nuclear DNA binding activity and suppressed tumor necrosis factor-␣ and interleukin-6 expression. For subarachnoid hemorrhage, mice underwent perforation of the anterior cerebral artery and were treated with intraperitoneal MnTnHex-2-PyP or vehicle for 3 days. Neurologic function was improved (P ϭ 0.02), and vasoconstriction of the anterior cerebral (P ϭ 0.0005), middle cerebral (P ϭ 0.003), and internal carotid (P ϭ 0.015) arteries was decreased by MnTnHex-2-PyP. Side-chain elongation preserved MnPorphyrin redox activity, but improved CNS bioavailability sufficient to cause improved outcome from acute CNS injury, despite delay in parenteral treatment onset of up to 6 h. This advance now allows consideration of MnPorphyrins for treatment of cerebrovascular disease.

The interaction of dihydrofolate (H2F) and NADPH with a fluorescent derivative of H 2F reductase (DHFR) was studied by using transient and single-molecule techniques. The fluorescent moiety Alexa 488 was attached to the structural loop that closes over the substrates after they are bound. Fluorescence quenching was found to accompany the binding of both substrates and the hydride transfer reaction. For the binding of H 2 F to DHFR, the simplest mechanism consistent with the data postulates that the enzyme exists as slowly interconverting conformers, with the substrate binding preferentially to one of the conformers. At pH 7.0, the binding reaction has a bimolecular rate constant of 1.8 ؋ 10 7 M ؊1 ⅐s ؊1 , and the formation of the initial complex is followed by a conformational change. The binding of NADPH to DHFR is more complex and suggests multiple conformers of the enzyme exist. NADPH binds to a different conformer than H 2F with a bimolecular rate constant of 2.6 -5.7 ؋ 10 6 M ؊1 ⅐s ؊1 , with the former value obtained from single-molecule kinetics and the latter from stopped-flow kinetics. Single-molecule studies of DHFR in equilibrium with substrates and products revealed a reaction with ensemble average rate constants of 170 and 470 s ؊1 at pH 8.5. The former rate constant has an isotope effect of >2 when NADPD is substituted for NADPH and probably is associated with hydride transfer. The results from stopped-flow and single-molecule methods are complementary and demonstrate that multiple conformations of both the enzyme and enzyme-substrate complexes exist. D ihydrofolate (H 2 F) reductase (DHFR) is a key enzyme for the biosynthesis of purines, thymidylate, and a number of amino acids. Its strategic location in metabolism has also made it a target for anticancer drugs. DHFR catalyzes the reaction of 7,8-H 2 F and NADPH to form 5,6,7,8-tetrahydrofolate. The mechanism of action of DHFR has been extensively probed to better understand the fundamental nature of enzyme catalysis. This is because of not only its physiological importance, but also its relatively easy purification and stability. These investigations include multiple structure determinations, steady-state and transient kinetic studies, theory, and single-molecule kinetics (cf. refs. 1-5).The structure of DHFR from Escherichia coli (molecular weight Ϸ18,000) is compact, with a loop that closes over the substrate binding sites. The mechanism of action of the enzyme involves multiple conformational changes that include domain rotation and interactions with structural loops (cf. refs. 1 and 2). Although the mechanism has been well characterized, it still remains an open question as to how the conformational changes that occur enhance the catalytic activity. Consequently, we have embarked on further investigation of the conformational coupling to catalysis by using transient and single-molecule kinetics.DHFR has a structural loop that closes over the substrates after they are bound. This is revealed by the x-ray structures and dynamic NMR measurements ...

A novel one-pot strategy is proposed to fabricate 3D porous graphene (3D GN) decorated with FeO nanoparticles (FeO NPs) by using hemin as iron source. During the process, graphene oxide was simultaneously reduced and self-assembled to form 3D graphene hydrogel while FeO NPs synthesized from hemin distributed uniformly on 3D GN. The preparation process is simple, facile, economical, and green. The obtained freeze-dried product (3D GH-5) exhibits outstanding peroxidase-like activity. Compared to the traditional 2D graphene-based nanocomposites, the introduced 3D porous structure dramatically improved the catalytic activity, as well as the catalysis velocity and its affinity for substrate. The high catalytic activity could be ascribed to the formation of FeO NPs and 3D porous graphene structures. Based on its peroxidase-like activity, 3D GH-5 was used for colorimetric determination of glucose with a low detection limit of 0.8 μM.