Two crystal structures of a potently sweet protein. Natural monellin at 2.75 A resolution and single-chain monellin at 1.7 A resolution

Two refined structures of the sweet-tasting protein monellin are presented. The structure of natural monellin has been refined at 2.75 A resolution. The final model consists of four monellin molecules in the asymmetric unit, encompassing 3136 non-hydrogen atoms. The crystallographic R-factor is 0.193 for the 8853 reflections between 6.0 A and 2.75 A resolution, and the root-mean-square deviations from ideality are 0.017 A for bond lengths and 3.6 degrees for bond angles. The refined structure generally confirms, with some difference in detail, the initial backbone model of monellin that was based on 3.0 A resolution data. Single-chain monellin (scm) was genetically engineered by fusing the two chains of monellin into a single 94-residue polypeptide. Using the refined monellin coordinates as a search model, the crystal structure of scm has been solved with the techniques of molecular replacement, and has been refined against data to 1.7 A resolution. The final model consists of two scm molecules per asymmetric unit, and includes 137 bound water molecules. The crystallographic R-factor for this model is 0.174 for the 15,053 reflections (magnitude of FO magnitude of > 2 sigma (FO)) between 6.0 A and 1.7 A resolution. The root-mean-square deviations from ideal bond lengths and angles are 0.015 A and 2.86 degrees, respectively, and the average coordinate error is approximately 0.2 A, as estimated from a Luzzati plot. The error in the model was also estimated by comparing the two molecules in the asymmetric unit. The most significant differences between the two molecules occur in loop regions and at the C terminus of the protein, and are generally correlated to differences in crystal packing contacts. Linking the two chains of monellin has not substantially altered the structure beyond the region immediately surrounding the new peptide bond. Like natural monellin, the conformation of scm is dominated by a 17-residue alpha-helix folded into the concave side of a twisted, five-strand anti-parallel beta-sheet. We expect that the availability of a high-resolution structure of scm, along with the convenience of producing site-specific mutants of this protein, will make scm a good model with which to probe the structural basis of sweetness.

Modified polyvinyl alcohol-benzoporphyrin derivative conjugates as phototoxic agents

Photosensitizing and biodistribution characteristics of a photosensitizer (benzoporphyrin derivative, monoacid ring A; BPD) conjugated to a macromolecule (modified polyvinyl alcohol; M-PVA, molecular weight = 10,000) were tested in vitro and in vivo. Modified PVA was loaded with BPD at molar ratios 1:12, 1:25, 1:50, 1:75 and 1:100. Most of the work was carried out with a conjugate having a 1:25 molar ratio. In vitro photosensitization was tested using A549 (human lung carcinoma), A432 (human epidermoid carcinoma) and P815 (mastocytoma of DBA/2 mice) cell lines. Photosensitization of M1 (rhabdomyosarcoma of DBA/2 mice) tumors was tested in an in vivo/in vitro assay, in which tumor-bearing mice were injected intravenously with free or conjugated 3H-BPD and 3 h later light activation of tumor cells was carried out in vitro. Biodistribution studies were carried out using M1 tumor-bearing DBA/2 mice and 3H-BPD either free or conjugated to M-PVA. The results of these studies showed that the conjugation of BPD to M-PVA resulted in the formation of a macromolecular photosensitizer that retained full photosensitizing activity of the photosensitizer molecules and at the same time gained new characteristics, advantageous for photodynamic treatment, especially in vivo. In vitro M-PVA-BPD conjugates were at least as efficient in photosensitization of tumor cells as an equivalent number of free BPD molecules, both in the presence and in the absence of serum. Although the biodistribution was in general comparable to free BPD, the conjugate (1:25) reached slightly higher levels in the blood, kidney, lung and spleen, and lower levels in the liver, brain, skin and muscle in comparison with free BPD.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ethanol ingestion on glucose transporter-1 protein and mRNA levels in rat brain

In the normal adult brain, glucose provides 90% of the energy requirement, as well as substrate for nucleic acid and lipid synthesis. We have previously observed that ethanol impairs hexose uptake by rat astrocytes in culture. In the present study, male Sprague-Dawley rats, 200-250 g, were fed liquid diet in which 36% of the calories were derived from ethanol (EF) for 4 weeks. Controls were fed ad libitum (AF) or pair-fed (PF) an equicaloric diet without ethanol. Blood glucose levels did not differ between the groups at the time of study. Glucose transport by brain plasma membranes was characterized by cytochalasin B binding and showed a slight increase in transporter number (mean +/- SEM of 4 experiments = 76.4 +/- 2.5 pmoles/mg protein in EF vs. 69.5 +/- 1.0 in PF) with no change in affinity (1.8 +/- 0.1 nM-1 in EF and 1.6 +/- 0.1 in PF). Glucose transporter, GLUT-1, was increased on Western blots. In contrast, Northern analysis of cortical tissue, using a rat brain glucose transporter cDNA insert (1.59 kb Bgl II fragment of pSPGT-1), showed a 23 to 35% decrease in steady-state levels of glucose transporter mRNA. GLUT-1 mRNA, localized in brain sections by in situ hybridization histochemistry, showed marked reductions in choroid plexus and hippocampus following ethanol treatment. Ethanol appears to have multiple effects on brain GLUT-1.

Visual functions and trace element metabolism in tobacco-toxic optic neuropathy

Visual functions and nutrition metabolic characteristics were studied in 8 subjects (16 eyes) with tobacco-toxic optic neuropathy (TTON). Their visual functions tested by psychophysical and electrophysiologic methods showed that 1: 1. central vision diminished in 16 eyes, 2. dyschromatopsias were found in 14 tested eyes, 3. bilateral symmetrical central or cecocentral scotomas were the visual field characteristics in all cases, 4. PVEP were severe abnormal in 3 spatial frequencies in all cases and 56.3% of 15' checkboard PVEP showed flat responses, which indicated the impairment of optic nerve dominated by the central field. However, the preserved visual responses could be obtained by FVEP test in 14 tested eyes even though their visual acuity were between the range of 0.02-0.2 and flat PVEP responses. The II and III wave latencies of primary stage were more prolonged than those of control group (P < 0.01), which further indicated the preferential demyelination corresponding to the papillomacular bundles, 5. ERG showed slightly attenuated amplitudes in 5 of 8 tested eyes, which indicated the secondary and mild retinal lesion. On the other hand, TTON occurred on a background of long-term, heavy smoking, drinking, emaciation and malnutrition bodies with low serum zinc level.

Effects of ethanol on glucose utilization by cultured mammalian embryos

We have previously observed correlations between placental glucose transfer and growth of fetuses of ethanol (EtOH)-fed and control rats. In the present study, whole mammalian embryos were used to define the interaction of glucose supply and the effects of EtOH on growth and differentiation. Rat embryos were cultured in 75% normal rat serum from day 9.5 to day 11.5 of gestation. EtOH produced dose-dependent reductions of embryo protein content (mean +/- SEM = 212 +/- 5, 171 +/- 11, 141 +/- 16, and 113 +/- 9 micrograms/embryo in the presence of 0, 25, 50, and 100 mM EtoH, respectively). Somite number was 25.7 +/- 0.3, 23.4 +/- 0.7, 21.8 +/- 0.7, and 21.1 +/- 0.4 under the same conditions. Exposure to ethanol during the first 24 hr in culture decreased embryo protein content to the same extent as exposure for the entire 48-hr culture period. After 46 hr in culture, control and ethanol-exposed embryos were incubated with 14C-glucose for 2 hr. Ethanol produced dose-dependent reductions of CO2 production, anabolic utilization, lactate release, and total glucose utilization. Glucose supplementation (300 mg/dl) significantly increased embryo protein content and each of these glucose utilization parameters. When glucose utilization was expressed relative to embryo protein content, incorporation of the label into embryonic tissues was significantly reduced by ethanol and increased by glucose supplementation. Embryo protein content correlated closely (r = 0.871, p less than 0.0001) with anabolic glucose utilization. Thus, ethanol directly affects embryo glucose utilization, both as an energy source and as a synthetic substrate, in addition to its effects on placental glucose transfer.

Electron spin echo envelope modulation studies of the Cu(II)-substituted derivative of isopenicillin N synthase: a structural and spectroscopic model

Electron spin echo envelope modulation spectroscopy (ESEEM) was used to study the active site structure of isopenicillin N synthase (IPNS) from Cephalosporium acremonium with Cu(II) as a spectroscopic probe. Fourier transform of the stimulated electron spin-echo envelope for the Cu(II)-substituted enzyme, Cu(II)IPNS, revealed two nearly magnetically equivalent, equatorially coordinated His imidazoles. The superhyperfine coupling constant, Aiso, for the remote 14N of each imidazole was 1.65 MHz. The binding of substrate to the enzyme altered the magnetic coupling so that Aiso is 1.30 MHz for one nitrogen and 2.16 MHz for the other. From a comparison of the ESEEM of Cu(II)IPNS in D2O and H2O, it is suggested that water is a ligand of Cu(II) and this is displaced upon the addition of substrate.

"Soft docking": matching of molecular surface cubes

Molecular recognition is achieved through the complementarity of molecular surface structures and energetics with, most commonly, associated minor conformational changes. This complementarity can take many forms: charge-charge interaction, hydrogen bonding, van der Waals' interaction, and the size and shape of surfaces. We describe a method that exploits these features to predict the sites of interactions between two cognate molecules given their three-dimensional structures. We have developed a "cube representation" of molecular surface and volume which enables us not only to design a simple algorithm for a six-dimensional search but also to allow implicitly the effects of the conformational changes caused by complex formation. The present molecular docking procedure may be divided into two stages. The first is the selection of a population of complexes by geometric "soft docking", in which surface structures of two interacting molecules are matched with each other, allowing minor conformational changes implicitly, on the basis of complementarity in size and shape, close packing, and the absence of steric hindrance. The second is a screening process to identify a subpopulation with many favorable energetic interactions between the buried surface areas. Once the size of the subpopulation is small, one may further screen to find the correct complex based on other criteria or constraints obtained from biochemical, genetic, and theoretical studies, including visual inspection. We have tested the present method in two ways. First is a control test in which we docked the components of a molecular complex of known crystal structure available in the Protein Data Bank (PDB). Two molecular complexes were used: (1) a ternary complex of dihydrofolate reductase, NADPH and methotrexate (3DFR in PDB) and (2) a binary complex of trypsin and trypsin inhibitor (2PTC in PDB). The components of each complex were taken apart at an arbitrary relative orientation and then docked together again. The results show that the geometric docking alone is sufficient to determine the correct docking solutions in these ideal cases, and that the cube representation of the molecules does not degrade the docking process in the search for the correct solution. The second is the more realistic experiment in which we docked the crystal structures of uncomplexed molecules and then compared the structures of docked complexes with the crystal structures of the corresponding complexes. This is to test the capability of our method in accommodating the effects of the conformational changes in the binding sites of the molecules in docking.(ABSTRACT TRUNCATED AT 400 WORDS)