Enrichment of enzyme activity on deformylation of 1-NFK-lysozyme.
The formamide linkage of an inactive lysozyme derivative (1-NFK-lysozyme), formed by selective ozonization of tryptophan 62 in hen egg-white lysozyme [EC 18.104.22.168] was hydrolyzed with dilute acid faster in the frozen state at about --10 degrees than at 20 degrees. On hydrolysis of 1-NFK-lysozyme the low lytic activity increased to approximately 80% of that of native lysozyme. It is suggested that the binding ability associated with kynurenine 62 in the lysozyme derivative formed by this hydrolysis may be responsible for increase in enzymatic activity. (+info)
Folding of apocytochrome c induced by the interaction with negatively charged lipid micelles proceeds via a collapsed intermediate state.
Unfolded apocytochrome c acquires an alpha-helical conformation upon interaction with lipid. Folding kinetic results below and above the lipid's CMC, together with energy transfer measurements of lipid bound states, and salt-induced compact states in solution, show that the folding transition of apocytochrome c from the unfolded state in solution to a lipid-inserted helical conformation proceeds via a collapsed intermediate state (I(C)). This initial compact state is driven by a hydrophobic collapse of the polypeptide chain in the absence of the heme group and may represent a heme-free analogue of an early compact intermediate detected on the folding pathway of cytochrome c in solution. Insertion into the lipid phase occurs via an unfolding step of I(C) through a more extended state associated with the membrane surface (I(S)). While I(C) appears to be as compact as salt-induced compact states in solution with substantial alpha-helix content, the final lipid-inserted state (Hmic) is as compact as the unfolded state in solution at pH 5 and has an alpha-helix content which resembles that of native cytochrome c. (+info)
Pathways of electron transfer in Escherichia coli DNA photolyase: Trp306 to FADH.
We describe the results of a series of theoretical calculations of electron transfer pathways between Trp306 and *FADH. in the Escherichia coli DNA photolyase molecule, using the method of interatomic tunneling currents. It is found that there are two conformationally orthogonal tryptophans, Trp359 and Trp382, between donor and acceptor that play a crucial role in the pathways of the electron transfer process. The pathways depend vitally on the aromaticity of tryptophans and the flavin molecule. The results of this calculation suggest that the major pathway of the electron transfer is due to a set of overlapping orthogonal pi-rings, which starts from the donor Trp306, runs through Trp359 and Trp382, and finally reaches the flavin group of the acceptor complex, FADH. (+info)
Localization and environment of tryptophans in soluble and membrane-bound states of a pore-forming toxin from Staphylococcus aureus.
The location and environment of tryptophans in the soluble and membrane-bound forms of Staphylococcus aureus alpha-toxin were monitored using intrinsic tryptophan fluorescence. Fluorescence quenching of the toxin monomer in solution indicated varying degrees of tryptophan burial within the protein interior. N-Bromosuccinimide readily abolished 80% of the fluorescence in solution. The residual fluorescence of the modified toxin showed a blue-shifted emission maximum, a longer fluorescence lifetime as compared to the unmodified and membrane-bound alpha-toxin, and a 5- to 6-nm red edge excitation shift, all indicating a restricted tryptophan environment and deeply buried tryptophans. In the membrane-bound form, the fluorescence of alpha-toxin was quenched by iodide, indicating a conformational change leading to exposure of some tryptophans. A shorter average lifetime of tryptophans in the membrane-bound alpha-toxin as compared to the native toxin supported the conclusions based on iodide quenching of the membrane-bound toxin. Fluorescence quenching of membrane-bound alpha-toxin using brominated and spin-labeled fatty acids showed no quenching of fluorescence using brominated lipids. However, significant quenching was observed using 5- and 12-doxyl stearic acids. An average depth calculation using the parallax method indicated that the doxyl-quenchable tryptophans are located at an average depth of 10 A from the center of the bilayer close to the membrane interface. This was found to be in striking agreement with the recently described structure of the membrane-bound form of alpha-toxin. (+info)
A novel epitope for the specific detection of exogenous prion proteins in transgenic mice and transfected murine cell lines.
Prion diseases are closely linked to the conversion of host-encoded cellular prion protein (PrPC) into its pathological isoform (PrPSc). PrP conversion experiments in scrapie infected tissue culture cells, transgenic mice, and cell-free systems usually require unique epitopes and corresponding monoclonal antibodies (MAbs) for the immunological discrimination of exogenously introduced and endogenous PrP compounds (e.g., MAb 3F4, which is directed to an epitope on hamster and human but not on murine PrP). In the current work, we characterize a novel MAb designated L42 that reacts to PrP of a variety of species, including cattle, sheep, goat, dog, human, cat, mink, rabbit, and guinea pig, but does not bind to mouse, hamster, and rat PrP. Therefore, MAb L42 may allow future in vitro conversion and transgenic studies on PrPs of the former species. The MAb L42 epitope on PrPC includes a tyrosine residue at position 144, whereas mouse, rat, and hamster PrPs incorporate tryptophane at this site. To verify this observation, we generated PrP expression vectors coding for authentic or mutated murine PrPCs (i.e., codon 144 encoding tyrosine instead of tryptophan). After transfection into neuroblastoma cells, MAb L42 did not react with immunoblotted wild-type murine PrPC, whereas L42 epitope-tagged murine PrPC was strongly recognized. Immunoblot and fluorescence-activated cell sorting data revealed that tagged PrPC was correctly posttranslationally processed and translocated to the cell surface. (+info)
Helical structure and packing orientation of the S2 segment in the Shaker K+ channel.
Six transmembrane segments, S1-S6, cluster around the central pore-forming region in voltage-gated K+ channels. To investigate the structural characteristics of the S2 segment in the Shaker K+ channel, we replaced each residue in S2 singly with tryptophan (or with alanine for the native tryptophan). All but one of the 23 Trp mutants expressed voltage-dependent K+ currents in Xenopus oocytes. The effects of the mutations were classified as being of low or high impact on channel gating properties. The periodicity evident in the effects of these mutations supports an alpha-helical structure for the S2 segment. The high- and low-impact residues cluster onto opposite faces of a helical wheel projection of the S2 segment. The low-impact face is also tolerant of single mutations to asparagine. All results are consistent with the idea that the low-impact face projects toward membrane lipids and that changes in S2 packing occur upon channel opening. We conclude that the S2 segment is a transmembrane alpha helix and that the high-impact face packs against other transmembrane segments in the functional channel. (+info)
A single hydrophobic residue confers barbiturate sensitivity to gamma-aminobutyric acid type C receptor.
Barbiturate sensitivity was imparted to the human rho1 homooligomeric gamma-aminobutyric acid (GABA) receptor channel by mutation of a tryptophan residue at position 328 (Trp328), which is located within the third transmembrane domain. Substitutions of Trp328 with a spectrum of amino acids revealed that nearly all hydrophobic residues produced receptor channels that were both directly activated and modulated by pentobarbital with similar sensitivities. Previous studies with ligand-gated ion channels (including GABA) have demonstrated that even conservative amino acid substitution within the agonist-dependent activation domain (N-terminal extracellular domain) can markedly impair agonist sensitivity. Thus, the lack of significant variation in pentobarbital sensitivity among the Trp328 mutants attests to an intrinsic difference between pentobarbital- and the GABA-dependent activation domain. Compared with the heterooligomeric alphabetagamma receptor channel, the mode of modulation for homooligomeric Trp328 mutants by pentobarbital was more dependent on the GABA concentration, yielding potentiation only at low concentrations of GABA (fractions of their respective EC50 values), yet causing inhibition at higher concentrations. Agonist-related studies have also demonstrated that residue 328 plays an important role in agonist-dependent activation, suggesting a functional interconnection between the GABA and pentobarbital activation domains. (+info)
Inhibition of myosin ATPase by metal fluoride complexes.
Magnesium (Mg2+) is the physiological divalent cation stabilizing nucleotide or nucleotide analog in the active site of myosin subfragment 1 (S1). In the presence of fluoride, Mg2+ and MgADP form a complex that traps the active site of S1 and inhibits myosin ATPase. The ATPase inactivation rate of the magnesium trapped S1 is comparable but smaller than the other known gamma-phosphate analogs at 1.2 M-1 s-1 with 1 mM MgCl2. The observed molar ratio of Mg/S1 in this complex of 1.58 suggests that magnesium occupies the gamma-phosphate position in the ATP binding site of S1 (S1-MgADP-MgFx). The stability of S1-MgADP-MgFx at 4 degrees C was studied by EDTA chase experiments but decomposition was not observed. However, removal of excess fluoride causes full recovery of the K+-EDTA ATPase activity indicating that free fluoride is necessary for maintaining a stable trap and suggesting that the magnesium fluoride complex is bonded to the bridging oxygen of beta-phosphate more loosely than the other known phosphate analogs. The structure of S1 in S1-MgADP-MgFx was studied with near ultraviolet circular dichroism, total tryptophan fluorescence, and tryptophan residue 510 quenching measurements. These data suggest that S1-MgADP-MgFx resembles the M**.ADP.Pi steady-state intermediate of myosin ATPase. Gallium fluoride was found to compete with MgFx for the gamma-phosphate site in S1-MgADP-MgFx. The ionic radius and coordination geometry of magnesium, gallium and other known gamma-phosphate analogs were compared and identified as important in determining which myosin ATPase intermediate the analog mimics. (+info)