Heat perturbation of bovine eye lens alpha-crystallin probed by covalently attached ratiometric fluorescent dye 4'-diethylamino-3-hydroxyflavone.

Bovine eye lens alpha-crystallin was covalently labeled with 6-bromomethyl-4'-diethylamino-3-hydroxyflavone and studied under native-like conditions and at the elevated temperature (60 degrees C) that is known to facilitate alpha-crystallin chaperone-like activity. This novel SH-reactive two-band ratiometric fluorescent probe is characterized by two highly emissive N*- and T*-bands; the latter appears due to excited state intramolecular proton transfer reaction. The positions of these bands and the ratio of their intensities for the alpha-crystallin-dye conjugate are the sensitive indicators of polarity of the dye environment and its participation in intermolecular hydrogen bonding. Although we found that the dye labels both the SH and the NH2 groups in alpha-crystallin, a recently developed procedure allowed us to distinguish between the heat-induced spectral changes of the dye molecules attached to SH and NH2 groups. We observed that at elevated temperature the environment of the SH-attached dye becomes more polar and flexible. The number of H-bond acceptor groups in the vicinity of the dye decreases. Since alpha-crystallin contains a single Cys residue within the C-terminal domain of its (alpha)A subunit (the (alpha)B subunit contains none), we can attribute the observed effects to temperature-induced changes in the C-terminal domain of this protein.

Trp42 rotamers report reduced flexibility when the inhibitor acetyl-pepstatin is bound to HIV-1 protease.

The Q7K/L331/L631 HIV-1 protease mutant was expressed in Escherichia coli and the effect of binding a substrate-analog inhibitor, acetyl-pepstatin, was investigated by fluorescence spectroscopy and molecular dynamics. The dimeric enzyme has four intrinsic tryptophans, located at positions 6 and 42 in each monomer. Fluorescence spectra and acrylamide quenching experiments show two differently accessible Trp populations in the apoenzyme with k(q1) = 6.85 x 10(9) M(-1) s(-1) and k(q2) = 1.88 x 10(9) M(-1) s(-1), that merge into one in the complex with k(q) = 1.78 x 10(9) M(-1) s(-1). 500 ps trajectory analysis of Trp X1/X2 rotameric interconversions suggest a model to account for the observed Trp fluorescence. In the simulations, Trp6/Trp6B rotameric interconversions do not occur on this timescale for both HIV forms. In the apoenzyme simulations, however, both Trp42s and Trp42Bs are flipping between X1/X2 states; in the complexed form, no such interconverions occur. A detailed investigation of the local Trp environments sampled during the molecular dynamics simulation suggests that one of the apoenzyme Trp42B rotameric interconversions would allow indole-quencher contact, such as with nearby Tyr59. This could account for the short lifetime component. The model thus interprets the experimental data on the basis of the conformational fluctuations of Trp42s alone. It suggests that the rotameric interconversions of these Trps, located relatively far from the active site and at the very start of the flap region, becomes restrained when the apoenzyme binds the inhibitor. The model is thus consistent with associating components of the fluorescence decay in HIV-1 protease to ground state conformational heterogeneity.

Tryptophan phosphorescence signals characteristic changes in protein dynamics at physiological temperatures.

The Arrhenius plot of the de-excitation rate of tryptophan triplet state deviates from linearity in the physiological temperature range for several proteins with buried tryptophans, similarly to the behaviour of enzyme activity. A model is presented featuring two de-excitation pathways whose effectiveness is regulated by protein dynamics.

Fluorescence measurements detect changes in scallop myosin regulatory domain.

Ca2+-induced conformational changes of scallop myosin regulatory domain (RD) were studied using intrinsic fluorescence. Both the intensity and anisotropy of tryptophan fluorescence decreased significantly upon removal of Ca2+. By making a mutant RD we found that the Ca2+-induced fluorescence change is due mainly to Trp21 of the essential light chain which is located at the unusual Ca2+-binding EF-hand motif of the first domain. This result suggests that Trp21 is in a less hydrophobic and more flexible environment in the Ca2+-free state, supporting a model for regulation based on the 2 A resolution structure of scallop RD with bound Ca2+ [Houdusse A. and Cohen C. (1996) Structure 4, 21-32]. Binding of the fluorescent probe, 8-anilinonaphthalene-1-sulphonate (ANS) to the RD senses the dissociation of the regulatory light chain (RLC) in the presence of EDTA, by energy transfer from a tryptophan cluster (Trp818, 824, 826, 827) on the heavy chain (HC). We identified a hydrophobic pentapeptide (Leu836-Ala840) at the head-rod junction which is required for the effective energy transfer and conceivably is part of the ANS-binding site. Extension of the HC component of RD towards the rod region results in a larger ANS response, presumably indicating changes in HC-RLC interactions, which might be crucial for the regulatory function of scallop myosin.

Effect of some potassium selective crown ethers on the permeability and structure of a phospholipid membrane.

The effect of some new crown ethers on the cation efflux and phase transition parameters of dipalmitoyl phosphatidylcholine liposomes was studied. The effects were correlated with the lipophilicity of the crown ethers. The results indicate that the presence of two crown ring structures in one crown either molecule is a prerequisite for the increase of ion permeability of liposomes. The effective crown ethers decrease the temperature, enthalpy and cooperativity of the gel-to-liquid crystalline phase transition. The crown ethers increase membrane permeability for potassium and, to a lesser extent, for rubidium and sodium. The ratio of permeability increase for potassium/rubidium significantly correlates with the lipophilicity of the crown ethers.

Physicochemical studies of two major subpopulations of low density lipoproteins by differential scanning calorimetry and n.m.r. spectroscopy.

Two subpopulations, layer 2 (density 1.025-1.029 g/ml) and layer 3 (density 1.032-1.043 g/ml) of low density lipoproteins (LDL) were isolated from fresh human plasma of normal lipidaemic subjects by density gradient ultracentrifugation. Chemical analyses demonstrated the ratios of triglyceride/cholesterol ester decreased with increasing densities of subfractions. These subfractions together with triglyceride-rich lipoproteins (layer 1, density less than 1.019 g/ml) were subjected to physicochemical studies by differential scanning calorimetry (d.s.c.) and nuclear magnetic resonance (n.m.r.) spectroscopy. The average transition temperature (Tt) of layer 2 was 34.20 +/- 0.83 degrees C and that of layer 3 was 37.25 +/- 0.35 degrees C. In addition, many of the layer 3, but not layer 2 and layer 1, samples showed structural alteration and gave rise to an average Tt of 39.18 +/- 1.24 degrees C. The structural alteration could be detected with polarizing light microscopy showing birefringent spherulites at body temperature. The peak Tt values obtained by d.s.c. were in good agreement with those by n.m.r. spectroscopy. These results demonstrate the physicochemical heterogeneity within the LDL density region and suggest that layer 3 subpopulation is much more labile than the others.