Structural mapping of aspartate transcarbamoylase by fluorescence energy-transfer measurements: determination of the distance between catalytic sites of different subunits

Real-time monitoring the spatiotemporal dynamics of intracellular cGMP in vascular smooth muscle cells

Real-time and noninvasive imaging of intracellular second messengers in mammalian cells, while -preserving their in vivo phenotype, requires biosensors of exquisite constitution. Here we provide the methodology for utilizing the single wavelength cGMP-biosensor δ-FlincG in aortic vascular smooth muscle cells.

Inter- and innermolecular reactions of chloro(phenyl)carbene

Supramolecular photolyses of 3-chloro-3-phenyl-3H-diazirine (8) were performed within cyclodextrin (CyD) hosts to determine whether these toroidal inclusion compounds could alter the reactivity of the ensuing carbene reaction intermediate, chloro(phenyl)carbene (9). Remarkably, no intramolecular products stemming from carbene 9 could be detected. Instead, modified CyDs were formed via so-called innermolecular reactions. Hence, diazirine 8 was photolyzed in various conventional solvents to gauge the intermolecular reactivity of carbene 9. Relevant results were used to rationalize the CyD innermolecular reaction products.

Fluorescence resonance energy transfers measurements on cell surfaces via fluorescence polarization

A method has been developed for the determination of the efficiency of fluorescence resonance energy transfer efficiency between moieties located on cell surfaces by performing individual cell fluorescence polarization (FP) measurements. The absolute value of energy transfer efficiency (E) is calculated on an individual cell basis. The examination of this methodology was carried out using model experiments on human T lymphocyte cells. The cells were labeled with fluorescein-conjugated Concanavalin A (ConA) as donor, or rhodamine-conjugated ConA as acceptor. The experiments and results clearly indicate that determination of E via FP measurements is possible, efficient, and more convenient than other methods.

Is pyridoxal 5'-phosphate an affinity label for phosphate-binding sites in proteins?: The case of bovine glutamate dehydrogenase

The effects of pyridoxal 5'-phosphate (PalP) on ox liver glutamate dehydrogenase (94% inactivation by 1.8 mM reagent at pH 7 and 25 degrees C) have been compared with those of three analogues, 5'-deoxypyridoxal (96% inactivation), pyridoxal 5'-sulphate (97%) and pyridoxal 5-methylsulphonate (94%), in order to establish whether PalP acts as an affinity label for this enzyme. Like PalP and unlike pyridoxal, which is a much less potent inactivator, none of the analogues has a free 5'-OH group to cyclize with the aldehyde function. The result with 5'-deoxypyridoxal shows that a negative charge, such as that of the phosphate group, is not required for efficient inactivation. With all four reagents, addition of an excess of cysteine or lysine led to 90-100% re-activation over 3-20 h. Dialysis also caused reactivation to a similar extent. A combination of 2.15 mM NADH, 1 mM GTP and 10 mM 2-oxoglutarate gave complete protection against PalP, but only partial protection against the analogues. 5'-Deoxypyridoxal still caused 20-25% inactivation in the presence of the protection mixture. Absorbance measurements after reduction with NaBH4 show the characteristic features of a reduced Schiff's base and allowed estimation of the extent of reaction. With all the reagents the protection mixture decreased incorporation by about 1 mol/mol, but levels of incorporation without protection varied from about 2 mol/mol for PalP up to about 5 mol/mol for 5'-deoxypyridoxal. The labelling at additional sites may explain the residual inactivation in the presence of potent protecting agents.

Orientation factor in steady-state and time-resolved resonance energy transfer measurements

Resonance energy transfer measurements provide a way to estimate distances between chromophores attached to different sites of macromolecules. There are two unknowns involved in resonance energy transfer measurements, the distance between two chromophores and their relative orientation. When static orientational disorder exists, the orientation factor, kappa 2, can vary from 0 to 4, leading to considerable uncertainty in estimation of distances. Fluorescence polarization anisotropy measurements can reduce the degree of uncertainty [Dale & Eisinger (1974) Biopolymers 13, 1573]. There may still be substantial error bounds for the average distance measurements. Time-resolved fluorescence measurements provide an "apparent" average distance and distance distribution containing contributions by both distance and orientation. The contribution of orientation to observed "apparent" average distance and distance distribution widths has been estimated for both simulated and real data. With a single unique distance as input in the simulation and with random but static orientation of donor and acceptor, the recovered average distance is very close to that of the input when the input distance is close to or larger than the Förster distance. The recovered width of apparent distance distribution can be substantial and it changes as a function of Förster distance to average distance ratio and as a function of Förster distance. Similar conclusions apply to the case where there is a real distance distribution. Motional averaging of the orientation was simulated by the Monte Carlo method to estimate the contribution of orientation when chromophores have certain degrees of mobility.(ABSTRACT TRUNCATED AT 250 WORDS)

Active-site-directed inactivation of wheat-germ aspartate transcarbamoylase by pyridoxal 5'-phosphate

Treatment of 1 microM wheat-germ aspartate transcarbamoylase with 1 mM-pyridoxal 5'-phosphate caused a rapid loss of activity, concomitant with the formation of a Schiff base. Complete loss of activity occurred within 10 min when the Schiff base was reduced with a 100-fold excess of NaBH4. Concomitantly, one amino group per chain was modified. No further residues were modified in the ensuing 30 min. The kinetics of inactivation were examined under conditions where the Schiff base was reduced before assay. Inactivation was apparently first-order. The pseudo-first-order rate constant, kapp., showed a hyperbolic dependence upon the concentration of pyridoxal 5'-phosphate, suggesting that the enzyme first formed a non-covalent complex with the reagent, modification of a lysine then proceeding within this complex. Inactivation of the enzyme by pyridoxal was 20 times slower than that by pyridoxal 5'-phosphate, indicating that the phosphate group was important in forming the initial complex. Partial protection against pyridoxal phosphate was provided by the leading substrate, carbamoyl phosphate, and nearly complete protection was provided by the bisubstrate analogue, N-phosphonoacetyl-L-aspartate, and the ligand-pair carbamoyl phosphate plus succinate. Steady-state kinetic studies, under conditions that minimized inactivation, showed that pyridoxal 5'-phosphate was also a competitive inhibitor with respect to the leading substrate, carbamoyl phosphate. Pyridoxal 5'-phosphate therefore appears to be an active-site-directed reagent. A sample of the enzyme containing one reduced pyridoxyl group per chain was digested with trypsin, and the labelled peptide was isolated and shown to contain a single pyridoxyl-lysine residue. Partial sequencing around the labelled lysine showed little homology with the sequence surrounding lysine-84, an active-centre residue of the catalytic subunit of aspartate transcarbamoylase from Escherichia coli, whose reaction with pyridoxal 5'-phosphate shows many similarities to the results described in the present paper. Arguably the reactive lysine is conserved between the two enzymes whereas the residues immediately surrounding the lysine are not. The same conclusion has been drawn in a comparison of reactive histidine residues in the two enzymes [Cole & Yon (1986) Biochemistry 25, 7168-7174].

Flow cytometric measurement of fluorescence resonance energy transfer on cell surfaces. Quantitative evaluation of the transfer efficiency on a cell-by-cell basis

A method has been developed for the determination of the efficiency (E) of the fluorescence resonance energy transfer between moieties on cell surfaces by use of a computer-controlled flow cytometer capable of dual wavelength excitation. The absolute value of E may be calculated on a single-cell basis. The analysis requires the measurement of samples stained with donor and acceptor conjugated ligands alone as well as together. In model experiments HK 22 murine lymphoma cells labeled with fluorescein-conjugated concanavalin A (Con A) and/or rhodamine conjugated Con A were used to determine energy transfer histograms. Using the analytic solution to energy transfer in two dimensions, a high surface density of Con A binding sites was found that suggests that the Con A receptor sites on the cell surface are to a degree preclustered . We call this technique flow cytometric energy transfer ( FCET ).

Fluorescence energy transfer in two dimensions. A numeric solution for random and nonrandom distributions

A method of Monte Carlo calculations has been applied to the problem of fluorescence energy transfer in two dimensions in order to provide a quantitative measure of the effects of nonideal mixing of lipid and protein molecules on the quenching profiles of membrane systems. These numerical techniques permit the formulation of a detailed set of equations that describes in a precise manner the quenching and depolarization properties of planar donor-acceptor distributions as a function of specific spectroscopic and organizational parameters. Because of the exact nature of the present numeric method, these results are used to evaluate critically the validity of previous approximate treatments existing in the literature. This method is also used to examine the effects of excluded volume interactions and distinct lattice structures on the expected transfer efficiencies. As a specific application, representative quenching profiles for protein-lipid mixtures, in which donor groups are covalently linked to the protein molecules and acceptor species are randomly distributed within lipid domains, have been obtained. It is found that the existence of phase-separated protein domains gives rise to a shielding effect that significantly decreases the transfer efficiencies with respect to those expected for an ideal distribution of protein molecules. The results from the present numerical study indicate that the experimental application of fluorescence energy transfer measurements in multicomponent membrane systems can be used to obtain organizational parameters that accurately reflect the lateral distribution of protein and lipid molecules within the bilayer membrane.

Calculation on fluorescence resonance energy transfer on surfaces

A general method for estimating fluorescence resonance energy transfer between distributions of donors and acceptors on surfaces is presented. Continued fraction approximants are obtained from equivalent power series expansions of the change in quantum yield in terms of the fluorescent lifetimes or the steady-state fluorescence. These approximants provide analytic equations for the analysis of energy transfer and error bounds for the approximants. Specific approximants are derived for five models of interest for membrane biochemistry: (a) an infinite plane, (b) parallel infinite planes, (c) the surface of a sphere, (d) the surfaces of concentric spheres, and (e) the surfaces of two separated spheres. Recent experimental results in the literature are analyzed with the equations obtained.