Circular Dichroism and Fluorescence Studies of Polyomavirus Major Capsid Protein VP1

The conformational changes of polymavirus (Py) major capsid protein VP₁ in solution by the solution pH, addition of calcium, and ionic strength were examined by circular dichroism (CD) and fluorescence spectroscopy. Comparison of the predicted secondary structures of PyVP₁ and simian virus (SV) 40 by the methods of Chou-Fasman, Gamier et al., and Yang method are presented. Hydropathicity, surface probability, and chain flexibility of PyVP₁ were computer-analyzed by the methods of Kyte and Doolittle, Emini et al., and Karplus and Schulz, respectively. The CD measurements indicate that the secondary structure of PyVP₁ is little dependent on its concentration, Ca²⁺ concentration, and ionic strength, but is strongly pH dependent. Fluorescence studies showed that emission spectra of PyVP₁ are also pH-dependent. At extreme acidic and alkaline pH, the fluorescence intensity of PyVP₁ is decreased and the emission maximum is red-shifted. The fluorescence of PyVP₁ is quenched by the presence of CsCl, KI, and acrylamide. The analyses of the modified Stern–Volmer plots indicate that five of seven tryptophan residues in PyVP₁ are located on the surface of the protein, among which two are accessible to Cs⁺ and the other three are accessible to I^–. The two others are buried more deeply in the interior of the protein molecule.On leave from National Taiwan University;     

Dual color microscopic imagery of cells expressing the green fluorescent protein and a red-shifted variant

The green fluorescent protein (GFP) from the jellyfish, Aequorea victoria, has become a versatile reporter for monitoring gene expression and protein localization in a variety of cells and organisms. GFP emits bright green light (λ_max = 510 nm) when excited with ultraviolet (UV) or blue light (λ_max = 395 nm, minor peak at 470 nm). The chromophore in GFP is intrinsic to the primary structure of the protein, and fluorescence from GFP does not require additional gene products, substrates or other factors. GFP fluorescence is stable, species-independent and can be monitored noninvasively using the techniques of fluorescence microscopy and flow cytometry [Chalfie et al., Science 263 (1994) 802–805; Stearns, Curr. Biol. 5 (1995) 262–264]. The protein appears to undergo an autocatalytic reaction to create the fluorophore [Heim et al., Proc. Natl. Acad. Sci. USA 91 (1994) 12501–12504] in a process involving cyclization of a Tyr⁶⁶ aa residue. Recently [Delagrave et al., Bio/Technology 13 (1995) 151–154], a combinatorial mutagenic strategy was targeted at aa 64 through 69, which spans the chromophore of A. victoria GFP, yielding a number of different mutants with redshifted fluorescence excitation spectra. One of these, RSGFP4, retains the characteristic green emission spectra (λ_max = 505 nm), but has a single excitation peak (λ_max = 490 nm). The fluorescence properties of RSGFP4 are similar to those of another naturally occurring GFP from the sea pansy, Renilla reniformis [Ward and Cormier, Photobiochem. Photobiol. 27 (1978) 389–396]. In the present study, we demonstrate by fluorescence microscopy that selective excitation of A. victoria GFP and RSGFP4 allows for spectral separation of each fluorescent signal, and provides the means to image these signals independently in a mixed population of bacteria or mammalian cells.     

Time resolved spectroscgpy of tryptopkyl fluorescence of yeast 3-phosphoglycerate kinase

The tryptophyl fluorescence emission of yeast 3-phosphoglycerate kinase decreases from pH 3.9 to pH 7.2 following a normal titration curve with an apparent pK of 4.7. The fluorescence decays have been determined at both extreme pH by photocounting pulse fluorimetry and have been found to vary with the emission wavelength. A quantitative analysis of these results according to a previously described method allows to determine the emission characteristics of the two tryptophan residues present in the protein molecule. At pH 3.9, one of the tryptophan residues is responsible for only 13% of the total fluorescence emission. This first residue has a lifetime τ₁= 0.6 ns and a maximum fluorescence wavelength λ_2max = 332 nm. The second tryptophan residue exhibits two lifetimes τ₂₁= 3.1 ns and τ₂₂= 7.0 ns (λ_2max= 338 nm). In agreement with the attribution of τ₂₁and τ₃₂ to the same tryptophan residue, the ratio β = C₂₁/C₂₂ of the normalized amplitudes is constant along the fluorescence emission spectrum. At pH 7.2, the two tryptophan residues contribute almost equally tc the protein fluorescence. The decay time of tryptophan 1 is 0.4 ns. The other emission parameters are the same as those determined at pH 3.9. We conclude that the fluorescence quenching in the range pH 3.9 to pH 8.0 comes essentially from the formation of a non emitting internal ground state complex between the tryptophan having the longest decay times and a neighbouring protein chemical group. The intrinsic pK of this group and the equilibrium constant of the irternal complex can be estimated. The quenching group is thought to be a carboxylate anion. Excitation transfers between the two tryptophyl residues of the protein molecule appear to have a small efficiency.     

Biotin binding changes the conformation and decreases tryptophan accessibility of streptavidin

Biotin binding reduces the tryptophan fluorescence emissions of streptavidin by 39%, blue shifts the emission peak from 333 to 329 nm, and reduces the bandwidth at half height from 53 to 46 nm. The biotin-induced emission difference spectrum resembles that of a moderately polar tryptophan. Streptavidin fluorescence can be described by two lifetime classes: 2.6 nsec (34%) and 1.3 nsec (66%). With biotin bound, lifetimes are 1.3 nsec (26%) and 0.8 nsec (74%). Biotin binding reduces the average fluorescence lifetime from 1.54 to 0.88 nsec. Biotin does not quench the fluorescence of indoles. The fluorescence changes are consistent with biotin binding causing a conformational change which moves tryptophans into proximity to portions of streptavidin which reduce the quantum yield and lifetimes. Fluorescence quenching by acrylamide revealed two classes of fluorophores. Analysis indicated a shielded component comprising 20–28% of the initial fluorescence with (K_SV+V)0.55 M^–1. The more accessible component has a predominance of static quenching. Measurements of fluorescence lifetimes at different acrylamide concentrations confirmed the strong static quenching. Since static quenching could be due to acrylamide binding to streptavidin, a dye displacement assay for acrylamide binding was constructed. Acrylamide does bind to streptavidin (Ka=5 M^–1), and probably binds within the biotin-binding site. In the absence of biotin, none of streptavidin's fluorescence is particularly accessible to iodide. In the presence of biotin, iodide neither quenches fluorescence nor alters emission spectra, and acrylamide access is dramatically reduced. We propose that the three tryptophans which always line the biotin site are sufficiently close to the surface of the binding site to be quenched by bound acrylamide. These tryptophans are shielded from iodide, most probably due to steric or ionic hindrances against diffusion into the binding site. Most of the shielding conferred by biotin binding can be attributed to the direct shielding of these residues and of a fourth tryptophan which moves into the binding site when biotin binds, as shown by X-ray studies (Weberet al., 1989).     

Role of Intracellular Ca2+ Levels in the Regulation of CD11a/CD18 Mediated Cell Adhesion

CD2, CD3, and MHC class II have been demonstrated to stimulate lymphocyte function-associated antigen (LFA)-1 (CD11a/CD18) mediated adhesion (Van Kooyk et al., 1989, Dustin and Springer, 1989; Mourad et al., 1990). Activation of LFA-1 may be mediated by different intracellular signals generated from these stimuli, since previous findings suggest that triggering of LFA-1 through CD2 or CD3 leads to sustained and transient cell adhesion respectively (Van Kooyk et al., 1989). We investigated the role of intracellular signalling pathways in more detail. The results demonstrate that, in addition to protein tyrosine kinase (PTK) and protein kinase C (PKC) mediated signalling, increase in cytosolic-free calcium ([Ca²⁺]_i) levels play a major role in the activation of LFA-1. The calcium iono-phore Ionomycin, which increases [Ca²⁺]_i is capable of directly activating LFA-1. Furthermore, activation of LFA-1 by triggering through CD2, CD3 or MHC class II is associated with an increase in [Ca²⁺]_i levels, with kinetics that directly correlate with cell adhesiveness. Moreover, entry of extracellular Ca²⁺ via Ca-channels is involved in both the CD3-and MHC class II, as well as part of the CD2 induced LFA-1 activation. Depletion of intracellular calcium results in unresponsiveness of LFA-1 to these stimuli, further demonstrating a regulatory role for [Ca²⁺]_i in LFA-1 mediated adhesion.     

Self-aggregation of a recombinant form of the propeptide NH<Subscript>2</Subscript>-terminal of the precursor of pulmonary surfactant protein SP-B: a conformational study

A recombinant form of the peptide N-terminally positioned from proSP-B (SP-B_N) has been produced in Escherichia coli as fusion with the Maltose Binding Protein, separated from it by Factor Xa cleavage and purified thereafter. This protein module is thought to control assembly of mature SP-B, a protein essential for respiration, in pulmonary surfactant as it progress through the progressively acidified secretory pathway of pneumocytes. Self-aggregation studies of the recombinant propeptide have been carried out as the pH of the medium evolved from neutral to moderately acid, again to neutral and finally basic. The profile of aggregation versus subsequent changes in pH showed differences depending on the ionic strength of the medium, low or moderate, and the presence of additives such as L-arginine (a known aggregation suppressor) and Ficoll 70 (a macromolecular crowder). Circular dichroism studies of SP-B_N samples along the aggregation process showed a decrease in α-helical content and a concomitant increase in β-sheet. Intrinsic fluorescence emission of SP-B_N was dominated by the emission of Trp residues in neutral medium, being its emission maximum shifted to red at low pH, suggesting that the protein undergoes a pH-dependent conformational change that increases the exposure of their Trp to the environment. A marked increase in the fluorescence emission of the extrinsic probe bis-ANS indicated the exposure of hydrophobic regions of SP-B_N at pH 5. The fluorescence of bis-ANS decreased slightly at low ionic strength, but to a great extent at moderate ionic strength when the pH was reversed to neutrality, suggesting that self-aggregation properties of the SP-B_N module could be tightly modulated by the conditions of pH and the ionic environment encountered by pulmonary surfactant during assembly and secretion.     

Fluorescence of free and protein-bound Auramine O

The spectral properties and binding of Auramine O were studied as a model for the binding of cationic ligands to proteins. The dye was fluorescent in H₂O with a quantum yield of 4 × 10^?5, but the emission became blue-shifted and more intense in less polar solvents, as in the case of more common fluorescent probes. Emission increased where dye motion was restricted, e.g., when bound to proteins, in glycerol solutions, dried on filter paper, or embedded in ice. The amount of solvent spectral shift was probably limited by the short lifetime of free dye emission, which was estimated to be of the order of picoseconds. Auramine O was bound by yeast alcohol dehydrogenase and serum albumins of different species. Fluorescence enhancement and equilibrium dialysis measurements showed the number of dyes bound per molecule of protein and the association constants to be 2 and 1.2 × 10⁴m^?1 for yeast alcohol dehydrogenase and 1 and 0.23–1.9 × 10⁴m^?1 for the albumins. The Auramine O complex with liver alcohol dehydrogenase, described by Conrad et al. [Biochemistry9, 1540–1546 (1970)], had peak emission at 520 nm, further to the red than any of the other complexes studied, suggesting a relatively polarizable binding environment. NaCl did not displace the dye, but enhanced its fluorescence in the complex. The fluorescence was sensitive to protein conformational changes brought about by urea. A literature survey suggests that cationic organic ligands bind strongly to the active site of only those enzymes which have cationic substrates, and bind only weakly to noncatalytic sites in other enzymes. The significance and advantages of cationic fluorescent probes of proteins are discussed.     

The pKa values of two histidine residues in human haemoglobin, the Bohr effect, and the dipole moments of alpha-helices

Studies of abnormal and chemically modified haemoglobins indicate that in 0.1 m-NaCl about 40% of the alkaline Bohr effect of human haemoglobin is contributed by the C-terminal histidine HC3(146)β. In deoxyhaemoglobin, the imidazole of this histidine forms a salt bridge with aspartate FG1(94)β, in oxyhaemoglobin or carbonmonoxyhaemoglobin it accepts a hydrogen bond from its own NH group instead. Kilmartin et al. (1973) showed that in 0.2 m-NaCl + 0.2 m-phosphate this change of ligation lowered the pK_a of the histidine from 8.0 in Hb³ to 7.1 in HbCO, but Russu et al. (1980) claimed that in bis-Tris buffer without added NaCl its pK_a in HbCO dropped no lower than 7.85, and that in this medium the C-terminal histidine made only a negligible contribution to the alkaline Bohr effect.We have compared the histidine resonances of HbCO A with those of three abnormal haemoglobins: HbCO Cowtown (His HC3(146)β → Leu), HbCO Wood (His FG4(97)β → Leu) and HbCO Malmø (His FG4(97)β → Gln). Our results show that the resonance assigned by Russu et al. to His HC3(146)β in fact belongs to His FG4(97)β. Although in Hb the pK_a of His HC3(146)β is 8.05 ± 0.05 independent of ionic strength, in HbCO its pK_a drops sharply with diminishing ionic strength, so that in the buffer employed by Russu et al. it has a pK_a of 6.2 and makes a contribution to the alkaline Bohr effect that is 57% larger than in the phosphate buffer employed by Kilmartin et al. (1973).In HbCO A, His FG4(97)β does not contribute to the Bohr effect, but in HbCO from which His HC3(146)β has been cleaved (HbCO des-His), His FG4(97)β is in equilibrium between two conformations with different pK_a values. This equilibrium varies with ionic strength and pH, and presumably also with degree of ligation of the haem moiety.In HbCO A, His FG4(97)β has a pK_a of 7.8 compared to the pK_a value of about 6.6 characteristic of free histidines at the surface of proteins. This high pK_a is accounted for by its interaction with the negative pole at the C terminus of helices F and FG. It corresponds to a free energy change of the same order as that observed in the interaction of histidines with carboxylate ions and confirms the strongly dipolar character of α-helices, which manifests itself even when they lie on the surface of the protein.     

Substrate specificity of pyrophosphate:fructose 6-phosphate 1-phosphotransferase from potato tuber

The aim of this work was to establish the precise ionic form of the reactants used by pyrophosphate:fructose-6-phosphate phosphotransferase. The enzyme was purified to near-homogeneity from potato (Solanum tuberosum L.) tubers. Changes in enzyme activity when the pH of the assay and the concentration of fructose 6-phosphate, pyrophosphate, and magnesium are varied independently indicate that fructose 6-phosphate²⁻ and MgP₂O₇²⁻ are the reacting species in the glycolytic direction. Analogous experiments with fructose 1,6-bisphosphate, inorganic phosphate, and magnesium demonstrate that the enzyme uses fructose 1,6-bisphosphate⁴⁻, HPO₄²⁻, and Mg²⁺ in the gluconeogenic direction. The ionic species used in the glycolytic direction are comparable with those required by bacterial ATP-dependent phosphofructokinase. This is consistent with the proposal that the active site of pyrophosphate:fructose-6-phosphate phosphotransferase in plants is equivalent to that of the bacterial phosphofructokinase (SM Carlisle et al. [1990] J Biol Chem 265: 18366-18371).     

Binding of phospholipids to β-Lactoglobulin and their transfer to lipid bilayers

The bovine milk lipocalin, β-Lactoglobulin (β-LG), has been associated with the binding and transport of small hydrophobic and amphiphilic compounds, whereby it is proposed to increase their bioavailability. We have studied the binding of the fluorescent phospholipid-derivative, NBD-didecanoylphosphatidylethanolamine (NBD-diC₁₀PE) to β-LG by following the increase in amphiphile fluorescence upon binding to the protein using established methods. The equilibrium association constant, K_B, was (1.2 ± 0.2) × 10⁶ M^− 1 at 25 °C, pH 7.4 and I = 0.15 M. Dependence of K_B on pH and on the monomer-dimer equilibrium of β-LG gave insight on the nature of the binding site which is proposed to be the hydrophobic calyx formed by the β-barrel in the protein. The monomer-dimer equilibrium of β-LG was re-assessed using fluorescence anisotropy of Tryptophan. The equilibrium constant for dimerization, K_D, was (7.0 ± 1.5) × 10⁵ M^− 1 at 25 °C, pH 7.4, and 0.15 M ionic strength. The exchange of NBD-diC₁₀PE between β-LG and POPC lipid bilayers was followed by the change in NBD fluorescence. β-LG was shown to be a catalyst of phospholipid exchange between lipid bilayers, the mechanism possibly involving adsorption of the protein at the bilayer surface.     

Extinction coefficients for use in equations for the spectrophotometric analysis of haemoglobin mixtures.

Values of the extinction coefficients for deoxyhaemoglobin, oxyhaemoglobin, carboxyhaemoglobin, as well as those of haemiglobin between pH 6.2 and 8.8, as published by Benesch et al. (1), are corrected, based on the internationally accepted value of 11.0 for ?_HiCN⁵⁴⁰.     

Differential scanning calorimetry and fluorescence study of lactoperoxidase as a function of guanidinium–HCl,urea, and pH

The stability of bovine lactoperoxidase to denaturation by guanidinium–HCl, urea, or high temperature was examined by differential scanning calorimetry (DSC) and tryptophan fluorescence. The calorimetric scans were observed to be dependent on the heating scan rate, indicating that lactoperoxidase stability at temperatures near T_m is controlled by kinetics. The values for the thermal transition, T_m, at slow heating scan rate were 66.8, 61.1, and 47.2 °C in the presence of 0.5, 1, and 2 M guanidinium–HCl, respectively. The extrapolated value for T_m in the absence of guanidinium–HCl is 73.7 °C, compared with 70.2 °C obtained by experiment; a lower experimental value without a denaturant is consistent with distortion of the thermal profile due to aggregation or other irreversible phenomenon. Values for the heat capacity, C_p, at T_m and E_a for the thermal transition decrease under conditions where T_m is lowered. At a given concentration, urea is less effective than guanidinium–HCl in reducing T_m, but urea reduces C_p relatively more. Both fluorescence and DSC indicate that thermally denatured protein is not random coil. A change in fluorescence around 35 °C, which was previously reported for EPR and CD measurements (Boscolo et al. Biochim. Biophys. Acta 1774 (2007) 1164–1172), is not seen by calorimetry, suggesting that a local and not a global change in protein conformation produces this fluorescence change.     

FAD binding properties of a cytosolic version of Escherichia coli NADH dehydrogenase-2

Respiratory NADH dehydrogenase-2 (NDH-2) of Escherichia coli is a peripheral membrane-bound flavoprotein. By eliminating its C-terminal region, a water soluble truncated version was obtained in our laboratory. Overall conformation of the mutant version resembles the wild-type protein. Considering these data and the fact that the mutant was obtained as an apo-protein, the truncated version is an ideal model to study the interaction between the enzyme and its cofactor. Here, the FAD binding properties of this version were characterized using far-UV circular dichroism (CD), differential scanning calorimetry (DSC), limited proteolysis, and steady-state and dynamic fluorescence spectroscopy. CD spectra, thermal unfolding and DSC profiles did not reveal any major difference in secondary structure between apo- and holo-protein. In addition, digestion site accessibility and tertiary conformation were similar for both proteins, as seen by comparable chymotryptic cleavage patterns. FAD binding to the apo-protein produced a parallel increment of both FAD fluorescence quantum yield and steady-state emission anisotropy. On the other hand, addition of FAD quenched the intrinsic fluorescence emission of the truncated protein, indicating that the flavin cofactor should be closely located to the protein Trp residues. Analysis of the steady-state and dynamic fluorescence data confirms the formation of the holo-protein with a 1:1 binding stoichiometry and an association constant K_A = 7.0(± 0.8) × 10⁴ M^− 1. Taken together, the FAD–protein interaction is energetically favorable and the addition of FAD is not necessary to induce the enzyme folded state. For the first time, a detailed characterization of the flavin:protein interaction was performed among alternative NADH dehydrogenases.     

Regulation of glutamine synthetase adenylylation and deadenylylation by the enzymatic uridylylation and deuridylylation of the PII regulatory protein

Regulation of glutamine synthetase activity in Escherichia coli is mediated by covalent attachment and detachment of an adenylyl group to each subunit of the enzyme [Kingdon, H. S. et al., Proc. Nat. Acad. Sci., 58, 1703, (1967); Wulff, K. D. et al., Biochem. Biophys. Res. Commun.28, 740, (1967)]. Adenylylation and deadenylylation of the enzyme are both catalyzed by a single adenylyltransferase (ATase) whose activity is modulated by various metabolites and by a regulatory protein, P_II [Shapiro, B. M., Biochemistry; Anderson, W. B. et al., Proc. Nat. Acad. Sci.67, 1761 (1970)].The present study confirms preliminary results [Brown, M. S. et al., Proc. Nat. Acad. Sci.68, 2949 (1971)] showing that: (1) the regulatory protein (P_II) exists in two interconvertible forms, P_IIA and P_IID, which, respectively, stimulate adenylylation and deadenylylation activity of ATase; (2) conversion of P_IIA to P_IID requires the presence of UTP, 2-oxoglutarate, ATP, and either Mg²⁺ or Mn²⁺; (3) this conversion involves covalent attachment of a uridine derivative to P_IIA. It is further established that the covalently bound uridine derivative is UMP which is derived from UTP in a reaction catalyzed by a specific uridylyltransferase (UTase). Removal of the covalently bound UMP from P_IID is catalyzed by a separate enzyme, referred to as the uridylyl-removing enzyme (UR-enzyme). This enzyme has an obligatory requirement for Mn²⁺.Regulation of glutamine synthetase activity in E. coli is thus facilitated by a highly sophisticated cascade system of proteins, consisting of an ATase, the regulatory protein (P_II), UTase, and the UR-enzyme. The activities of these various components is rigorously controlled by various metabolites, including glutamine, 2-oxoglutarate, ATP, P_i, UTP, and the divalent cations, Mn²⁺ and Mg²⁺.     

Thermal,Chemical and pH Induced Unfolding of Turmeric Root Lectin: Modes of Denaturation

Curcuma longa rhizome lectin, of non-seed origin having antifungal, antibacterial and α-glucosidase inhibitory activities, forms a homodimer with high thermal stability as well as acid tolerance. Size exclusion chromatography and dynamic light scattering show it to be a dimer at pH 7, but it converts to a monomer near pH 2. Circular dichroism spectra and fluorescence emission maxima are virtually indistinguishable from pH 7 to 2, indicating secondary and tertiary structures remain the same in dimer and monomer within experimental error. The tryptophan environment as probed by acrylamide quenching data yielded very similar data at pH 2 and pH 7, implying very similar folding for monomer and dimer. Differential scanning calorimetry shows a transition at 350.3 K for dimer and at 327.0 K for monomer. Thermal unfolding and chemical unfolding induced by guanidinium chloride for dimer are both reversible and can be described by two-state models. The temperatures and the denaturant concentrations at which one-half of the protein molecules are unfolded, are protein concentration-dependent for dimer but protein concentration-independent for monomer. The free energy of unfolding at 298 K was found to be 5.23 Kcal mol⁻¹ and 14.90 Kcal mol⁻¹ for the monomer and dimer respectively. The value of change in excess heat capacity upon protein denaturation (ΔC_p) is 3.42 Kcal mol⁻¹ K⁻¹ for dimer. The small ΔC_p for unfolding of CLA reflects a buried hydrophobic core in the folded dimeric protein. These unfolding experiments, temperature dependent circular dichroism and dynamic light scattering for the dimer at pH 7 indicate its higher stability than for the monomer at pH 2. This difference in stability of dimeric and monomeric forms highlights the contribution of inter-subunit interactions in the former.     

Kinetics of nucleotide binding to chloroplast coupling factor (CF1)

Studies of the kinetics of association and dissociation of the formycin nucleotides FTP and FDP with CF₁ were carried out using the enhancement of formycin fluorescence. The protein used, derived from lettuce chloroplasts by chloroform induced release, contains only 4 types of subunit and has a molecular weight of 280 000.In the presence of 1.25 mM MgCl₂, 1 mol of ATP or FTP is bound to the latent enzyme, with K_d = 10^?7 or 2 · 10^?7, respectively. The fluorescence emission (λ_max 340 nm) of FTP is enhanced 3-fold upon binding, and polarization of fluorescence is markedly increased. The fluorescence changes have been used to follow FTP binding, which behaves as a bimolecular process with K₁ = 2.4 · 10⁴ M^?1 · s^?1. FTP is displaced by ATP in a process apparently involving unimolecular dissociation of FTP with k_?1 = 3 · 10^?3 s^?1. The ratio of rates is comparable to the equilibrium constant and no additional steps have been observed.The protein has 3 sites for ADP binding. Rates of ADP binding are similar in magnitude to those for FTP. ADP and ATP sites are at least partly competitive with one another.The kinetics of nucleotide binding are strikingly altered upon activation of the protein as an ATPase. The rate of FTP binding increases to at least 10⁶ M^?1 · s^?1. This suggests that activation involves lowering of the kinetic barriers to substrate and product binding-dissociation and has implications for the mechanism of energy transduction in photophosphorylation.