[Show abstract][Hide abstract] ABSTRACT: The electron transfer reactions between several chromium(III), iron(III) and cobalt(III) coordination compounds and spinach plastocyanin (PC) were studied. The ligands coordinated to the metal ions are derivatives of benzimidazole. Kinetic studies were carried out in dimethyl sulphoxide–H2O (25:75%) and the reaction mechanism is discussed. For comparison with previous studies, the reaction of [Co(phen)3]3+ with PC was studied both in aqueous buffer solution and in dimethyl sulphoxide–H2O (25:75%); the results indicated that the electron transfer is accelerated in reactions carried out in the latter medium. [Fe(2gb)3](NO3)3, 2gb=2-guanidinobenzimidazole, and [Fe(ntb)Cl2]Cl, ntb=tris(benzimidazolyl)methylamine oxidised PC following a simple second order outer sphere mechanism. The rate constants for electron transfer are 1.4×104±1.1×102 and 706.2±12.7 M−1 s−1, respectively. Cobalt(III) and chromium(III) benzimidazolic compounds behaved as inhibitors to the electron transfer process. NMR studies indicated that the conformation of the protein does not change in DMSO–H2O (25:75% v/v) when compared with that in aqueous buffer.
[Show abstract][Hide abstract] ABSTRACT: A set of plastocyanin (Pc) mutants, probing the small acidic patch (Glu59, Glu60, and Asp61) and a nearby residue, Gln88, has been constructed to provide further insight into the electron transfer process between Pc and photosystem 1. The negatively charged residues were changed into their neutral counterparts or to a positive lysine. All mutant proteins exhibited electron transfer kinetics qualitatively similar to those of the wild type protein over a wide range of Pc concentrations. The kinetics were slightly faster for the Gln88Lys mutant, while they were significantly slower for the Glu59Lys mutant. The data were analyzed with two different models: one involving a conformational change of the Pc-photosystem 1 complex that precedes the electron transfer step (assumed to be irreversible) [Bottin, H., and Mathis, P. (1985) Biochemistry 24, 6453-6460] and another where no conformational change occurs, the electron transfer step is reversible, and dissociation of products is explicitly taken into account [Drepper, F., Hippler, M., Nitschke, W., and Haehnel, W. (1996) Biochemistry 35, 1282-1295]. Both models can account for the observed kinetics in the limits of low and high Pc concentrations. To discriminate between the models, the effects of added magnesium ions on the kinetics were investigated. At a high Pc concentration (0.7 mM), the ionic strength dependence was found to be consistent with the model involving a conformational change but not with the model where the electron transfer is reversible. One residue in the small acidic patch, Glu60, seems to be responsible for the major part of the ionic strength dependence of the kinetics.
[Show abstract][Hide abstract] ABSTRACT: The binding of Ag- and Cd-substituted plastocyanin to reduced photosystem 1 of spinach has been studied through the rotational correlation time of plastocyanin measured by the technique of perturbed angular correlation of gamma-rays (PAC). Ag and Cd are used as models for native Cu(I) and Cu(II), respectively. A dissociation constant of 5 microM was found for Ag-plastocyanin, whereas the dissociation constant was at least 24 times higher for Cd-plastocyanin. PAC was further used to characterize the structure of the metal site of Cd- and Ag-plastocyanin. The Cd spectra are characteristic of a planar configuration of one cysteine and two histidines. However, the spectra show an unusual peak broadening and a high degree of internal motion, interpreted as motion of one of the histidines within the plane. (111)Ag decays to (111)Cd, followed by the emission of two gamma-rays used for the PAC experiment. The (111)Ag PAC spectra indicate that one of the coordinating histidines has a different position in the Ag protein than in the Cd protein but that the decay of Ag to Cd causes a relaxation of the position of this histidine to the position in the Cd protein within 20 ns. Binding of Ag-plastocyanin to photosystem I stabilized the Ag metal site structure so that no relaxation was observed on a time scale of 100 ns. This stabilization of the Ag structure upon binding indicates that the metal site structure is involved in regulating how the dissociation constant for plastocyanin depends on the charge of the metal ion.
[Show abstract][Hide abstract] ABSTRACT: This is a comparative study of the photoinduced (so-called forward) electron-transfer reaction 3Zncyt/pc(II) --> Zncyt+/pc(I), between the triplet state of zinc cytochrome c (3Zncyt) and cupriplastocyanin [pc(II)], and the thermal (so-called back) electron-transfer reaction Zncyt+/pc(I) --> Zncyt/pc(II), between the cation (radical) of zinc cytochrome c (Zncyt+) and cuproplastocyanin [pc(I)], which follows it. Both reactions occur between associated (docked) reactants, and the respective unimolecular rate constants are kF and kB. Our previous studies showed that the forward reaction is gated by a rearrangement of the diprotein complex. Now we examine the back reaction and complare the two. We study the effects of temperature (in the range 273.3-302.9 K) and viscosity (in the range 1.00-17.4 cP) on the rate constants and determine enthalpies (DeltaH), entropies (DeltaS), and free energies (DeltaG) of activation. We compare wild-type spinach plastocyanin, the single mutants Tyr83Leu and Glu59Lys, and the double mutant Glu59Lys/Glu60Gln. The rate constant kB for wild-type spinach plastocyanin and its mutants markedly depends on viscosity, an indication that the back reaction is also gated. The activation parameters DeltaH and DeltaS show that the forward and back reactions have similar mechanisms, involving a rearrangement of the diprotein complex from the initial binding configuration to the reactive configuration. The rearrangements of the complexes 3Zncyt/pc(II) and Zncyt+/pc(I) that gate their respective reactions are similar but not identical. Since the back reaction of all plastocyanin variants is faster than the forward reaction, the difference in free energy between the docking and the reactive configuration is smaller for the back reaction than for the forward reaction. This difference is explained by the change in the electrostatic potential on the plastocyanin surface as Cu(II) is reduced to Cu(I). It is the smaller DeltaH that makes DeltaG smaller for the back reaction than for the forward reaction.
[Show abstract][Hide abstract] ABSTRACT: The crystal structure of plastocyanin from spinach has been determined using molecular replacement, with the structure of plastocyanin from poplar as a search model. Successful crystallization was facilitated by site-directed mutagenesis in which residue Gly8 was substituted with Asp. The region around residue 8 was believed to be too mobile for the wild-type protein to form crystals despite extensive screening. The current structure represents the oxidized plastocyanin, copper (II), at low pH (approximately 4.4). In contrast to the similarity in the core region as compared to its poplar counterpart, the structure shows some significant differences in loop regions. The most notable is the large shift of the 59-61 loop where the largest shift is 3.0 A for the C(alpha) atom of Glu59. This results in different patterns of electrostatic potential around the acidic patches for the two proteins.
Protein Science 10/1998; 7(10):2099-105. DOI:10.1002/pro.5560071006 · 2.85 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The unimolecular rate constant for the photoinduced electron-transfer reaction 3Zncyt/pc(II) --> Zncyt+/pc(I) within the electrostatic complex of zinc cytochrome c and spinach cupriplastocyanin is kF. We report the effects on kF of the following factors, all at pH 7.0: 12 single mutations on the plastocyanin surface (Leu12Asn, Leu12Glu, Leu12Lys, Asp42Asn, Asp42Lys, Glu43Asn, Glu59Gln, Glu59Lys, Glu60Gln, Glu60Lys, Gln88Glu, and Gln88Lys), the double mutation Glu59Lys/Glu60Gln, temperature (in the range 273.3-302.9 K), and solution viscosity (in the range 1. 00-116.0 cP) at 283.2 and 293.2 K. We also report the effects of the plastocyanin mutations on the association constant (Ka) and the corresponding free energy of association (DeltaGa) with zinc cytochrome c at 298.2 K. Dependence of kF on temperature yielded the activation parameters DeltaH, DeltaS, and DeltaG. Dependence of kF on solution viscosity yielded the protein friction and confirmed the DeltaG values determined from the temperature dependence. The aforementioned intracomplex reaction is not a simple electron-transfer reaction because donor-acceptor electronic coupling (HAB) and reorganizational energy (lambda), obtained by fitting of the temperature dependence of kF to the Marcus equation, deviate from the expectations based on precedents and because kF greatly depends on viscosity. This last dependence and the fact that certain mutations affect Ka but not kF are two lines of evidence against the mechanism in which the electron-transfer step is coupled with the faster, but thermodynamically unfavorable, rearrangement step. The electron-transfer reaction is gated by the slower, and thus rate determining, structural rearrangement of the diprotein complex; the rate constant kF corresponds to this rearrangement. Isokinetic correlation of DeltaH and DeltaS parameters and Coulombic energies of the various configurations of the Zncyt/pc(II) complex consistently show that the rearrangement is a facile configurational fluctuation of the associated proteins, qualitatively the same process regardless of the mutations in plastocyanin. Correlation of kF with the orientation of the cupriplastocyanin dipole moment indicates that the reactive configuration of the diprotein complex involves the area near the residue 59, between the upper acidic cluster and the hydrophobic patch. Kinetic effects and noneffects of plastocyanin mutations show that the rearrangement from the initial (docking) configuration, which involves both acidic clusters, to the reactive configuration does not involve the lower acidic cluster and the hydrophobic patch but involves the upper acidic cluster and the area near the residue 88.
[Show abstract][Hide abstract] ABSTRACT: Six different spinach plastocyanin mutants have been constructed by site-directed mutagenesis and expressed in Escherichia coli to probe the importance of the two acidic patches in the interaction with photosystem I. The mutants were: Asp42Lys, Glu43Asn, Glu43Lys, Glu43Gln/Asp44Asn, Glu59Lys/Glu60Gln and Glu43Asn/Glu59Lys/Glu60Gln and they have been characterised by optical absorption and EPR spectroscopy, redox titrations and isoelectric focusing. The electron transfer to photosystem I was investigated by flash-induced time-resolved absorption measurements at 830 nm. The kinetics were interpreted with a model that incorporates a rate-limiting conformational change from inactive to active forms of the plastocyanin-photosystem I complex. All mutations resulted in a displacement of the equilibrium towards the inactive conformation. The strongest impairment of the electron transfer was found for mutations in the larger acidic patch, in particular upon modification of residues 43 or 44. However, mutations of residues 59 and 60 in the smaller acidic patch also resulted in a lower reactivity.
[Show abstract][Hide abstract] ABSTRACT: The small electron transporting copper protein, azurin, has been studied in order to investigate the interplay between the oxidation states of the metal and its coordination geometry. The results show that the metal coordination geometry for Ag(I) in Ag(I) substituted wild type azurin is only slightly different from the geometry for Cd(II) in cadmium substituted azurin both being similar to the geometry for copper in native azurin. Furthermore, the coordination geometry for Ag(I) in the Met121 to Leu substituted mutant of azurin is also similar to the geometry of copper in native azurin. In contrast, previously published results show that Cd(II) substituted Met121Leu-azurin exhibits two different coordination geometries for Cd(II), one again similar to the wild type geometry and another very flexible and distinctly different from wild type azurin. These results have been obtained by Perturbed Angular Correlation of γ-rays spectroscopy using the two radioactive isotopes 111Ag(I) and 111mCd(II) as probes of a monovalent and a divalent ion, respectively. The technique also revealed that the metal-coordination geometry for Ag(I) in wild type azurin relaxes to the coordination geometry of Cd(II) on a time scale of 100 ns after the decay from 111Ag(I) to 111Cd(II). We suggest that the role of Met121 is to maintain the rigid tricoordinated metal coordination geometry independent of the oxidation state of the metal.
Journal of the American Chemical Society 01/1997; 119(1). DOI:10.1021/ja962499w · 12.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study, by flash kinetic spectrophotometry on the microsecond time scale, the effects of ionic strength and viscosity on the kinetics of oxidative quenching of the triplet state of zinc cytochrome c (3Zncyt) by the wild-type form and the following nine mutants of cupriplastocyanin: Leu12Glu, Leu12Asn, Phe35Tyr, Gln88Glu, Tyr83Phe, Tyr83His, Asp42Asn, Glu43Asn, and the double mutant Glu59Lys/Glu60Gln. The unimolecular rate constants for the quenching reactions within the persistent diprotein complex, which predominates at low ionic strengths, and within the transient diprotein complex, which is involved at higher ionic strengths, are equal irrespective of the mutation. Evidently, the two complexes are the same. In both reactions, the rate-limiting step is rearrangement of the diprotein complex from a configuration optimal for docking to the one optimal for the subsequent electron-transfer step, which is fast. We investigate the effects of plastocyanin mutations on this rearrangement, which gates the overall electron-transfer reaction. Conversion of the carboxylate anions into amide groups in the lower acidic cluster (residues 42 and 43), replacement of Tyr83 with other aromatic residues, and mutations in the hydrophobic patch in plastocyanin do not significantly affect the rearrangement. Conversion of a pair of carboxylate anions into a cationic and a neutral residue in the upper acidic cluster (residues 59 and 60) impedes the rearrangement. Creation of an anion at position 88, between the upper acidic cluster and the hydrophobic patch, facilitates the rearrangement. The rate constant for the rearrangement smoothly decreases as the solution viscosity increases, irrespective of the mutation. Fittings of this dependence to the modified Kramers's equation and to an empirical equation show that zinc cytochrome c follows the same trajectory on the surfaces of all the plastocyanin mutants but that the obstacles along the way vary as mutations alter the electrostatic potential. Mutations that affect protein association (i.e., change the binding constant) do not necessarily affect the reaction between the associated proteins (i.e., the rate constant) and vice versa. All of the kinetic and thermodynamic effects and noneffects of mutations consistently indicate that in the protein rearrangement the basic patch of zinc cytochrome c moves from a position between the two acidic clusters to a position at or near the upper acidic cluster.
[Show abstract][Hide abstract] ABSTRACT: A series of plastocyanin mutants have been constructed by site-directed mutagenesis and expressed in Escherichia coli to elucidate the interaction between plastocyanin and photosystem 1 in the photosynthetic electron-transfer chain. Leu-12 has been replaced with alanine, asparagine, glutamate, and lysine, while Tyr-83 has been exchanged for histidine, phenylalanine, and leucine. Phe-35, Asp-42, and Gln-88 have been mutated to tyrosine, asparagine, and glutamate, respectively. The mutations that have been introduced do not seem to place any strain on the tertiary structure according to optical absorption and electron paramagnetic resonance (EPR) spectroscopic studies. However, there are changes in the reduction potential for the Leu-12 mutants that cannot be accounted for by electrostatic interactions alone. For some of the mutants, the pI shifts, in accordance with the changes in the number of titratable groups. Only the Leu-12 mutants show any major change in their photosystem 1 kinetics, while the mutants in the acidic patch show minor changes, suggesting that both the hydrophobic and acidic patches make contact with photosystem 1 but that the electron transfer occurs at the hydrophobic interface, most probably via the His-87 residue. The kinetics are best described with a model in which a rate-limiting conformational change occurs in the plastocyanin-photosystem 1 complex [Bottin, H., & Mathis, P. (1985) Biochemistry 24, 6453-6460; Sigfridsson, K., Hansson, O., Karlsson, B.G., Baltzer L., Nordling, M., & Lundberg, L. G. (1995) Biochim. Biophys. Acta 1228, 28-36], where the changes observed are attributed to changes in the dynamics within the electron-transfer complex.
[Show abstract][Hide abstract] ABSTRACT: Tyrosine-83 in spinach plastocyanin (Pc) has been modified by site-directed mutagenesis to a histidine. An NMR titration yields a pK value of 8.44 for this residue. The high value is probably due to the acidic residues close to this site. The reduction potential is increased by 35 mV at pH 7.5, but only slightly, if at all, at pH 8.9. EPR and optical absorption bands associated with the copper site are not affected by the mutation, either at pH 7.5 or at pH 8.9. The electron transfer (ET) to Photosystem I (PS I), as studied by a flash-photolysis technique, is pH dependent for the mutant, being slower than the wild type at pH 7.5 but more similar to it at pH 8.9. The data have been interpreted with a model that includes a rate-limiting conformational change in the Pc-PS I complex which precedes the intracomplex ET (Bottin, H. and Mathis, P. (1985) Biochemistry 24, 6453–6460). The slower kinetics at the lower pH for the mutant is attributed to a dual effect of the protonation of the His-83 residue: (i) A destabilization of the ‘close’ bound conformation, i.e., the one competent in electron transfer, and (ii) a smaller intracomplex ET rate constant, partly due to a smaller driving force for ET. From this it is concluded that the Tyr-83 residue is not a part of the ET pathway to PS I.
[Show abstract][Hide abstract] ABSTRACT: The geometries of the metal sites in cadmium-substituted azurins have been investigated by Cd perturbed angular correlation (PAC). The study includes wild type azurin as well as Met mutants of azurin, where methionine has been substituted by Ala, Asn, Asp, Gln, Glu, and Leu.
The nuclear quadrupole interaction of wild type azurin analyzed in the angular overlap model is well described as coordination
of His, His, and Cys and cannot be described by coordination of Met and/or Gly.
For most of the mutants, there exist two coordination geometries of the cadmium ion. With the exception of the Glu and Asp
mutants, one of the conformations is similar to the wild type conformation. The other coordination geometries are either best
described by a coordinating water molecule close to the original methionine position or by coordination by the substituting
amino acid. These experiments show that even though the methionine does not coordinate it plays an important role for the
geometry of the metal site.
The nuclear quadrupole interaction of stellacyanin was also measured. The value resembles the most prominent nuclear quadrupole
interaction of the Met Gln mutant of Alcaligenes denitrificans azurin, indicating that the structures of the two metal sites are similar.
[Show abstract][Hide abstract] ABSTRACT: The reduction of plastocyanin by cytochromes c and f has been investigated with mutants of spinach plastocyanin in which individual, highly conserved surface residues have been modified. These include Leu-12 and Phe-35 in the 'northern' hydrophobic patch and Tyr-83 and Asp-42 in the 'eastern' acidic patch. The differences observed all involved binding rather than the intrinsic rates of electron transfer. The Glu-12 and Ala-12 mutants showed small but significant decreases in binding constant with cytochrome c, even though the cytochrome is not expected to make contact with the northern face of plastocyanin. These results, and small changes in the EPR parameters, suggested that these mutations cause small conformational changes in surface residues on the eastern face of plastocyanin, transmitted through the copper centre. In the case of cytochrome f, the Glu-12 and Ala-12 mutants also bound less strongly, but Leu12Asn showed a marked increase in binding constant, suggesting that cytochrome f can hydrogen bond directly to Asn-12 in the reaction complex. A surprising result was that the kinetics of reduction of Asp42Asn were not significantly different from wild type, despite the loss of a negative charge.
[Show abstract][Hide abstract] ABSTRACT: Plastocyanin (Pc) has been modified by site-directed mutagenesis at two separate electron-transfer (ET) sites: Leu-12-Glu at a hydrophobic patch, and Tyr-83-His at an acidic patch. The reduction potential at pH 7.5 is decreased by 26 mV in Pc(Leu-12-Glu) and increased by 35 mV in Pc(Tyr-83-His). The latter mutant shows a 2-fold slower intracomplex ET to photosystem I (PSI) as expected from the decreased driving force. The affinity for PSI is unaffected for this mutant but is drastically decreased for Pc(Leu-12-Glu). It is concluded that the hydrophobic patch is more important for the ET to PSI.
[Show abstract][Hide abstract] ABSTRACT: The reduction rate of P680+, the oxidized form of the primary electron donor of Photosystem II, has been studied with flash absorption spectroscopy at 830 nm. Photosystem II membranes, partially depleted of the intrinsic managanese of the oxygen-evolving complex, were used. The reduction rate of P680+ was measured as a function of the concentration of free Mn2+, which was stabilized by metal-ion buffer systems consisting of chelators and metal-chelator complexes. Increasing the Mn2+ concentration induced an 18 to 35 μs decay component in the P680+ reduction kinetics and diminished the amplitude of the 4 to 8 μs decay component. A dissociation constant of approx. 50 μM was obtained for the observed Mn2+ binding site. Other transition metal ions affected the reduction kinetics of P680+ at lower concentrations. Thus, photooxidation of Mn2+ is not required for the detection of its binding at this site. To account for the kinetic effect, it is proposed that the bound metal ion interacts electrostatically with the tyrosine residue YZ, the intrinsic electron donor to P680+.
[Show abstract][Hide abstract] ABSTRACT: Photosynthetic organisms absorb solar energy using chlorophyll-containing antennas and convert it to chemical energy in the form of charge-separated radical pairs. Experimental studies of photosystem 2 from plants and cyanobacteria are often interpreted in terms of a model in which the excited antenna is in equilibrium with the charge-separated states. Assigning a single state for the excited antenna is a simplification since it is known that the excitation energy migrates over the antenna for some time until it is trapped in special reaction centers where charge separation takes place. In the present work, existing random-walk models for excitation-energy transfer are extended to a case where charge recombination is allowed for. The treatment is based on Laplace and discrete Fourier transform techniques. It is shown that the simple equilibrium model approximates the situation well, provided that the overall trapping and detrapping rates are appropriately scaled to the rate constants for charge separation and recombination in the reaction center. Expressions for the quantum yields of charge separation and of fluorescence are derived and the consistency between the present random-walk model and the general notion of antenna entropy is demonstrated.
Chemical Physics 01/1991; 149(3):361-372. DOI:10.1016/0301-0104(91)90036-S · 1.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Flash-induced absorbance changes in the nanosecond to millisecond time ranges have been measured in the Photosystem II reaction center complex consisting of D1 and D2 subunits and cytochrome b-559, in the presence of 2,5-dibromo-3-methyl-6-isopropylbenzoquinone (DBMIB). The results indicate that DBMIB largely quenches the primary radical pair (P-680+ Pheo−) and the formation of the triplet (3P-680). Long-lived absorption signals in the red and near-infrared (bleaching at 680 nm and broad increase at 740–830 nm) and in the green (peak at 560 nm) can be attributed to the oxidation of P-680 and to the reduction of cytochrome b-559. These data show that addition of DBMIB induces stabilization of P-680+ and a rapid (perhaps submicrosecond) reduction of cytochrome b-559. The signals attributed to P-680+ and to reduced cytochrome decay in parallel (), showing that the cytochrome reduces P-680+. The stabilization occurred also in the presence of plastoquinone-3 and (with DBMIB) at −29°C in a viscous solution containing 60% glycerol at a low concentration of quinone, suggesting that the quinone reconstitutes the function of QA and thus mediates electron transport from the reduced pheophytin a to the intrinsic cytochrome.
[Show abstract][Hide abstract] ABSTRACT: Electron paramagnetic resonance (EPR) and absorption spectroscopy have been used to study the low temperature photochemical behavior of the Photosystem II D-1/D-2/ cytochrome b559 reaction center complex. The reaction center displays large triplet state EPR signals which are attenuated after actinic illumination at low temperatures in the presence of sodium dithionite. Concomitant with the triplet attenuation is the buildup of a structured radical signal with an effective g value of 2.0046 and a peak-to-peak width of 11.9 G. The structure in the signal is suggestive of it being comprised in part of the anion radical of pheophytin a. This assignment is corroborated by low temperature optical absorbance measurements carried out after actinic illumination at the low temperatures which show absorption bleachings at 681 nm, 544 nm and 422 nm and an absorbance buildup at 446 nm indicating the formation of reduced pheophytin.
Photosynthesis Research 05/1989; 20(3):279-289. DOI:10.1007/BF00034070 · 3.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The reaction of ammonia with the oxygen-evolving system was investigated using EPR. Two sites with distinct binding properties were found. One site, previously known to be responsible for the modification by ammonia of the multiline EPR signal from the S2 state and believed to be accessible in this state only, was found to bind ammonia also in the S1 state although weaker. The second binding site, identified by the effect of bound ammonia on the shape and position of the g = 4.1 EPR signal, was also found to be accessible in both the S1 and S2 states. The apparent dissociation constants for ammonia at the two sites in the S1 and S2 states were determined. In neither state did the binding the ammonia account for the observed inhibition of oxygen evolution, suggesting that binding to other S states plays an important role in the inhibition. Chloride, which is known to interfere with ammonia-induced inhibition of oxygen evolution, was found to compete with ammonia at the site associated with the modification of the g = 4.1 EPR signal. The broadening of the hyperfine lines of the multiline EPR signal, seen in the presence of 17O-labeled water, was still observed after the modification of the signal by ammonia. This indicates that ammonia has not completely displaced water bound to the catalytic site in the S2 state. The results of the binding studies are interpreted in terms of a two state — two site model, where the two states are identified by their EPR signals, the multiline and the g = 4.1 signal, respectively, and the two sites identified by the effects of ammonia on these signals and where the equilibrium between the two states is regulated by the binding of ligands to the sites.