[Cytochrome b2 (L-lactate dehydrogenase): equilibrium and elementary processes of the substrate-enzyme combining reaction]

Reaction of the Hansenula anomala flavocytochrome b2 and cytochrome b2 core with inorganic outer sphere redox compounds

The oxidation of reduced cytochrome b2 core and flavocytochrome b2 by three inorganic outer sphere compounds, Fe(CN)6(3-), Co(phen)3(3+) and Mn(CyDTA) (H2O)-, has been studied by stopped-flow. The reaction with Fe(CN)6(3-) is very rapid; the second order rate constants at 10 degrees C (pH 7) and I = 0.02 M are k = 1 x 10(8) M-1 s-1 and 1 x 10(7) M-1 s-1 for cytochrome b2 core and flavocytochrome b2, respectively. The reaction between cytochrome b2 core and Co(phen)3(3+), too fast at pH 7.0, has been characterized at 10 degrees C and pH 4.0; the second order rate constant is k = 2 x 10(7) M-1 s-1 and becomes 4 x 10(8) M-1 s-1 at pH 6.5. The reaction between flavocytochrome b2 and Co(phen)3(3+) has a second order rate constant k = 2 x 10(7) M-1 s-1 at pH 7.0, 10 degrees C. The oxidation of both proteins by Mn(CyDTA)(H2O)- is characterized by a second order rate constant k = 2.8 x 10(6) M-1 s-1 and 2.3 x 10(5) M-1 s-1 for cytochrome b2 core and flavocytochrome b2, respectively, at pH 7.0 and 10 degrees C. The reactivity of the b2 heme towards the outer sphere oxidants is higher than that reported for heme c in bacterial and eukaryotic cytochrome c. The larger delta E and the larger accessibility of the b2 heme can account for this result. The flavodehydrogenase domain seems to modulate the electron transfer also to these inorganic compounds, as found previously in the case of macromolecular electron acceptors.

Kinetics of electron transfer between two Hansenula anomala flavocytochrome b2 derivatives and two simple copper proteins (azurin and stellacyanin)

Two derivatives of Hansenula anomala flavocytochrome b2 have been prepared, one deprived of the flavin prosthetic group (deflavocytochrome b2), and the other consisting of the heme-b-carrying globule (b2 core). The redox potential of the heme in the two derivatives is -5 (+/- 5) mV and -10 (+/- 5) mV respectively, fairly similar to the value of -20 (+/- 5) mV reported for the holoenzyme, indicating a minor effect of the flavin and of the flavodehydrogenase domain on heme potential. The kinetics of azurin and stellacyanin reduction by both derivatives have been investigated. At pH 7.0, I = 0.2 M and 20 degrees C the second-order rate constants are: k = 8 X 10(5) M-1 S-1 for azurin reduction by deflavocytochrome b2; k = 1.6 X 10(6) M-1 S-1 for azurin reduction by b2 core; k = 1 X 10(7) M-1 S-1 for stellacyanin reduction by deflavocytochrome b2; k = 3 X 10(7) M-1 S-1 for stellacyanin reduction by b2 core. The change in pH markedly affects the kinetics in the case of azurin, but has no effect on stellacyanin reduction. The change in ionic strength has a significant effect when deflavocytochrome b2 is the reductant, indicating that the flavodehydrogenase domain plays a role in the stabilization of the transient kinetic complex by means of electrostatic interactions. The kinetic results are discussed in the framework of the Marcus theory.

Spontaneous dissociation of a cytochrome core and a biglobular flavoprotein after mild trypsinolysis of the bifunctional Saccharomyces cerevisiae flavocytochrome b2

Saccharomyces cerevisiae flavocytochrome b2 is known as a bifunctional enzyme which behaves as the association of an FMN flavodehydrogenase with its specific acceptor, a b5-like cytochrome. Mild trypsinolysis gives rise to three complementary fragments (n, X, beta'), both prosthetic groups being still bound. After such proteolysis the separation of a biglobular flavoprotein domain (carrying FMN) from a cytochrome domain (with the heme) is obtained by molecular sieving under non-denaturing conditions. The marked lack of affinity between the tetrameric flavoprotein (X, beta')4 and the monomeric cytochrome core (n) leads to the hypothesis that the two domains are not tightly associated in the native molecule and might more relative to each other. Their respective mobility is possibly required for the catalytic mechanism. The comparison with previous trypsinolysis studies on the flavocytochrome b2 from Hansenula anomala suggests the presence of two common zones of hypersensitivity to proteases, along the protomeric polypeptide chain, and strongly supports the validity of the triglobular model for both flavocytochromes.

Flavocytochrome b2 (Baker's yeast). Deuterium isotope effect studied by rapid-kinetic methods as a probe for the mechanism of electron transfer

The use of DL-[2-2H]lactate in steady-state measurements of ferricyanide reduction by flavocytochrome b2 at 30 degrees C has previously yielded an isotope effect of 5 [F. Lederer (1974) Eur. J. Biochem. 46, 393--399]. We report here studies carried out at 5 degrees C with L-[2-2H]lactate, where flavin and heme reduction were observed in the stopped-flow apparatus, in the absence of acceptor. The generally biphasic reduction curves were analysed according to a new mathematical treatment which allowed us to derive microscopic constants from initial reduction rates. It has thus been possible to determine an isotope effect of 8 on flavin reduction, 6 on heme reduction, compared to 4 in the steady state. Consequently, two slightly rate-limiting steps occur after the first one where the alpha-hydrogen is abstracted. It has also been possible to calculate the substrate association and dissociation rate constants for intact enzyme. The studies were carried out in parallel on intact and cleaved cytochrome b2. The results suggest that proteolysis affects essentially the steps involved in flavin reduction, and not intramolecular electron transfer steps. Moreover, the experimental data obtained at low rates of electron entry have led us to reexamine a previously proposed scheme for electron transfer [Capeillère-Blandin, Bray, Iwatsubo and Labeyrie (1975) Eur. J. Biochem. 54, 549--566]. An alternative model based on computer-simulation studies will be presented in a paper in this journal.

Sulfite binding to a flavodehydrogenase, cytochrome b2 from baker's yeast

Baker's yeast L-lactate dehydrogenase (flavocytochrome b2) is a typical flavodehydrogenase, in that it accepts two electrons from the substrate but has a monoelectronic acceptor. Yet it forms a red semiquinone [Capeillère Blandin et al. Eur. J. Biochem. 54, 549--566 (1975)] and it is shown in this paper that it forms a reversible covalent complex with sulfite (Kd = 1.4 muM). This complex can be observed by difference spectroscopy and provides a convenient tool for visualizing the flavin chromophore, usually hidden behind the intense heme absorbance. A number of anions (D-lactate, oxalate and pyruvate) are inhibitors of the enzymatic reaction and induce spectral perturbations of the flavin spectrum. It is concluded that probably two positive charges exist at the active site: one which stabilizes the red semiquinone and one which attracts organic anions and sulfite. It is also concluded that the correlation between reactivity with sulfite and reactivity with oxygen among flavo-proteins may not be as general as previously proposed [Massey et al. J. Biol. Chem. 244, 3999--4006 (1969)].

Bromopyruvate as an affinity label for baker's yeast flavocytochrome b2. Kinetic study of the inactivation reaction

Bromopyruvate was shown to completely inactivate cytochrome b2 in a reaction that obeyed the kinetic criteria required for affinity labels: it inactivated flavocytochrome b2 according to saturation kinetics, and the inactivation reaction was competitively inhibited by the substrate or competitive inhibitors. Inactivation was irreversible. The behaviour of both forms of flavocytochrome b2 (lintact and proteolytically cleaved) was examined. It was found that the reduced cleaved enzyme was not inactivated by bromopyruvate; this phenomenon can probably be ascribed to a structural change undergone upon reduction. The value of the lactate dissociation constant of intact cytochrome b2 cytochrome b2 was determined in competition experiments with bromopyruvate. By comparison with the divergent published values for the Ks of the cleaved from, it appears that only those that differ from the Km by a factor of two or three are reasonable. This study opens the way for the identification of an active site residue and localization in the peptide chain of the bifunctional enzyme.

Binding studies of NADPH to NADP-specific L-glutamate dehydrogenase from Saccharomyces cerevisiae

Optical characteristics of enzyme-reduced coenzyme complexes of yeast NADP-specific glutamate dehydrogenase have been investigated in the presence and absence of product (L-glutamate) and in the presence or absence of phosphate. The phosphate effect, pointed out in a previous work, is found again: inorganic phosphate (Pi) destabilizes the binary complex (E - NADPH), the dissociation constant of which is equal to 14 muM, a value much higher than that determined in Tris-HCl buffer: Kd = 0.9 muM. Concerning the role of phosphate some assumptions are drawn up with respect to a similar behaviour of Pi toward yeast glutamate dehydrogenase and ADP toward the beef liver enzyme. In the same way, L-glutamate induces a stabilization of the binary complex; this latter effect is unchanged in the presence of phosphate, yet it is less marked than in the case of beef liver glutamate dehydrogenase. Protein fluorescence, nucleotide fluorescence and circular dichroism measurements allowed the determination of three identical and independent NADPH binding sites per hexameric active unit. In analogy with beef liver enzyme, it seems that yeast glutamate dehydrogenase is a good model to study anticooperativity in ligand binding.

Flavocytochrome b2: simulation studies of the electron-transfer reactions among the prosthetic groups

Simulation studies by digital computer were undertaken in order to test and clarify the interpretations deduced from experimental data concerning the electron transfer mechanism from L-lactate to flavocytochrome b2, which were presented in a preceding paper in this journal. The reaction scheme proposed as the "best" one is composed of 7 steps. It allows the best fitting of the time courses established for the oxidized flavin (Flox), the flavin semiquinone (Flsq), the fully reduced flavin (Flred), and the reduced haem (Hred); it can be extended to 1 s. This scheme also allows a good simulation of the general shape of preequilibrium titration curves obtained at a 200-ms reaction time for Hred and Flsq, and a valuable simulation of the reduction electron paramagnetic resonance time course established for Hred and Flsq at low lactate concentration. The agreement between experimental and simulated curves led to an estimation of some rate constants experimentally unknown, relative (in particular) to the electron exchange between flavin and haem and between couples of flavins. Another interest of these stimulation studies was to point out the obligatory involvement of a slow final step to perform the flavocytochrome b2 full reduction; this step could be controlled by some conformational change of the protein.

Flavocytochrome b2: kinetic studies by absorbance and electron-paramagnetic-resonance spectroscopy of electron distribution among prosthetic groups

The reduction by L-lactate of the prosthetic groups of flavocytochrome b2 (L-lactate cytochrome c oxidoreductase from aerobic yeast, a tetrameric molecule containing one haem and one flavin mononucleotide per protomer) was reinvestigated. It was confirmed that the enzyme ultimately takes up 3 electrons per protomer from this 2-electron donor. Stopped-flow absorbance data at an haem isosbestic point to follow the oxidized flavin and in a haem band indicate that, under the conditions used, haem and flavin reduction time courses are indistinguishable, both being biphasic (phases I and II). Comparison with electron paramagnetic resonance data (Fe3+ haem and flavosemiquinone signals) led to a complete description at 24 degrees C of the time courses of the various reduction states of the prosthetic groups. It has been previously demonstrated (Morton and Sturtevant, 1964) that, after the formation of the enzyme-substrate complex, the electron transfer to the enzyme takes place as the first and rate-limiting step of the turnover. In the present study, an initial burst of fully reduced flavin, of small amplitude, is detected at the very beginning of phase I (before 6 ms). The redox forms which accumulate thereafter till the end of phase I (30-35 ms) are the reduced haem (up to 80%), the flavin semiquinone (up to 50%) and the fully reduced flavin (from 25% up to 35%); the total of electrons distributed at the end of phase I is about 2 per protomer meaning that, in this phase, each enzyme site acts as a 2-electron and not a 3-electron acceptor. A 2-electron flow as the limiting step during phase I with the rate constant kI accounts for the steady-state electron flow during catalysis. Phase I is followed by the much slower phase II which corresponds to the entry of the third electron and cannot be involved in the turnover. The interpretation of the results are given as a scheme, with the proper rate constants, allowing a satisfactory fitting of experimental data by simulation. Among the elementary steps required are a rapid distribution of one electron from reduced flavin to the haem, a rapid interprotomers dismutation between couples of flavin semiquinone regenerating two oxidized flavin per tetramer. The very low reactivity of the latter for the entry of the third electron per protomer is tentatively explained by the occurrence of a slow additional step limiting the final reduction reaction. It was observed that, over phase I and the beginning of phase II, from 15 to 200 ms, all the redox species remain apparently under equilibrium conditions. Parallel studies (titrations of flavocytochrome b2 by L-lactate) showed that the set of equilibrium parameters relative to haem and flavin species is significantly different in the "final" equilibrium (after 30 s) from that in the time interval 15-200 ms. Such an anomaly suggests a conformation change takes place very slowly in the molecule after the acceptance of the first two electrons per protomer.

Interactions substrat-flavine-hemoproteine et stabilite thermique du cytochrome b(2) (L-lactate deshydrogenase de la levure)

We studied the thermal inactivation of yeast cytochrome b(2), the native flavohaemoprotein (FHP(n)) and its derivatives, the haemoprotein (HP) obtained by dissociation of FMN from FHP(n) and the reconstituted flavohaemoprotein (FHP(r)) (haemoprotein + FMN). The initial velocity of inactivation is faster in the presence of FMN. The substrate (L-lactate) or the competitive inhibitor (oxolate) protects only the flavohaemoproteins (FHP(n) and FHP(r)) from thermal inactivation by decreasing this velocity. In each case, the "protection constant", K(p), is determined.Several interpretations can be proposed to explain the lack of protection of the haemoprotein by its substrate: 1) the structure of the substrate-binding site depends on the presence of the FMN; 2) the substrate binding site exists, but the affinity is too small to be detected; 3) a hypothetical scheme is proposed, suggesting that the protein can exist under different thermally sensitive forms following whether FMN or substrate is present.