Chemical synthesis and some properties of 6-substituted flavins

A number of derivatives of riboflavin and of 3-methyllumiflavin substituted in the 6 position have been synthesized starting with 6-nitro flavins, reduction to the 6-amino flavin, and diazotization, followed by reaction with the appropriate nucleophile. The absorption spectra, oxidation-reduction potentials, and the electron spin resonance spectra of the radical cationic forms of several of these synthetic compounds have been determined, including 6-S-cysteinyl-3-methyllumiflavin and 6-S-cysteinylriboflavin. The latter has been shown to be identical with the dephosphorylated form of the aminoacyl flavin isolated from trimethylamine dehydrogenase [Steenkamp, D. J., Kenney, W. C. & Singer, T. P. (1978) J. Biol. Chem. 253, 2812-2817; Steenkamp, D. J., McIntire, W., & Kenney, W. C. (1978) J. Biol. Chem. 253, 2818-2824] in regard to absorption specturm, photochemical properties, and mobility in high-voltage electrophoresis and in thin-layer chromatography. An unusually pronounced interaction between the amino group and the isoalloxazine ring system was deduced from the absorption spectra of 6-amino-3-methyllumiflavin and 6-aminoriboflavin.     

A rate-limiting conformational change of the flavin in p-hydroxybenzoate hydroxylase is necessary for ligand exchange and catalysis: studies with 8-mercapto- and 8-hydroxy-flavins

The FAD of p-hydroxybenzoate hydroxylase (PHBH) is known to exist in two conformations. The FAD must be in the in-position for hydroxylation of p-hydroxybenzoate (pOHB), whereas the out-position is essential for reduction of the flavin by NADPH. In these investigations, we have used 8-mercapto-FAD and 8-hydroxy-FAD to probe the movement of the flavin in catalysis. Under the conditions employed, 8-mercapto-FAD (pK(a) = 3.8) and 8-hydroxy-FAD (pK(a) = 4.8) are mainly anionic. The spectral characteristics of the anionic forms of these flavins are very sensitive to their environment, making them sensitive probes for detecting movement of the flavin during catalysis. With these flavin analogues, the enzyme hydroxylates pOHB efficiently, but at a rate much slower than that of enzyme with FAD. Reaction of oxygen with reduced forms of these modified enzymes in the absence of substrate appears to proceed through the formation of the flavin-C4a-hydroperoxide intermediate, as with normal enzyme, but the decay of this intermediate is so fast compared to its formation that very little accumulates during the reaction. However, after elimination of H2O2 from the flavin-C4a-hydroperoxide, a perturbed oxidized enzyme spectrum is observed (Eox*), and this converts slowly to the spectrum of the resting oxidized form of the enzyme (Eox). In the presence of pOHB, PHBH reconstituted with 8-mercapto-FAD also shows the additional oxidized intermediate (Eox*) after the usual oxygenated C4a-intermediates have formed and decayed in the course of the hydroxylation reaction. This Eox* to Eox step is postulated to be due to flavin movement. Furthermore, binding of pOHB to resting (Eox) follows a three-step equilibrium mechanism that is also consistent with flavin movement being the rate-limiting step. The rate for the slowest step during pOHB binding is similar to that observed for the conversion of Eox* to Eox during the oxygen reaction in the absence or presence of substrate. Steady-state kinetic analysis of PHBH substituted with 8-mercapto-FAD demonstrated that the apparent k(cat) is also similar to the rate of Eox* conversion to Eox. Presumably, the protein environment surrounding the flavin in Eox* differs slightly from that of the final resting form of the enzyme (Eox).     

Differences in protein structure of xanthine dehydrogenase and xanthine oxidase revealed by reconstitution with flavin active site probes

The native flavin, FAD, was removed from chicken liver xanthine dehydrogenase and milk xanthine oxidase by incubation with CaCl2. The deflavoenzymes, still retaining their molybdopterin and iron-sulfur prosthetic groups, were reconstituted with a series of FAD derivatives containing chemically reactive or environmentally sensitive substituents in the isoalloxazine ring system. The reconstituted enzymes containing these artificial flavins were all catalytically active. With both the chicken liver dehydrogenase and the milk oxidase, the flavin 8-position was found to be freely accessible to solvent. The flavin 6-position was also freely accessible to solvent in milk xanthine oxidase, but was significantly less exposed to solvent in the chicken liver dehydrogenase. Pronounced differences in protein structure surrounding the bound flavin were indicated by the spectral properties of the two enzymes reconstituted with flavins containing ionizable -OH or -SH substituents at the flavin 6- or 8-positions. Milk xanthine oxidase either displayed no preference for binding of the neutral or anionic flavin (8-OH-FAD) or a slight preference for the anionic form of the flavin (6-hydroxy-FAD, 6-mercapto-FAD, and possibly 8-mercapto-FAD). On the other hand, the chicken liver dehydrogenase had a dramatic preference for binding the neutral (protonated) forms of all four flavins, perturbing the pK of the ionizable substituent greater than or equal to 4 pH units. These results imply the existence of a strong negative charge in the flavin binding site of the dehydrogenase, which is absent in the oxidase.     

6-Thiocyanatoflavins and 6-mercaptoflavins as active-site probes of flavoproteins

6-Thiocyanatoflavins have been found to be susceptible to nucleophilic displacement reactions with sulfite and thiols, yielding respectively the 6-S-SO3--flavin and 6-mercaptoflavin, with rate constants at pH 7.0, 20 degrees C, of 55 M-1 min-1 for sulfite and 1000 M-1 min-1 for dithiothreitol. The 6-SCN-flavin binds tightly to riboflavin-binding protein as the riboflavin derivative, to apoflavodoxin, apo-lactate oxidase, and apo-Old Yellow Enzyme as the FMN derivative, and to apo-D-amino acid oxidase as the FAD derivative. The riboflavin-binding protein derivative is inaccessible to dithiothreitol attack, and the lactate oxidase and D-amino acid oxidase derivatives show only limited accessibility. However, the flavodoxin and Old Yellow Enzyme derivatives react readily with dithiothreitol, indicating that the flavin 6-position is exposed to solvent in these proteins. The lactate oxidase and D-amino acid oxidase derivatives convert slowly but spontaneously to the 6-mercaptoflavin enzyme forms in the absence of any added thiol, indicating the presence of a thiol residue in the flavin binding site of these proteins. The reaction rates have been investigated of 6-mercaptoflavins with iodoacetamide, N-ethylmaleimide, methyl methanethiosulfonate, H2O2, and m-chloroperbenzoate, in both the free and protein-bound state. The results confirm the conclusions drawn from the studies with 6-SCN-flavins described above and from 6-N3-flavins [Massey, V., Ghisla, S., & Yagi, K. (1986) Biochemistry (preceding paper in this issue)]. The spectral properties of the protein-bound 6-mercaptoflavin vary widely among the five proteins studied and show stabilization of the neutral flavin with flavodoxin and riboflavin-binding protein and of the anionic species by Old Yellow Enzyme, lactate oxidase, and D-amino acid oxidase. In the case of the latter two enzymes, the stabilization appears to be due to interaction of the negatively charged flavin with a positively charged protein residue located near the flavin pyrimidine ring. This positively charged residue appears to be responsible also for the strong stabilization of the two-electron oxidation state of the mercaptoflavin as the 6-S-oxide. With the other flavoproteins studied this oxidation level is stabilized as the 6-sulfenic acid or 6-sulfenate.     

Characterization of 2-nitrophenol uptake system of Pseudomonas putida B2

Uptake of substituted nitrophenols from the bulk solution into the cytoplasm limited reaction rates by Pseudomonas putida B2. Initial enzymatic conversion of 2-nitrophenol (ONP) to catechol is by an intracellular soluble enzyme, nitrophenol oxygenase [Zeyer J and Kearney PC. 1984. J Agric Food Chem 32: 238–242]. Addition of N-ethylmaleimide (NEM) to cell suspensions led to a decrease in specific reaction rates for ONP, dependent on the ratio of NEM to cellular protein. Maximal NEM inhibition resulted in an 80–90% decrease in the ONP reaction rate which could not be reversed following dilution. Cell-free enzyme extract isolated from NEM-inactivated cells demonstrated less than 20% loss of the specific ONP reaction rates. NEM apparently acted by inhibiting a protein which facilitated uptake of nitrophenol into the cytoplasm, prior to the first catabolic enzyme. Both intact organisms and protoplasts exhibited the same 80–90% decrease in reaction rate which established that NEM inhibition was localized in the plasma membrane. NEM elicited variable effects on reaction rates for a series of ring substituted 2-nitrophenols. The data indicated that uptake of substituted 2-nitrophenols involved at least two transport systems, one sensitive to NEM inactivation and a second insensitive uptake process. Received 05 November 1996/ Accepted in revised form 29 May 1997     

Reactions of the 4a-hydroperoxide of liver microsomal flavin-containing monooxygenase with nucleophilic and electrophilic substrates

Liver microsomal flavin-containing monooxygenase (MFMO) has been shown to exhibit a stable 4a-flavin hydroperoxide intermediate in the absence of oxygenatable substrate (Poulsen, L. L., and Ziegler, D. M. (1979) J. Biol. Chem. 254, 6449-6455; Beaty, N. B., and Ballou, D. P. (1981) J. Biol. Chem. 256, 4619-4625). The reaction of this intermediate with an assortment of substrates was studied by stopped flow techniques. The first observed spectral change is a small blue shift in the absorbance peak of the 4a-flavin intermediate. The rate of this spectral change is dependent on the concentration of the substrate. This small spectral change is succeeded by a large increase in the absorbance at 450 nm. The rate of appearance of oxidized flavin is independent of substrate concentration but does increase at higher pH. Steady state turnover rates also greater at higher pH, consistent with earlier observations that the formation of oxidized flavin is rate determining in catalysis. Upon oxygenation by MFMO, thiobenzamide and iodide each undergo a spectral change which is dependent on substrate concentration. The spectral changes corresponding to oxygenation of these substrates occur at the same rates as do the initial small spectral changes contributed by the flavin chromophore as observed with all substrates. However, no substrate tested to date shows any effect on the rate of formation of oxidized flavin. Previous work has shown MFMO to catalyze the oxygenation of a variety of nitrogen- and sulfur-containing hydrophobic compounds. Two new classes of compounds are shown here to be substrates for this enzyme. The nucleophilic anions, iodide and thiocyanate, catalyze the decomposition of the 4a-flavin hydroperoxide. Organic boronic acids (e.g. phenylboronic acid and butylboronic acid) also appear to be oxygenated with no striking differences in kinetic characteristics from those of nucleophilic substrates. These organic boronic acids are classic electrophiles and suggest that like peracids, the 4a-flavin hydroperoxide is capable of oxygenating both nucleophiles and electrophiles (Lee, J. B., and Uff, B. C. (1967) Quart. Rev. 21, 429-457).     

Purification and properties of milk xanthine dehydrogenase.

Milk xanthine oxidase (XO) has been prepared in a dehydrogenase form (XDH) by purifying the enzyme in the presence of 2.5 mM dithiothreitol. Unlike XO, which reacts rapidly only with oxygen and not with NAD, the XDH form of the enzyme reacts rapidly with NAD. XDH has a turnover number for the NAD-dependent conversion of xanthine to urate of 380 mol/min/mol at pH 7.5, 25 degrees C, with a Km = < or = 1 microM for xanthine and a Km = 7 microM for NAD, but has very little O2-dependent activity. There is evidence that the two forms of the enzyme have different flavin environments: XDH stabilizes the neutral form of the flavin semiquinone and XO does not. Further, XDH binds the artificial flavin 8-mercapto-FAD in its neutral form, shifting the pK of this flavin by 5 pH units, while XO binds 8-mercapto-FAD in its benzoquinoid anionic form. XDH can be converted back to the XO form by the addition of three to four equivalents of the disulfide-forming reagent 4,4'-dithiodipyridine, suggesting that, in the XDH form of the enzyme, disulfide bonds are broken; this may cause a conformational change which creates a binding site for NAD and changes the protein structure near the flavin.     

Active-site structural comparison of streptococcal NADH peroxidase and NADH oxidase. Reconstitution with artificial flavins.

The apoproteins of the streptococcal NADH peroxidase (H2O2----2H2O) and NADH oxidase (O2----2H2O) stabilize the neutral forms of 6-hydroxy- and 6-mercapto-FAD, respectively. The redox behavior of the 6-hydroxy-FAD peroxidase closely mimics that of the native enzyme with both dithionite and NADH. Both oxidase and peroxidase preferentially stabilize the N(1)-protonated p-quinonoid species of 8-mercapto-FAD, and the 8-position of the bound flavin is accessible to solvent in both proteins. The 8-mercapto-FAD peroxidase yields an EH2 spectrum on reduction virtually identical to that seen with 8-mercapto-FAD glutathione reductase, but no distinct EH2.NADH form appears. The dramatic decreases in reactivity at the flavin 2- and 4-positions for both the peroxidase and the oxidase, assessed with the reconstituted 2- and 4-thio-FAD enzymes, suggest that these positions are buried by elements of both protein structures. Furthermore, reconstitution of the peroxidase with the higher potential 2- and 4-thioflavins yields enzyme forms which are fully reducible with 1.4 eq of NADH/FAD, giving rise to stable thio-FADH2.NAD+ complexes. This behavior closely mimics that of the native NADH oxidase and provides further evidence supporting the hypothesis that a major functional distinction between the two structurally related proteins is determined by the redox potential and/or NADH reactivity of the bound flavin coenzyme.     

Implication of arginine-131 and arginine-303 in the substrate site of adenylosuccinate synthetase of Escherichia coli by affinity labeling with 6-(4-bromo-2,3-dioxobutyl)thioadenosine 5'-monophosphate

Adenylosuccinate synthetase from Escherichia coli is inactivated in a biphasic reaction by 6-(4-bromo-2,3-dioxobutyl)thioadenosine 5'-monophosphate (6-BDB-TAMP) at pH 7.0 and 25 degrees C. The initial fast-phase inactivation is not affected by the presence of active-site ligands and can be completely eliminated by blocking Cys291 of the enzyme with N-ethylmaleimide (NEM). Reaction of the NEM-treated enzyme with 6-BDB-[32P]TAMP results in 2 mol of reagent incorporated/mol of enzyme subunit. The inactivation kinetics of the slow-phase exhibit an apparent KI of 40.6 microM and kmax of 0.0228 min-1. Active-site ligands, either adenylosuccinate or IMP and GTP, completely prevent inactivation of the enzyme by 6-BDB-TAMP, whereas IMP or IMP and aspartate is much less effective in protection. 6-BDB-TAMP-inactivated enzyme has a 3-fold increase in Km for aspartate with no change in Km for IMP or GTP. Protease digestion of 6-BDB-[32P]TAMP inactivated enzyme reveals that both Arg131 and Arg303 are modified by the affinity-labeling reagent. The crystal structure [Poland, B. W., Fromm, H. J., and Honzatko, R. B. (1996) J. Mol. Biol. 264, 1013-1027] and site-directed mutagenesis [Kang, C., Sun, N., Poland, B. W., Gorrell, A., and Fromm, H. J. (1997) J. Biol. Chem. 272, 11881-11885] of E. coli adenylosuccinate synthetase show that Arg303 interacts with the carboxyl group of aspartate and the 2'-OH of the ribose of IMP and Arg131 is involved in stabilizing aspartate in the active site of the enzyme. We conclude that 6-BDB-TAMP functions as a reactive adenylosuccinate analogue in modifying both Arg131 and Arg303 in the active site of adenylosuccinate synthetase.     

Bacterial luciferase: demonstration of a catalytically competent altered conformational state following a single turnover

Ziegler-Nicoli et al. [Ziegler-Nicoli, M., Meighen, E. A., & Hastings, J. W. (1974) J. Biol. Chem. 249, 2385-2392] reported that a highly reactive cysteinyl residue on the alpha subunit of bacterial luciferase resides in or near the flavin binding site such that the enzyme-flavin complex is protected from inactivation by alkylating reagents. These authors also observed that injection of reduced flavin mononucleotide (FMNH2) into an air-equilibrated solution of enzyme protected the enzyme from alkylation for much longer than the lifetime of the 4a-peroxydihydroflavin intermediate resulting from reaction of enzyme-bound FMNH2 with O2. Two related explanations were offered: either the product flavin mononucleotide dissociated from the enzyme much more slowly following a catalytic cycle than would be predicted from the Kd measured by equilibrium binding or the enzyme itself, without bound flavin, was in an altered conformational state in which the thiol was less reactive following a catalytic cycle. Either explanation involves a slow return of the enzyme to its initial state following a catalytic cycle. We have investigated this phenomenon in more detail and found that rapid removal of the flavin from the enzyme by chromatography following catalytic turnover did not return the enzyme to its original state of susceptibility to either alkylating reagents or proteolytic enzymes. The flavin-free enzyme returned to the susceptible conformation with a half-time of ca. 25 min at 0 degree C. Inactivation of the enzyme at intermediate times of relaxation by either a proteolytic enzyme or an alkylating reagent showed biphasic kinetics, indicative of a mixture of the protected and susceptible forms.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cysteine 532 and cysteine 545 are the N-ethylmaleimide-reactive residues of the Neurospora plasma membrane H+-ATPase

Previous studies from this laboratory (Brooker, R. J., and Slayman, C. W. (1983) J. Biol. Chem. 258, 222-226; Davenport, J. W., and Slayman, C. W. (1988) J. Biol. Chem. 263, 16007-16013) have used the sulfhydryl reagent N-ethylmaleimide (NEM) to define two sites on the Neurospora plasma membrane H+-ATPase: a "fast" site which reacts in several minutes with no loss of enzymatic activity and a "slow" site which reacts in tens of minutes to produce complete inactivation of the enzyme. The slow site is protected when MgATP or MgADP is bound to the catalytic site of the ATPase. The present study demonstrates that the fluorescent reagent 5-[2-iodoacetamido)ethyl)-1-aminonaphthalenesulfonic acid (IAEDANS) can be used to label five of the eight cysteine residues of the Neurospora ATPase (Cys376, Cys409, Cys472, Cys532, Cys545). Tryptic peptides bearing those residues have been purified by high performance liquid chromatography and located within the known primary structure of the ATPase by amino acid analysis and/or sequencing. By pretreating the enzyme with NEM in the presence or absence of MgADP before incubation with IAEDANS, it has been possible to identify the fast NEM site as Cys545 and the slow MgADP-protectable NEM site as Cys532. Both residues lie within the central hydrophilic domain of the protein, close to a highly conserved stretch of amino acids that may be involved in nucleotide binding. However, all five IAEDANS-reactive cysteines can be nearly completely modified by the less bulky sulfhydryl reagent methyl methanethiosulfonate with less than 20% inhibition of enzyme activity; thus, none of the five cysteines can be considered to play a direct role in the reaction cycle of the ATPase.     

para-Hydroxybenzoate hydroxylase containing 6-hydroxy-FAD is an effective enzyme with modified reaction mechanisms

The flavin prosthetic group (FAD) of p-hydroxybenzoate hydroxylase (EC 1.14.13.2) from Pseudomonas fluorescens, was replaced by 6-hydroxy-FAD (an extra hydroxyl group on the carbon at position 6 of the isoalloxazine ring of FAD). The catalytic cycle of this modified enzyme was analyzed and compared to the function of native (FAD) enzyme. Transient state kinetic analyses of the multiple changes in the chemical state of the flavin were the principal methods used to probe the mechanism. Four known substrates of the native enzyme were used to probe the reaction. With the natural substrate, p-hydroxybenzoate, the 6-hydroxy-FAD enzyme activity was 12-15% of native enzyme, due to a slower release of product from the enzyme, and less than one product molecule was formed per NADPH oxidized, due to an increased rate of nonproductive decomposition of the transient peroxyflavin essential to the catalytic pathway. More extensive changes in mechanism were observed with the substrates, 2,4-dihydroxybenzoate and p-aminobenzoate. The results suggest that, during catalysis, when the reduced state of FAD is ready for oxygen reaction, the substrate is located below and close to the C-4a/N-5 edge of the isoalloxazine ring. The nature of the high extinction, transient state of flavin, formed upon transfer of oxygen to substrate is discussed. It is not a flavin cation, and is unlikely to be an oxygen-substituted analogue of N-3/C-4 dihydroflavin.     

Lysine-21 of Leuconostoc mesenteroides glucose 6-phosphate dehydrogenase participates in substrate binding through charge-charge interaction.

Leuconostoc mesenteroides glucose 6-phosphate dehydrogenase (G6PD) was isolated in high yield and purified to homogeneity from a newly constructed strain of Escherichia coli which lacks its own glucose 6-phosphate dehydrogenase gene. Lys-21 is one of two lysyl residues in the enzyme previously modified by the affinity labels pyridoxal 5'-phosphate and pyridoxal 5'-diphosphate-5'-adenosine, which are competitive inhibitors of the enzyme with respect to glucose 6-phosphate (LaDine, J.R., Carlow, D., Lee, W.T., Cross, R.L., Flynn, T.G., & Levy, H.R., 1991, J. Biol. Chem. 266, 5558-5562). K21R and K21Q mutants of the enzyme were purified to homogeneity and characterized kinetically to determine the function of Lys-21. Both mutant enzymes showed increased Km-values for glucose 6-phosphate compared to wild-type enzyme: 1.4-fold (NAD-linked reaction) and 2.1-fold (NADP-linked reaction) for the K21R enzyme, and 36-fold (NAD-linked reaction) and 53-fold (NADP-linked reaction) for the K21Q enzyme. The Km for NADP+ was unchanged in both mutant enzymes. The Km for NAD+ was increased 1.5- and 3.2-fold, compared to the wild-type enzyme, in the K21R and K21Q enzymes, respectively. For the K21R enzyme the kcat for the NAD- and NADP-linked reactions was unchanged. The kcat for the K21Q enzyme was increased in the NAD-linked reaction by 26% and decreased by 30% in the NADP-linked reaction from the values for the wild-type enzyme. The data are consistent with Lys-21 participating in the binding of the phosphate group of the substrate to the enzyme via charge-charge interaction.     

FAD analogues as prosthetic groups of human glutathione reductase. Properties of the modified enzyme species and comparisons with the active site structure

Human glutathione reductase (NADPH + GSSG + H+ in equilibrium with NADP+ + 2 GSH) is a suitable enzyme for correlating spectroscopic properties and chemical reactivities of protein-bound FAD analogues with structural data. FAD, the prosthetic group of the enzyme, was replaced by FAD analogues, which were modified at the positions 8, 1, 2, 4, 5 and 6, respectively, of the isoalloxazine ring. When compared with a value of 100% for native glutathione reductase, the specific activities of most enzyme species ranged from 40% to 17%, in the order of the prosthetic groups 8-mercapto-FAD greater than 8-azido-FAD = 8-F-FAD = 8-C1-FAD greater than 4-thio-FAD = 1-deaza-FAD greater than 2-thio-FAD. The enzymic activities indicate a correct orientation of the bound analogues. The enzyme species containing 5-deaza-FAD and 6-OH-FAD, respectively, had no more glutathione reductase activity than the FAD-free apoenzyme. 5-Deaza-FAD X glutathione reductase was crystallized for X-ray diffraction analysis. Detailed studies were focussed on position 8 of the flavin. 8-Cl-FAD X glutathione reductase and 8-F-FAD X glutathione reductase reacted only poorly with HS- to give 8-mercapto-FAD X glutathione reductase, which suggests that the region around Val61 hinders the halogen anion from leaving the tetrahedral intermediate. Other experiments showed that position 8 is accessible to certain solvent-borne reagents. 8-Mercapto-FAD X glutathione reductase, for instance, reacted readily and stoichiometrically with the thiol reagent methylmethanethiosulfonate. 8-Mercapto-FAD X glutathione reductase does not exhibit a long wavelength charge transfer absorption band upon reduction, as it is the case for the 2-electron-reduced FAD-containing enzyme. This behaviour indicates that the charge transfer interaction between flavin and the thiolate of Cys63 in the native enzyme is not per se essential for catalysis. The absorption spectrum of the blue anionic 8-mercapto-FAD bound to glutathione reductase suggests that the protein concurs to the stabilization of a negative charge in the pyrimidine subnucleus. In light of the protein structure this effect is attributed to the dipole moment of alpha-helix 338-354 which starts out close to the N(1)/C(2)/O(2 alpha) region of the flavin. 1-Deaza-FAD binds as tightly as FAD to the apoenzyme. The resulting holoenzyme was found to be enzymically active but structurally unstable. In this respect 1-deaza-FAD . glutathione reductase mimics the properties of the enzyme species found in inborn glutathione reductase deficiency.(ABSTRACT TRUNCATED AT 400 WORDS)     

Studies of the mechanism of phenol hydroxylase: mutants Tyr289Phe, Asp54Asn, and Arg281Met

Three residues in the active site of the flavoprotein phenol hydroxylase (PHHY) were independently changed by site-directed mutagenesis. One of the mutant forms of PHHY, Tyr289Phe, is reduced by NADPH much slower than is the wild-type enzyme, although it has a slightly higher redox potential than the wild-type enzyme. In the structure of the wild-type enzyme, residue Tyr289 is hydrogen-bonded with the FAD when the latter is at the "out" position but has no direct contact with the flavin when it is "in". The oxidative half-reaction of PHHY is not significantly affected by this mutation, contrary to the concept that Tyr289 is a critical residue in the hydroxylation reaction [Enroth, C., Neujahr, H., Schneider, G., and Lindqvist, Y. (1998) Structure 6, 605-617; Ridder, L., Mullholland, A. J., Rietjens, I. M. C. M., and Vervoort, J. (2000) J. Am. Chem. Soc. 122, 8728-8738]. Tyr289 may help stabilize the FAD in the out conformation where it can be reduced by NADPH. For the Asp54Asn mutant form of PHHY, the initial step of the oxidative half-reaction is significantly slower than for the wild-type enzyme. Asp54Asn utilizes less than 20% of the reduced flavin for hydroxylating the substrate with the remainder forming H(2)O(2). Similar changes are observed when Arg281, a residue between Asp54 and the solvent, is mutated to Met. These two residues are suggested to be part of the active site environment the enzyme provides for the flavin cofactor to function optimally in the oxidative half-reaction. In the construction of the mutant forms of PHHY, it was determined that 11 of the previously reported amino acid residues in the sequence of PHHY were incorrect.     

Cysteinyl peptides of pig heart NADP-dependent isocitrate dehydrogenase that are modified upon inactivation by N-ethylmaleimide

Pig heart NADP-specific isocitrate dehydrogenase is inactivated by N-ethylmaleimide (NEM) (Colman, R. F., and Chu, R. (1970) J. Biol. Chem. 245, 601-607), and is completely protected against inactivation, but not against the incorporation of NEM, by isocitrate plus Mn2+. We have now treated the enzyme with [3H]NEM in the absence and presence of isocitrate plus Mn2+, digested it with trypsin, and isolated and sequenced the labeled Cys peptides. In the inactive enzyme, two major peptides, SSGGFVWACK and DLAGCIHGLSNVK, and two minor peptides, CATITPDEAR and EPIICK, were labeled at Cys. Upon reaction with [3H]NEM in the presence of isocitrate plus Mn2+, full catalytic activity was retained and only DLAGCIHGLSNVK was labeled; the Cys of this peptide is therefore not essential for catalysis. The modification of SSGGFVWACK appears to be the major cause of inactivation by NEM. The Cys in SSGGFVWACK may have a catalytic role, most likely in the strengthened binding of Mn2+ in the presence of isocitrate. Isocitrate dehydrogenase was carboxymethylated under denaturing conditions with [14C]iodoacetate and digested with trypsin; 6 unique labeled Cys peptides, containing 6 unique Cys residues, were purified and sequenced. Six corresponding peptides were isolated from enzyme treated under denaturing conditions with [3H]NEM. These results eliminate the previous uncertainty regarding the number of Cys residues in the enzyme. A comparison of the sequences of the NH2-terminal 30 residues and the 6 Cys peptides of the pig heart NADP-dependent isocitrate dehydrogenase with the Escherichia coli NADP enzyme provides evidence for great dissimilarity between the two enzymes.     

Properties of lipoamide dehydrogenase altered by site-directed mutagenesis at a key residue (I184Y) in the pyridine nucleotide binding domain.

The binding of pyridine nucleotide to human erythrocyte glutathione reductase, an enzyme of known three-dimensional structure, requires some movement of the side chain of Tyr197. Moreover, this side chain lies very close to the isoalloxazine ring of the FAD cofactor. The analogous residue, Ile184, in the homologous enzyme Escherichia coli lipoamide dehydrogenase has been altered by site-directed mutagenesis to a tyrosine residue (I184Y) [Russell, G. C., Allison, N., Williams, C. H., Jr., & Guest, J.R. (1989) Ann. N.Y. Acad. Sci. 573, 429-431]. Characterization of the altered enzyme shows that the rate of the pyridine nucleotide half-reaction has been markedly reduced and that the spectral properties have been changed to mimic those of glutathione reductase. Therefore, Ile184 is shown to be an important residue in modulating the properties of the flavin in lipoamide dehydrogenase. Turnover in the dihydrolipoamide/NAD+ reaction is decreased by 10-fold and in the NADH/lipoamide reaction by 2-fold in I184Y lipoamide dehydrogenase. The oxidized form of I184Y shows remarkable changes in the fine structure of the visible absorption and circular dichroism spectra and also shows nearly complete quenching of FAD fluorescence. The spectral properties of the altered enzyme are thus similar to those of glutathione reductase and very different from those of wild-type lipoamide dehydrogenase. On the other hand, spectral evidence does not reveal any change in the amount of charge-transfer stabilization at the EH2 level. Stopped-flow data indicate that, in the reduction of I184Y by NADH, the first step, reduction of the flavin, is only slightly slowed but the subsequent two-electron transfer to the disulfide is markedly inhibited.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanism-based inactivation of mitochondrial monoamine oxidase by N-(1-methylcyclopropyl)benzylamine

Three different radioactively labeled N-(1-methylcyclopropyl)benzylamines [N-(1-Me)CBA] were synthesized and used to show which atoms of the inactivator remain bound to monoamine oxidase (MAO) after inactivation. Organic chemical reactions were employed to elucidate the structure of the enzyme adduct and clarify the mechanism of inactivation. Following inactivation and dialysis, the benzyl substituent is lost, but the methyl group and cyclopropyl carbons remain attached to the enzyme even after further dialysis against solutions containing 1 mM benzylamine or 8 M urea. Treatment of inactivated enzyme with sodium cyanoborohydride prior to dialysis results in the retention of the benzyl group, suggesting an imine linkage. One hydride from sodium boro[3H]hydride is incorporated into the dialyzed inactivated enzyme consistent with a ketone functional group. When Pronase-digested N-(1-Me)CBA-inactivated MAO is treated with basic potassium triiodide, iodoform is isolated, indicating the presence of a methyl ketone. During inactivation, the optical spectrum of the covalently bound active site flavin changes from that of oxidized to reduced flavin. After urea denaturation, the flavin remains reduced, suggesting covalent linkage of the inactivator to the cofactor. On the basis of previous results [Silverman, R. B., Hoffman, S. J., & Catus, W. B., III (1980) J. Am. Chem. Soc. 102, 7126-7128], it is proposed that the mechanism of inactivation involves transfer of one electron from N-(1-Me)CBA to the flavin, resulting in an amine radical cation and a flavin radical. Then, either the cyclopropyl ring is attacked by the flavin radical or the cyclopropyl ring opens, and the radical generated is captured by the flavin radical. The product of this mechanism is the imine of benzylamine and 4-flavinyl-2-butanone, the proposed enzyme-inactivator adduct.     

Catalytic function of tyrosine residues in para-hydroxybenzoate hydroxylase as determined by the study of site-directed mutants

The role of protein residues in activating the substrate in the reaction catalyzed by the flavoprotein p-hydroxybenzoate hydroxylase was studied. X-ray crystallography (Schreuder, H. A., Prick, P.A.J., Wieringa, R.K., Vriend, G., Wilson, K.S., Hol, W.G. J., and Drenth, J. (1989) J. Mol. Biol. 208, 679-696) indicates that Tyr-201 and Tyr-385 form a hydrogen bond network with the 4-OH of p-hydroxybenzoate. Therefore, site directed mutants were constructed, converting each of these tyrosines into phenylalanines. Spectral (visible and fluorescence) properties, reduction potentials, and binding constants are very similar to those of wild type, indicating that there are no major structural changes in the mutants. In the absence of substrate, the mutants and wild type exhibit similar pH-dependent changes in the FAD spectrum. However, the enzyme-substrate complex of Tyr-201----Phe lacks an ionization observed in both wild type and Tyr-385----Phe, which preferentially bind the phenolate form of substrates. Tyr-201----Phe shows no preference, indicating that Tyr-201 is required to ionize the substrate. The mutants have less than 6% the activity of the wild type enzyme. The effects on catalysis were studied by stopped flow techniques. Reduction of FAD by NADPH is slower by 10-fold in Tyr-201----Phe and 100-fold in Tyr-385----Phe. When the reduced Tyr-201----Phe-p-hydroxybenzoate complex reacts with oxygen, a long-lived flavin-C(4a)-hydroperoxide is observed, which slowly eliminates H2O2 with very little hydroxylation. Thus, the role of Tyr-201 is to activate the substrate by stabilizing the phenolate. Tyr-385----Phe reacts with oxygen to form 25% oxidized enzyme, and 75% flavin hydroperoxide, which successfully hydroxylates the substrate. This mutant also hydroxylates the product (3, 4-dihydroxybenzoate) to form gallic acid.     

Mechanism of action of methanol oxidase, reconstitution of methanol oxidase with 5-deazaflavin, and inactivation of methanol oxidase by cyclopropanol

Methanol oxidase isolated from Hansenula polymorpha contains two distinct flavin cofactors in approximately equal amounts. One has been identified as authentic FAD and the other as a modified form of FAD differing only in the ribityl portion of the ribityldiphosphoadenosine side chain. The significance of this finding is as yet unknown. Previous studies have shown that cyclopropanol irreversibly inactivates methanol oxidase [Mincey, T., Tayrien, G., Mildvan, A. S., & Abeles, R. H. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 7099-7101]. We have now established that inactivation is accompanied by covalent modification of the flavin cofactor. The stoichiometry of this reaction is 1 mol of cyclopropanol/mol of active flavin. The structure of the covalent adduct was determined by NMR, IR, and UV spectral studies to be an N5,C4a-cyclic 4a,5-dihydroflavin. Reduction of the covalent adduct with NaBH4 at pH 9.0 before removal from the enzyme converted it to the 1-(ribityldiphosphoadenosine)-substituted 4-(3-hydroxypropyl)-2,3-dioxoquinoxaline. Cyclopropyl ring cleavage accompanies inactivation, and covalent bond formation occurs between a methylene carbon of cyclopropanol and N5 of flavin. Methanol oxidase was also reconstituted with 5-deazaflavin adenine dinucleotide (dFAD). Reconstituted enzyme did not catalyze the oxidation of alcohols to the corresponding aldehydes, nor did reduced reconstituted enzyme catalyze the reverse reaction. Incubation of reconstituted enzyme with cyclopropanol resulted in an absorbance decrease at 399 nm, but no irreversible covalent modification of the deazaflavin cofactor. A reversible addition complex between cyclopropanol and dFAD is formed. The structure of that complex was not definitively established, but it is likely that it is formed through the addition of cyclopropoxide to C5 of dFAD. The failure of dFAD-reconstituted methanol oxidase to catalyze the oxidation of substrate, as well as the lack of reaction with cyclopropanol, supports a radical mechanism for alcohol oxidation and cyclopropanol inactivation. Methanol oxidase catalyzes the oxidation of cyclopropylcarbinol to the corresponding aldehyde. No ring-opened products were detected. The failure to form ring-opened products has been used as an argument against radical processes [MacInnes, I., Nonhebel, D. C., Orsculik, S. T., & Suckling, C. J. (1982) J. Chem. Soc., Chem. Commun., 121-122]. We present arguments against this interpretation.