Characterization of a Structurally Distinct ATP-Dependent Reactivating Factor of Adenosylcobalamin-Dependent Lysine 5,6-Aminomutase

Several anaerobic bacterial species, including the Gram-negative oral bacterium Fusobacterium nucleatum, ferment lysine to produce butyrate, acetate, and ammonia. The second step of the metabolic pathway─isomerization of β-l-lysine to erythro-3,5-diaminohexanoate─is catalyzed by the adenosylcobalamin (AdoCbl) and pyridoxal 5'-phosphate (PLP)-dependent enzyme, lysine 5,6-aminomutase (5,6-LAM). Similar to other AdoCbl-dependent enzymes, 5,6-LAM undergoes mechanism-based inactivation due to loss of the AdoCbl 5'-deoxyadenosyl moiety and oxidation of the cob(II)alamin intermediate to hydroxocob(III)alamin. Herein, we identified kamB and kamC, two genes responsible for ATP-dependent reactivation of 5,6-LAM. KamB and KamC, which are encoded upstream of the genes corresponding to α and β subunits of 5,6-LAM (kamD and kamE), co-purified following coexpression of the genes in Escherichia coli. KamBC exhibited a basal level of ATP-hydrolyzing activity that was increased 35% in a reaction mixture that facilitated 5,6-LAM turnover with β-l-lysine or d,l-lysine. Ultraviolet-visible (UV-vis) spectroscopic studies performed under anaerobic conditions revealed that KamBC in the presence of ATP/Mg2+ increased the steady-state concentration of the cob(II)alamin intermediate in the presence of excess β-l-lysine. Using a coupled UV-visible spectroscopic assay, we show that KamBC is able to reactivate 5,6-LAM through exchange of the damaged hydroxocob(III)alamin for AdoCbl. KamBC is also specific for 5,6-LAM as it had no effect on the rate of substrate-induced inactivation of the homologue, ornithine 4,5-aminomutase. Based on sequence homology, KamBC is structurally distinct from previously characterized B12 chaperones and reactivases, and correspondingly adds to the list of proteins that have evolved to maintain the cellular activity of B12 enzymes.

Expanding the β-substitution reactions of serine synthase through mutagenesis of aromatic active site residues

The Gram-negative bacterium, Fusobacterium nucleatum, possesses a fold II type pyridoxal 5'-phosphate-dependent enzyme that catalyzes the reversible β-replacement of l-cysteine and l-serine, generating H2S and H2O, respectively. This enzyme, termed serine synthase (FN1055), contains an active site Asp232 that serves as a general base in the activation of a water molecule for nucleophilic attack of the ⍺-aminoacrylate intermediate. A network of hydrophobic residues surrounding Asp232 are key to catalysis as they increase the basicity of the side chain. However, these residues severely restrict the range of nucleophilic substrates that can react with the ⍺-aminoacrylate, making the enzyme an ineffective biocatalyst for noncanonical amino acid biosynthesis. Herein, we systematically substituted four aromatic active residues (Trp99, Phe125, Phe148 and Phe234) to an alanine to determine their catalytic importance in serine/cysteine synthase reactions and if their substitution could broaden the scope of nucleophiles that could react with the ⍺-aminoacrylate intermediate. All four single site mutants W99A, F125A, F148A, and F234A could form the ⍺-aminoacrylate intermediate upon reaction with either l-cysteine or l-serine; however, the rate constant associated with the elimination of the β-hydroxyl group from l-serine was 150 to 200-fold lower in the F125A and F148A variants. Substitution of Phe125 and Phe148, situated ∼3-4 Å from the general base, also abolished the serine synthase reaction due to their inability to activate a water molecule for nucleophilic attack of the ⍺-aminoacrylate. Overall, the mutational studies indicate that the clustering of aromatic residues disproportionately benefits the serine synthase reaction as they increase the binding affinity for l-cysteine, decrease the binding of the product, l-serine, and promote the activation of a water molecule. Notably, the aminoacrylate species present in F125A and F148A was able to react with thiophenol, signifying that serine synthase has biocatalytic potential in the synthesis of noncanonical amino acids.

A new member of the flavodoxin superfamily from Fusobacterium nucleatum that functions in heme trafficking and reduction of anaerobilin

Fusobacterium nucleatum is an opportunistic oral pathogen that is associated with various cancers. To fulfill its essential need for iron, this anaerobe will express heme uptake machinery encoded at a single genetic locus. The heme uptake operon includes HmuW, a class C radical SAM-dependent methyltransferase that degrades heme anaerobically to release Fe2+ and a linear tetrapyrrole called anaerobilin. The last gene in the operon, hmuF encodes a member of the flavodoxin superfamily of proteins. We discovered that HmuF and a paralog, FldH, bind tightly to both FMN and heme. The structure of Fe3+-heme-bound FldH (1.6 Å resolution) reveals a helical cap domain appended to the ⍺/β core of the flavodoxin fold. The cap creates a hydrophobic binding cleft that positions the heme planar to the si-face of the FMN isoalloxazine ring. The ferric heme iron is hexacoordinated to His134 and a solvent molecule. In contrast to flavodoxins, FldH and HmuF do not stabilize the FMN semiquinone but instead cycle between the FMN oxidized and hydroquinone states. We show that heme-loaded HmuF and heme-loaded FldH traffic heme to HmuW for degradation of the protoporphyrin ring. Both FldH and HmuF then catalyze multiple reductions of anaerobilin through hydride transfer from the FMN hydroquinone. The latter activity eliminates the aromaticity of anaerobilin and the electrophilic methylene group that was installed through HmuW turnover. Hence, HmuF provides a protected path for anaerobic heme catabolism, offering F. nucleatum a competitive advantage in the colonization of anoxic sites of the human body.

Biosynthesis of meso-lanthionine in Fusobacterium nucleatum

The opportunistic oral pathogen, Fusobacterium nucleatum contains meso-lanthionine as the diaminodicarboxylic acid in the pentapeptide crosslink of the peptidoglycan layer. The diastereomer, l,l-lanthionine is formed by lanthionine synthase, a PLP-dependent enzyme that catalyzes the β-replacement of l-cysteine with a second equivalent of l-cysteine. In this study, we explored possible enzymatic mechanisms for the formation of meso-lanthionine. Our inhibition studies with lanthionine synthase, described herein, revealed that meso-diaminopimelate, a bioisostere of meso-lanthionine, is a more potent inhibitor of lanthionine synthase compared to the diastereomer, l,l-diaminopimelate. These results suggested that lanthionine synthase could also form meso-lanthionine by the β-replacement of l-cysteine with d-cysteine. Through steady-state and pre-steady state kinetic analysis, we confirm that d-cysteine reacts with the ⍺-aminoacylate intermediate with a k_on that was 2-3-fold faster and K_d value that was 2-3fold lower compared to l-cysteine. However, given that intracellular levels of d-cysteine levels are assumed to be significantly lower than that of l-cysteine, we also determined if the gene product, FN1732, with low sequence identity to diaminopimelate epimerase could convert l,l-lanthionine to meso-lanthionine. Using diaminopimelate dehydrogenase in a coupled spectrophotometric assay, we show that FN1732 can convert l,l-lanthionine to meso-lanthionine with a k_cat of 0.07 ± 0.001 s^-1 and a K_M of 1.9 ± 0.1 mM. In summary, our results provide two possible enzymatic mechanisms for the biosynthesis of meso-lanthionine in F. nucleatum.

Sequence Divergence in the Arginase Domain of Ornithine Decarboxylase/Arginase in Fusobacteriacea Leads to Loss of Function in Oral Associated Species

A number of species within the Fusobacteriaceae family of Gram-negative bacteria uniquely encode for an ornithine decarboxylase/arginase (ODA) that ostensibly channels l-ornithine generated by hydrolysis of l-arginine to putrescine formation. However, two aspartate residues required for coordination to a catalytically obligatory manganese cluster of arginases are substituted for a serine and an asparagine. Curiously, these natural substitutions occur only in a clade of Fusobacterium species that inhabit the oral cavity. Herein, we expressed and isolated full-length ODA from the opportunistic oral pathogen Fusobacterium nucleatum along with the individual arginase and ornithine decarboxylase components. The crystal structure of the arginase domain reveals that it adopts the classical α/β arginase-fold, but metal ions are absent in the active site. As expected, the ureohydrolase activity with l-arginine was not detected for wild-type ODA or the isolated arginase domain. However, engineering of the complete metal coordination environment through site-directed mutagenesis restored Mn²⁺ binding capacity and arginase activity, although the catalytic efficiency for l-arginine was low (60-100 M^-1 s^-1). Full-length ODA and the isolated ODC component were able to decarboxylate both l-ornithine and l-arginine to form putrescine and agmatine, respectively, but k_cat/K_M of l-ornithine was ∼20-fold higher compared to l-arginine. We discuss environmental conditions that may have led to the natural selection of an inactive arginase in the oral associated species of Fusobacterium.

Steady-state and pre-steady state kinetic analysis of ornithine 4,5-aminomutase

Ornithine 4,5-aminomutase (4,5-OAM) is a pyridoxal 5'-phosphate and adenosylcobalamin-dependent enzyme that catalyzes a 1,2-rearrangement of the terminal amine of d-ornithine to form (2R, 4S)-diaminopentanoate. The gene encoding ornithine 4,5-aminomutase is clustered with other genes that function in the oxidative l-ornithine metabolic pathway present in a number of anaerobic bacteria. This chapter discusses the methodology for measuring 4,5-OAM activity using NAD⁺-dependent diaminopentanoate dehydrogenase, which functions downstream of 4,5-OAM in the l-ornithine metabolic pathway. The use of ornithine racemace, which functions upstream of 4,5-OAM, for the synthesis of d,l-ornithine-3,3,4,4,5,5-d₆ is also presented. Finally, this chapter describes the anaerobic stopped-flow spectrophotometric analysis of 4,5-OAM. Information is provided on the integration of a stopped-flow system in the anaerobically-maintained glove, the preparation of anaerobic solutions, and the experimental approach.

A Fold Type II PLP-Dependent Enzyme from Fusobacterium nucleatum Functions as a Serine Synthase and Cysteine Synthase

Serine synthase (SS) from Fusobacterium nucleatum is a fold type II pyridoxal 5'-phosphate (PLP)-dependent enzyme that catalyzes the β-replacement of l-cysteine with water to form l-serine and H₂S. Herein, we show that SS can also function as a cysteine synthase, catalyzing the β-replacement of l-serine with bisulfide to produce l-cysteine and H₂O. The forward (serine synthase) and reverse (cysteine synthase) reactions occur with comparable turnover numbers and catalytic efficiencies for the amino acid substrate. Reaction of SS with l-cysteine leads to transient formation of a quinonoid species, suggesting that deprotonation of the Cα and β-elimination of the thiolate group from l-cysteine occur via a stepwise mechanism. In contrast, the quinonoid species was not detected in the formation of the α-aminoacrylate intermediate following reaction of SS with l-serine. A key active site residue, D232, was shown to stabilize the more chemically reactive ketoenamine PLP tautomer and also function as an acid/base catalyst in the forward and reverse reactions. Fluorescence resonance energy transfer between PLP and W99, the enzyme's only tryptophan residue, supports ligand-induced closure of the active site, which shields the PLP cofactor from the solvent and increases the basicity of D232. These results provide new insight into amino acid metabolism in F. nucleatum and highlight the multiple catalytic roles of D232 in a new member of the fold type II family of PLP-dependent enzymes.

S224 Presents a Catalytic Trade-off in PLP-Dependent l-Lanthionine Synthase from Fusobacterium nucleatum

Lanthionine synthase from the oral bacterium Fusobacterium nucleatum is a fold type II pyridoxal-5'-phosphate (PLP)-dependent enzyme that catalyzes the β-replacement of l-cysteine by a second molecule of l-cysteine to form H₂S and l-lanthionine. The meso-isomer of the latter product is incorporated into the F. nucleatum peptidoglycan layer. Herein, we investigated the catalytic role of S224, which engages in hydrogen-bond contact with the terminal carboxylate of l-lanthionine in the closed conformation of the enzyme. Unexpectedly, the S224A variant elicited a 7-fold increase in the turnover rate for H₂S and lanthionine formation and a 70-fold faster rate constant for the formation of the α-aminoacrylate intermediate compared to the wild-type enzyme. Presteady state kinetic analysis further showed that the reaction between S224A and l-cysteine leads to the formation of the more reactive ketoenamine tautomer of the α-aminoacrylate. The α-aminoacrylate with the protonated Schiff base is not an observable intermediate in the analogous reaction with the wild type, which may account for its attenuated kinetic properties. However, the S224A substitution is detrimental to other aspects of the catalytic cycle; it facilitates the α,β-elimination of l-lanthionine, and it weakens the enzyme's catalytic preference for the formation of l-lanthionine over that of l-cystathionine.

Structural insight into the high reduction potentials observed for Fusobacterium nucleatum flavodoxin

Flavodoxins are small flavin mononucleotide (FMN)-containing proteins that mediate a variety of electron transfer processes. The primary sequence of flavodoxin from Fusobacterium nucleatum, a pathogenic oral bacterium, is marked with a number of distinct features including a glycine to lysine (K13) substitution in the highly conserved phosphate-binding loop (T/S-X-T-G-X-T), variation in the aromatic residues that sandwich the FMN cofactor, and a more even distribution of acidic and basic residues. The E_ox/sq (oxidized/semiquinone; -43 mV) and E_sq/hq (semiquinone/hydroquinone; -256 mV) are the highest recorded reduction potentials of known long-chain flavodoxins. These more electropositive values are a consequence of the apoprotein binding to the FMN hydroquinone anion with ~70-fold greater affinity compared to the oxidized form of the cofactor. Inspection of the FnFld crystal structure revealed the absence of a hydrogen bond between the protein and the oxidized FMN N5 atom, which likely accounts for the more electropositive E_ox/sq . The more electropositive E_sq/hq is likely attributed to only one negatively charged group positioned within 12 Å of the FMN N1. We show that natural substitutions of highly conserved residues partially account for these more electropositive reduction potentials.

Kinetic characterization of acetone monooxygenase from Gordonia sp. strain TY-5

Acetone monooxygenase (ACMO) is a unique member of the Baeyer–Villiger monooxygenase (BVMO) family based on its ability to act on small ketones, such as acetone. Herein, we performed a kinetic analysis of ACMO from the propane-utilizing bacterium Gordonia sp. strain TY-5 to assess its preference for smaller ketone substrates. Steady state kinetic analysis of ACMO with a range of linear (C3–C7) and cyclic ketones (C4–C6) reveals that the enzyme elicits the highest catalytic efficiency towards butanone and cyclobutanone. Stopped-flow and inhibition studies further revealed that ACMO has a relatively weak binding affinity for the coenzyme with a dissociation constant of 120 μM. We show through mutagenesis that sequence variation in the residue that coordinates to the 2′-phosphate of NADP(H) partially accounts for the weaker binding affinity observed. As for shown for related BVMOs, NADP⁺ stabilizes the C4a-peroxyflavin intermediate in ACMO; however, the rate of its formation is considerably slower in ACMO. The observed rate constant for NADPH-dependent flavin reduction was 60 s⁻¹ at 25 °C, and experiments performed with 4(R)-[4-²H]NADPH confirm that the C4-pro-R-hydride from the nicotinamide ring is transferred to the FAD. The latter experimental result suggests that the nicotinamide ring rotates within the active site to carry out its two functional roles: reduction of the FAD cofactor and stabilization of the C4a-peroxyflavin adduct.

Active site arginine controls the stereochemistry of hydride transfer in cyclohexanone monooxygenase

Cyclohexanone monooxygenase (CHMO) uses NADPH and O₂ to insert oxygen into an array of (a)cyclic ketones to form esters or lactones. Herein, the role of two conserved active site residues (R327 and D57) in controlling the binding mode of NADP(H) was investigated. Wild type CHMO elicits a kinetic isotope effect (KIE) of 4.7 ± 0.1 and 1.1 ± 0.1 with 4(R)-[4-²H]NADPH and 4(S)-[4-²H]NADPH, respectively, consistent with transfer of the proR hydrogen to FAD. Strikingly, the R327K variant appears to lack stereospecificity for hydride transfer as a KIE of 1.5 ± 0.1 and 2.5 ± 0.1 was observed for the proR and proS deuterated forms of NADPH. ¹H NMR of the NADP⁺ products confirmed that the R327K variant abstracts either the proR or proS hydrogen from NADPH. While the D57A variant retained stereospecificity for the proR hydrogen, this substitution resulted in slow decomposition of the C4a-peroxyflavin intermediate in the presence of cyclohexanone. Based on published structures of a related flavin monooxygenase, we suggest that elimination of the hydrogen bond between D57 and R327 in the D57A variant causes R327 to adopt a substrate-induced conformation that slows substrate access to the active site, thereby prolonging the lifetime of the C4a-peroxyflavin intermediate.

Role of active site loop in coenzyme binding and flavin reduction in cytochrome P450 reductase

Cytochrome P450 reductase (CPR) contains a loop within the active site (comprising Asp(634), Ala(635), Arg(636) and Asn(637); human CPR numbering) that relocates upon NADPH binding. Repositioning of the loop triggers the reorientation of an FAD-shielding tryptophan (Trp(679)) to a partially stacked conformer, reducing the energy barrier for displacement of the residue by the NADPH nicotinamide ring: an essential step for hydride transfer. We used site-directed mutagenesis and kinetic analysis to investigate if the amino acid composition of the loop influences the catalytic properties of CPR. The D634A and D634N variants elicited a modest increase in coenzyme binding affinity coupled with a 36- and 10-fold reduction in cytochrome c(3+) turnover and a 17- and 3-fold decrease in the pre-steady state rate of flavin reduction. These results, in combination with a reduction in the kinetic isotope effect for hydride transfer, suggest that diminished activity is due to destabilization of the partially stacked conformer of Trp(677) and slower release of NADP(+). In contrast, R636A, R636S and an A635G/R636S double mutant led to a modest increase in cytochrome c(3+) reduction, which is linked to weaker coenzyme binding and faster interflavin electron transfer. A potential mechanism by which Arg(636) influences catalysis is discussed.

Kinetic analysis of electron flux in cytochrome P450 reductases reveals differences in rate-determining steps in plant and mammalian enzymes

Herein, we compare the kinetic properties of CPR from Arabidopsis thaliana (ATR2), with CPR from Artemisia annua (aaCPR) and human CPR (hCPR). While all three CPR forms elicit comparable rates for cytochrome c(3+) turnover, NADPH reduction of the FAD cofactor is ∼50-fold faster in aaCPR and ATR2 compared to hCPR, with a kobs of ∼500 s(-1) (6 °C). Stopped-flow analysis of the isolated FAD-domains reveals that NADP(+)-FADH2 charge-transfer complex formation is also significantly faster in the plant enzymes, but the rate of its decay is comparable for all three proteins. In hCPR, transfer of a hydride ion from NADPH to FAD is tightly coupled to subsequent FAD to FMN electron transfer, indicating that the former catalytic event is slow relative to the latter. In contrast, interflavin electron transfer is slower than NADPH hydride transfer in aaCPR and ATR2, occurring with an observed rate constant of ∼50 s(-1). Finally, the transfer of electrons from FMN to cytochrome c(3+) is rapid (>10(3) s(-1)) in all three enzymes and does not limit catalytic turnover. In combination, the data reveal differences in rate-determining steps between plant CPR and their mammalian equivalent in mediating the flux of reducing equivalents from NADPH to external electron acceptors.

Isotope effects for deuterium transfer and mutagenesis of Tyr187 provide insight into controlled radical chemistry in adenosylcobalamin-dependent ornithine 4,5-aminomutase

Adenosylcobalamin-dependent ornithine 4,5-aminomutase (OAM) from Clostridium sticklandii utilizes pyridoxal 5'-phosphate (PLP) to interconvert d-ornithine to 2,4-diaminopentanoate via a multistep mechanism that involves two hydrogen transfer steps. Herein, we uncover features of the OAM catalytic mechanism that differentiate it from its homologue, the more catalytically promiscuous lysine 5,6-aminomutase. Kinetic isotope effects (KIEs) with dl-ornithine-3,3,4,4,5,5-d6 revealed a diminished (D)kcat/Km of 2.5 ± 0.4 relative to a (D)kcat of 7.6 ± 0.5, suggesting slow release of the substrate from the active site. In contrast, a KIE was not observed on the rate constant associated with Co-C bond homolysis as this step is likely "gated" by the formation of the external aldimine. The role of tyrosine 187, which lies planar to the PLP pyridine ring, was also investigated via site-directed mutagenesis. The 25- and 1260-fold reduced kcat values for Y187F and Y187A, respectively, are attributed to a slower rate of external aldimine formation and a diminution of adenosylcobalamin Co-C bond homolysis. Notably, electron paramagnetic resonance studies of Y187F suggest that the integrity of the active site is maintained as cob(II)alamin and the PLP organic radical (even at lower concentrations) remain tightly exchange-coupled. Modeling of d-lysine and l-β-lysine into the 5,6-LAM active site reveals interactions between the substrate and protein are weaker than those in OAM and fewer in number. The combined data suggest that the level of protein-substrate interactions in aminomutases not only influences substrate specificity, but also controls radical chemistry.

Proximal FAD histidine residue influences interflavin electron transfer in cytochrome P450 reductase and methionine synthase reductase

Cytochrome P450 reductase (CPR) and methionine synthase reductase (MSR) transfer reducing equivalents from NADPH to FAD to FMN. In CPR, hydride transfer and interflavin electron transfer are kinetically coupled steps, but in MSR the two catalytic steps are represented by two distinct kinetic phases leading to transient formation of the FAD hydroquinone. In human CPR, His(322) forms a hydrogen-bond with the highly conserved Asp(677), a member of the catalytic triad. The catalytic triad is present in MSR, but Ala(312) replaces the histidine residue. To examine if this structural variation accounts for differences in their kinetic behavior, reciprocal substitutions were created. Substitution of His(322) for Ala in CPR does not affect the rate of NADPH hydride transfer or the FAD redox potentials, but does impede interflavin electron transfer. For MSR, swapping Ala(312) for a histidine residue resulted in the kinetic coupling of hydride and interflavin electron transfer, and eliminated the formation of the FAD hydroquinone intermediate. For both enzymes, placement of the His residue in the active site weakens coenzyme binding affinity. The data suggest that the proximal FAD histidine residue accelerates proton-coupled electron transfer from FADH2 to the higher potential FMN; a mechanism for this catalytic role is discussed.

Kinetic analysis of cytochrome P450 reductase from Artemisia annua reveals accelerated rates of NADH-dependent flavin reduction

Cytochrome P450 reductase from Artemisia annua (aaCPR) is a diflavin enzyme that has been employed for the microbial synthesis of artemisinic acid (a semi-synthetic precursor of the anti-malarial drug, artemisinin) based on its ability to transfer electrons to the cytochrome P450 monooxygenase, CYP71AV1. We have isolated recombinant aaCPR (with the N-terminal transmembrane motif removed) from Escherichia coli and compared its kinetic and thermodynamic properties with other CPR orthologues, most notably human CPR. The FAD and FMN redox potentials and the macroscopic kinetic constants associated with cytochrome c(3+) reduction for aaCPR are comparable to that of other CPR orthologues, with the exception that the apparent binding affinity for the oxidized coenzyme is ~ 30-fold weaker compared to human CPR. CPR from A. annua shows a 3.5-fold increase in uncoupled NADPH oxidation compared to human CPR and a strong preference (85 100-fold) for NADPH over NADH. Strikingly, reduction of the enzyme by the first and second equivalent of NADPH is much faster in aaCPR, with rates of > 500 and 17 s(-1) at 6 °C. We also optically detect a charge-transfer species that rapidly forms in < 3 ms and then persists during the reductive half reaction. Additional stopped-flow kinetic studies with NADH and (R)-[4-(2) H]NADPH suggest that the accelerated rate of flavin reduction is attributed to the relatively weak binding affinity of aaCPR for NADP(+) .

Aromatic substitution of the FAD-shielding tryptophan reveals its differential role in regulating electron flux in methionine synthase reductase and cytochrome P450 reductase

Methionine synthase reductase (MSR) and cytochrome P450 reductase (CPR) transfer reducing equivalents from NADPH via an FAD and FMN cofactor to a redox partner protein. In both enzymes, hydride transfer from NADPH to FAD requires displacement of a conserved tryptophan that lies coplanar to the FAD isoalloxazine ring. Swapping the tryptophan for a smaller aromatic side chain revealed a distinct role for the residue in regulating MSR and CPR catalysis. MSR W697F and W697Y showed enhanced catalysis, noted by increases in kcat and k(cat)/K(m)(NADPH) for steady-state cytochrome c(3+) reduction and a 10-fold increase in the rate constant (k(obs1)) associated with hydride transfer. Elevated primary kinetic isotope effects on k(obs1) for W697F and W697Y suggest that preceding isotopically insensitive steps like displacement of W697 are less rate determining. MSR W697Y, but not MSR W697F, showed detectable formation of the disemiquinone intermediate, indicating that the polarity of the aromatic side chain influences the rate of interflavin electron transfer. By contrast, the CPR variants (W676F and W676Y) displayed modest decreases in cytochrome c(3+) reduction, a 30- and 3.5-fold decrease in the rate of FAD reduction, accumulation of a FADH2 -NADP(+) charge-transfer complex and dramatically suppressed rates of interflavin electron transfer. We conclude for MSR that hydride transfer is 'gated' by the free energy required to disrupt dispersion forces between the FAD isoalloxazine ring and W697. By contrast, the bulky indole ring of W676 accelerates catalysis in CPR by lowering the energy barrier for displacement of the oxidized nicotinamide ring coplanar with the FAD.

Mutagenesis of a conserved glutamate reveals the contribution of electrostatic energy to adenosylcobalamin co-C bond homolysis in ornithine 4,5-aminomutase and methylmalonyl-CoA mutase

Binding of substrate to ornithine 4,5-aminomutase (OAM) and methylmalonyl-CoA mutase (MCM) leads to the formation of an electrostatic interaction between a conserved glutamate side chain and the adenosyl ribose of the adenosylcobalamin (AdoCbl) cofactor. The contribution of this residue (Glu338 in OAM from Clostridium sticklandii and Glu392 in human MCM) to AdoCbl Co-C bond labilization and catalysis was evaluated by substituting the residue with a glutamine, aspartate, or alanine. The OAM variants, E338Q, E338D, and E338A, showed 90-, 380-, and 670-fold reductions in catalytic turnover and 20-, 60-, and 220-fold reductions in k(cat)/K(m), respectively. Likewise, the MCM variants, E392Q, E392D, and E392A, showed 16-, 330-, and 12-fold reductions in k(cat), respectively. Binding of substrate to OAM is unaffected by the single-amino acid mutation as stopped-flow absorbance spectroscopy showed that the rates of external aldimine formation in the OAM variants were similar to that of the native enzyme. The decrease in the level of catalysis is instead linked to impaired Co-C bond rupture, as UV-visible spectroscopy did not show detectable AdoCbl homolysis upon binding of the physiological substrate, d-ornithine. AdoCbl homolysis was also not detected in the MCM mutants, as it was for the native enzyme. We conclude from these results that a gradual weakening of the electrostatic energy between the protein and the ribose leads to a progressive increase in the activation energy barrier for Co-C bond homolysis, thereby pointing to a key role for the conserved polar glutamate residue in controlling the initial generation of radical species.

Tryptophan 697 modulates hydride and interflavin electron transfer in human methionine synthase reductase

Human methionine synthase reductase (MSR), a diflavin oxidoreductase, plays a vital role in methionine and folate metabolism by sustaining methionine synthase (MS) activity. MSR catalyzes the oxidation of NADPH and shuttles electrons via its FAD and FMN cofactors to inactive MS-cob(II)alamin. A conserved aromatic residue (Trp697) positioned next to the FAD isoalloxazine ring controls nicotinamide binding and catalysis in related flavoproteins. We created four MSR mutants (W697S, W697H, S698Δ, and S698A) and studied their associated kinetic behavior. Multiwavelength stopped-flow analysis reveals that NADPH reduction of the C-terminal Ser698 mutants occurs in three resolvable kinetic steps encompassing transfer of a hydride ion to FAD, semiquinone formation (indicating FAD to FMN electron transfer), and slow flavin reduction by a second molecule of NADPH. Corresponding experiments with the W697 mutants show a two-step flavin reduction without an observable semiquinone intermediate, indicating that W697 supports FAD to FMN electron transfer. Accelerated rates of FAD reduction, steady-state cytochrome c(3+) turnover, and uncoupled NADPH oxidation in the S698Δ and W697H mutants may be attributed to a decrease in the energy barrier for displacement of W697 by NADPH. Binding of NADP(+), but not 2',5'-ADP, is tighter for all mutants than for native MSR. The combined studies demonstrate that while W697 attenuates hydride transfer, it ensures coenzyme selectivity and accelerates FAD to FMN electron transfer. Moreover, analysis of analogous cytochrome P450 reductase (CPR) variants points to key differences in the driving force for flavin reduction and suggests that the conserved FAD stacking tryptophan residue in CPR also promotes interflavin electron transfer.

Large-scale domain dynamics and adenosylcobalamin reorientation orchestrate radical catalysis in ornithine 4,5-aminomutase

D-ornithine 4,5-aminomutase (OAM) from Clostridium sticklandii converts D-ornithine to 2,4-diaminopentanoic acid by way of radical propagation from an adenosylcobalamin (AdoCbl) to a pyridoxal 5'-phosphate (PLP) cofactor. We have solved OAM crystal structures in different catalytic states that together demonstrate unusual stability of the AdoCbl Co-C bond and that radical catalysis is coupled to large-scale domain motion. The 2.0-A substrate-free enzyme crystal structure reveals the Rossmann domain, harboring the intact AdoCbl cofactor, is tilted toward the edge of the PLP binding triose-phosphate isomerase barrel domain. The PLP forms an internal aldimine link to the Rossmann domain through Lys(629), effectively locking the enzyme in this "open" pre-catalytic conformation. The distance between PLP and 5'-deoxyadenosyl group is 23 A, and large-scale domain movement is thus required prior to radical catalysis. The OAM crystals contain two Rossmann domains within the asymmetric unit that are unconstrained by the crystal lattice. Surprisingly, the binding of various ligands to OAM crystals (in an oxygen-free environment) leads to transimination in the absence of significant reorientation of the Rossmann domains. In contrast, when performed under aerobic conditions, this leads to extreme disorder in the latter domains correlated with the loss of the 5'-deoxyadenosyl group. Our data indicate turnover and hence formation of the "closed" conformation is occurring within OAM crystals, but that the equilibrium is poised toward the open conformation. We propose that substrate binding induces large-scale domain motion concomitant with a reconfiguration of the 5'-deoxyadenosyl group, triggering radical catalysis in OAM.

Cobalamin uptake and reactivation occurs through specific protein interactions in the methionine synthase-methionine synthase reductase complex

Human methionine synthase reductase (MSR), a diflavin enzyme, restores the activity of human methionine synthase through reductive methylation of methionine synthase (MS)-bound cob(II)alamin. Recently, it was also reported that MSR enhances uptake of cobalamin by apo-MS, a role associated with the MSR-catalysed reduction of exogenous aquacob(III)alamin to cob(II)alamin [Yamada K, Gravel RA, TorayaT & Matthews RG (2006) Proc Natl Acad Sci USA103, 9476-9481]. Here, we report the expression and purification of human methionine synthase from Pichia pastoris. This has enabled us to assess the ability of human MSR and two other structurally related diflavin reductase enzymes (cytochrome P450 reductase and the reductase domain of neuronal nitric oxide synthase) to: (a) stimulate formation of holo-MS from aquacob(III)alamin and the apo-form of MS; and (b) reactivate the inert cob(II)alamin form of MS that accumulates during enzyme catalysis. Of the three diflavin reductases studied, cytochrome P450 reductase had the highest turnover rate (55.5 s(-1)) for aquacob(III)alamin reduction, and the reductase domain of neuronal nitric oxide synthase elicited the highest specificity (k(cat)/K(m) of 1.5 x 10(5) m(-1) s(-1)) and MSR had the lowest K(m) (6.6 microm) for the cofactor. Despite the ability of all three enzymes to reduce aquacob(III)alamin, only MSR (the full-length form or the isolated FMN domain) enhanced the uptake of cobalamin by apo-MS. MSR was also the only diflavin reductase to reactivate the inert cob(II)alamin form of purified human MS (K(act) of 107 nm) isolated from Pichia pastoris. Our work shows that reactivation of cob(II)alamin MS and incorporation of cobalamin into apo-MS is enhanced through specific protein-protein interactions between the MSR FMN domain and MS.

Mechanism of radical-based catalysis in the reaction catalyzed by adenosylcobalamin-dependent ornithine 4,5-aminomutase

We report an analysis of the reaction mechanism of ornithine 4,5-aminomutase, an adenosylcobalamin (AdoCbl)- and pyridoxal L-phosphate (PLP)-dependent enzyme that catalyzes the 1,2-rearrangement of the terminal amino group of D-ornithine to generate (2R,4S)-2,4-diaminopentanoic acid. We show by stopped-flow absorbance studies that binding of the substrate D-ornithine or the substrate analogue D-2,4-diaminobutryic acid (DAB) induces rapid homolysis of the AdoCbl Co-C bond (781 s(-1), D-ornithine; 513 s(-1), DAB). However, only DAB results in the stable formation of a cob(II)alamin species. EPR spectra of DAB and [2,4,4-(2)H(3)]DAB bound to holo-ornithine 4,5-aminomutase suggests strong electronic coupling between cob(II)alamin and a radical form of the substrate analog. Loading of substrate/analogue onto PLP (i.e. formation of an external aldimine) is also rapid (532 s(-1), D-ornithine; 488 s(-1), DAB). In AdoCbl-depleted enzyme, formation of the external aldimine occurs over long time scales (approximately 50 s) and occurs in three resolvable kinetic phases, identifying four distinct spectral intermediates (termed A-D). We infer that these represent the internal aldimine (lambda(max) 416 nm; A), two different unliganded PLP states of the enzyme (lambda(max) at 409 nm; B and C), and the external aldimine (lambda(max) 426 nm; D). An imine linkage with d-ornithine and DAB generates both tautomeric forms of the external aldimine, but with D-ornithine the equilibrium is shifted toward the ketoimine state. The influence of this equilibrium distribution of prototropic isomers in driving homolysis and stabilizing radical intermediate states is discussed. Our work provides the first detailed analysis of radical-based catalysis in this Class III AdoCbl-dependent enzyme.

Mechanism of coenzyme binding to human methionine synthase reductase revealed through the crystal structure of the FNR-like module and isothermal titration calorimetry

Human methionine synthase reductase (MSR) is a 78 kDa flavoprotein that regenerates the active form of cobalamin-dependent methionine synthase (MS). MSR contains one FAD and one FMN cofactor per polypeptide and functions in the sequential transfer of reducing equivalents from NADPH to MS via its flavin centers. We report the 1.9 A crystal structure of the NADP+-bound FNR-like module of MSR that spans the NADP(H)-binding domain, the FAD-binding domain, the connecting domain, and part of the extended hinge region, a feature unique to MSR. The overall fold of the protein is similar to that of the corresponding domains of the related diflavin reductase enzymes cytochrome P450 reductase and neuronal nitric oxide synthase (NOS). However, the extended hinge region of MSR, which is positioned between the NADP(H)/FAD- and FMN-binding domains, is in an unexpected orientation with potential implications for the mechanism of electron transfer. Compared with related flavoproteins, there is structural variation in the NADP(H)-binding site, in particular regarding those residues that interact with the 2'-phosphate and the pyrophosphate moiety of the coenzyme. The lack of a conserved binding determinant for the 2'-phosphate does not weaken the coenzyme specificity for NADP(H) over NAD(H), which is within the range expected for the diflavin oxidoreductase family of enzymes. Isothermal titration calorimetry reveals a binding constant of 37 and 2 microM for binding of NADP+ and 2',5'-ADP, respectively, for the ligand-protein complex formed with full-length MSR or the isolated FNR module. These values are consistent with Ki values (36 microM for NADP+ and 1.4 microM for 2',5'-ADP) obtained from steady-state inhibition studies. The relatively weaker binding of NADP+ to MSR compared with other members of the diflavin oxidoreductase family might arise from unique electrostatic repulsive forces near the 5'-pyrophosphate moiety and/or increased hydrophobic stacking between Trp697 and the re face of the FAD isoalloxazine ring. Small structural permutations within the NADP(H)-binding cleft have profound affects on coenzyme binding, which likely retards catalytic turnover of the enzyme in the cell. The biological implications of an attenuated mechanism of MS reactivation by MSR on methionine and folate metabolism are discussed.

Crystal structure and solution characterization of the activation domain of human methionine synthase

Human methionine synthase (hMS) is a multidomain cobalamin-dependent enzyme that catalyses the conversion of homocysteine to methionine by methyl group transfer. We report here the 1.6 A crystal structure of the C-terminal activation domain of hMS. The structure is C-shaped with the core comprising mixed alpha and beta regions, dominated by a twisted antiparallel beta sheet with a beta-meander region. These features, including the positions of the active-site residues, are similar to the activation domain of Escherichia coli cobalamin-dependent MS (MetH). Structural and solution studies suggest a small proportion of hMS activation domain exists in a dimeric form, which contrasts with the monomeric form of the E. coli homologue. Fluorescence studies show that human activation domain interacts with the FMN-binding domain of human methionine synthase reductase (hMSR). This interaction is enhanced in the presence of S-adenosyl-methionine. Binding of the D963E/K1071N mutant activation domain to the FMN domain of MSR is weaker than with wild-type activation domain. This suggests that one or both of the residues D963 and K1071 are important in partner binding. Key differences in the sequences and structures of hMS and MetH activation domains are recognized and include a major reorientation of an extended 3(10)-containing loop in the human protein. This structural alteration might reflect differences in their respective reactivation complexes and/or potential for dimer formation. The reported structure is a component of the multidomain hMS : MSR complex, and represents an important step in understanding the impact of clinical mutations and polymorphisms in this key electron transfer complex.

Protein interactions in the human methionine synthase-methionine synthase reductase complex and implications for the mechanism of enzyme reactivation

Methionine synthase (MS) is a cobalamin-dependent enzyme. It transfers a methyl group from methyltetrahydrofolate to homocysteine forming methionine and tetrahydrofolate. On the basis of sequence similarity with Escherichia coli cobalamin-dependent MS (MetH), human MS comprises four discrete functional modules that bind from the N- to C-terminus, respectively, homocysteine, methyltetrahydrofolate, cobalamin, and S-adenosylmethionine (AdoMet). The C-terminal activation domain also interacts with methionine synthase reductase (MSR), a NADPH-dependent diflavin oxidoreductase required for the reductive regeneration of catalytically inert cob(II)alamin (which is formed every 200-1000 catalytic cycles of MS) to cob(I)alamin. We have investigated complex formation between the (i) MS activation domain and MSR and (ii) MS activation domain and the isolated FMN-binding domain of MSR. We show that the MS activation domain interacts directly with the FMN-binding domain of MSR. Binding is weakened at high ionic strength, emphasizing the importance of electrostatic interactions at the protein-protein interface. Mutagenesis of conserved lysine residues (Lys1071 and Lys987) in the human activation domain weakens this protein interaction. Chemical cross-linking demonstrates complex formation mediated by acidic residues (FMN-binding domain) and basic residues (activation domain). The activation domain and isolated FMN-domain form a 1:1 complex, but a 1:2 complex is formed with activation domain and MSR. The midpoint reduction potentials of the FAD and FMN cofactors of MSR are not perturbed significantly on forming this complex, implying that electron transfer to cob(II)alamin is endergonic. The kinetics of electron transfer in MSR and the MSR-activation domain complex are similar. Our studies indicate (i) conserved binding determinants, but differences in protein stoichiometry, between human MS and bacterial MetH in complex formation with redox partners; (ii) a substantial endergonic barrier to electron transfer in the reactivation complex; and (iii) a lack of control on the thermodynamics and kinetics of electron transfer in MSR exerted by complex formation with activation domain. The structural and functional consequences of complex formation are discussed in light of the known crystal structure of human activation domain and the inferred conformational heterogeneity of the multidomain MSR-MS complex.

Kinetic and thermodynamic characterization of the common polymorphic variants of human methionine synthase reductase

Human methionine synthase reductase (MSR) is a protein containing both FAD and FMN, and it reactivates methionine synthase that has lost activity due to oxidation of cob(I)alamin to cob(II)alamin. In this study, anaerobic redox titrations were employed to determine the midpoint reduction potentials for the flavin cofactors in two highly prevalent polymorphic variants of MSR, I22/L175 and M22/S175. The latter is a genetic determinant of plasma homocysteine levels and has been linked to premature coronary artery disease, Down's syndrome, and neural tube defects. The I22/L175 polymorphism has been described in a homocystinuric patient. Interestingly, this polymorphism is in the extended linker region between the two flavin domains, which may mediate or facilitate interaction with methionine synthase. In MSR I22/L175, the FMN potentials are -103 mV (oxidized/semiquinone) and -175 mV (semiquinone/hydroquinone) at pH 7.0 and 25 degrees C, and the corresponding FAD potentials are -252 and -285 mV, respectively. For the M22/S175 variants, the values of the four midpoint potentials are -114 mV (FMN oxidized/semiquinone), -212 mV (FMN semiquinone/hydroquinone), -236 mV (FAD oxidized/semiquinone), and -264 mV (FAD semiquinone/hydroquinone). The midpoint potential values in the two variants are generally comparable to those originally determined for the MSR I22/S175 variant [Wolthers, K. R. (2003) Biochemistry 42, 3911-3920], with relatively minor variations in the different redox couples. In each case, blue neutral flavin semiquinone species are stabilized on both flavins, and are characterized by a broad absorption band in the long wavelength region. In addition, stopped-flow absorption and fluorescence spectroscopy were used to study the pre-steady state reduction kinetics by NADPH of the two polymorphic variants. The reversible kinetic model proposed for wild-type MSR was validated for the I22/L175 and M22/S175 variants. Thus, the biochemical penalties associated with these polymorphisms, which result in less effective methionine synthase activation, do not appear to result from differences in their reduction kinetics. It is likely that differences in their relative affinities for the redox partner, methionine synthase, underlie the differences in the relative efficiencies of reductive activation exhibited by the variants.

Electron transfer in human methionine synthase reductase studied by stopped-flow spectrophotometry

Human methionine synthase reductase (MSR) is a key enzyme in folate and methionine metabolism as it reactivates the catalytically inert cob(II)alamin form of methionine synthase (MS). Electron transfer from MSR to the cob(II)alamin cofactor coupled with methyl transfer from S-adenosyl methionine returns MS to the active methylcob(III)alamin state. MSR contains stoichiometric amounts of FAD and FMN, which shuttle NADPH-derived electrons to the MS cob(II)alamin cofactor. Herein, we have investigated the pre-steady state kinetic behavior of the reductive half-reaction of MSR by anaerobic stopped-flow absorbance and fluorescence spectroscopy. Photodiode array and single-wavelength spectroscopy performed on both full-length MSR and the isolated FAD domain enabled assignment of observed kinetic phases to mechanistic steps in reduction of the flavins. Under single turnover conditions, reduction of the isolated FAD domain by NADPH occurs in two kinetically resolved steps: a rapid (120 s(-1)) phase, characterized by the formation of a charge-transfer complex between oxidized FAD and NADPH, is followed by a slower (20 s(-1)) phase involving flavin reduction. These two kinetic phases are also observed for reduction of full-length MSR by NADPH, and are followed by two slower and additional kinetic phases (0.2 and 0.016 s(-1)) involving electron transfer between FAD and FMN (thus yielding the disemiquinoid form of MSR) and further reduction of MSR by a second molecule of NADPH. The observed rate constants associated with flavin reduction are dependent hyperbolically on NADPH and [4(R)-2H]NADPH concentration, and the observed primary kinetic isotope effect on this step is 2.2 and 1.7 for the isolated FAD domain and full-length MSR, respectively. Both full-length MSR and the separated FAD domain that have been reduced with dithionite catalyze the reduction of NADP+. The observed rate constant of reverse hydride transfer increases hyperbolically with NADP+ concentration with the FAD domain. The stopped-flow kinetic data, in conjunction with the reported redox potentials of the flavin cofactors for MSR [Wolthers, K. R., Basran, J., Munro, A. W., and Scrutton, N. S. (2003) Biochemistry, 42, 3911-3920], are used to define the mechanism of electron transfer for the reductive half-reaction of MSR. Comparisons are made with similar stopped-flow kinetic studies of the structurally related enzymes cytochrome P450 reductase and nitric oxide synthase.

Molecular dissection of human methionine synthase reductase: determination of the flavin redox potentials in full-length enzyme and isolated flavin-binding domains

Human methionine synthase reductase (MSR) catalyzes the NADPH-dependent reductive methylation of methionine synthase. MSR is 78 kDa flavoprotein belonging to a family of diflavin reductases, with cytochrome P450 reductase (CPR) as the prototype. MSR and its individual flavin-binding domains were cloned as GST-tagged fusion proteins for expression and purification from Escherichia coli. The isolated flavin domains of MSR retain UV-visible and secondary structural properties indicative of correctly folded flavoproteins. Anaerobic redox titrations on the individual domains assisted in assignment of the midpoint potentials for the high- and low-potential flavin. For the isolated FMN domain, the midpoint potentials for the oxidized/semiquinone (ox/sq) couple and semiquinone/hydroquinone (sq/hq) couple are -112 and -221 mV, respectively, at pH 7.0 and 25 degrees C. The corresponding couples in the isolated FAD domain are -222 mV (ox/sq) and -288 mV (sq/hq). Both flavins form blue neutral semiquinone species characterized by broad absorption peaks in the long-wavelength region during anaerobic titration with sodium dithionite. In full-length MSR, the values of the FMN couples are -109 mV (ox/sq) and -227 mV (sq/hq), and the corresponding couple values for FAD are -254 mV (ox/sq) and -291 mV (sq/hq). Separation of the MSR flavins does not perturb their thermodynamic properties, as midpoint potentials for all four couples are similar in isolated domains and in full-length MSR. The redox properties of MSR are discussed in relation to other members of the diflavin oxidoreductase family and the mechanism of electron transfer.

Identification and in vitro biological activities of hop proanthocyanidins: inhibition of nNOS activity and scavenging of reactive nitrogen species

Oligomeric proanthocyanidins constitute a group of water-soluble polyphenolic tannins that are present in the female inflorescences (up to 5% dry wt) of the hop plant (Humulus lupulus). Humans are exposed to hop proanthocyanidins through consumption of beer. Proanthocyanidins from hops were characterized for their chemical structure and their in vitro biological activities. Chemically, they consist mainly of oligomeric catechins ranging from dimers to octamers, with minor amounts of catechin oligomers containing one or two gallocatechin units. The chemical structures of four procyanidin dimers (B1, B2, B3, and B4) and one trimer, epicatechin-(4beta-->8)-catechin-(4alpha-->8)-catechin (TR), were elucidated using mass spectrometry, NMR spectroscopy, and chemical degradation. When tested as a mixture, the hop oligomeric proanthocyanidins (PC) were found to be potent inhibitors of neuronal nitric oxide synthase (nNOS) activity. Among the oligomers tested, procyanidin B2 was most inhibitory against nNOS activity. Procyanidin B3, catechin, and epicatechin were noninhibitory against nNOS activity. PC and the individual oligomers were all strong inhibitors of 3-morpholinosydnonimine (SIN-1)-induced oxidation of LDL, with procyanidin B3 showing the highest antioxidant activity at 0.1 microg/mL. The catechin trimer (TR) exhibited antioxidant activity more than 1 order of magnitude greater than that of alpha-tocopherol or ascorbic acid on a molar basis.

Neuronal nitric oxide synthase: substrate and solvent kinetic isotope effects on the steady-state kinetic parameters for the reduction of 2,6-dichloroindophenol and cytochrome c(3+)

The neuronal nitric oxide synthase (nNOS) basal and calmodulin- (CaM-) stimulated reduction of 2,6-dichloroindophenol (DCIP) and cytochrome c(3+) follow ping-pong mechanisms [Wolthers and Schimerlik (2001) Biochemistry 40, 4722-4737]. Primary deuterium [NADPH(D)] and solvent deuterium isotope effects on the kinetic parameters were studied to determine rate-limiting step(s) in the kinetic mechanisms for the two substrates. nNOS was found to abstract the pro-R (A-side) hydrogen from NADPH. Values for (D)V and (D)(V/K)(NADPH) were similar for the basal (1.3-1.7) and CaM-stimulated (1.5-2.1) reduction of DCIP, while (D)V (2.1-2.8) was higher than (D)(V/K)(NADPH) (1.1-1.5) for cytochrome c(3+) reduction with and without CaM. This suggests that the rate of the reductive half-reaction (NADPH oxidation) rather than that of the oxidative half-reaction (reduction of DCIP or cytochrome c(3+)) limits the overall reaction rate. A value for (D)(V/K)(NADPH) close to 1 indicates the intrinsic isotope effect on hydride transfer is suppressed by a slower step in the reductive half-reaction. The oxidative half-reaction is insensitive to NADPD isotope effects as both (D)(V/K)(DCIP) and (D)(V/K)(cytc) equal 1 within experimental error. Large solvent kinetic isotope effects (SKIE) observed for (V/K)(cytc) for basal (approximately 8) and CaM-stimulated (approximately 31) reduction of cytochrome c(3+) suggest that proton uptake from the solvent limits the rate of the oxidative half-reaction. This step does not severely limit the overall reaction rate as (D2O)V equaled 2 and (D2O)(V/K)(NADPH) was between 0.9 and 1.3 for basal and CaM-stimulated cytochrome c(3+) reduction.

Effects of Ca(2+)-activated calmodulin on neuronal nitric oxide synthase reductase activity and binding of substrates: pH dependence of kinetic parameters

The pH dependence of basal and calmodulin- (CaM-) stimulated neuronal nitric oxide synthase (nNOS) reduction of 2,6-dichloroindophenol (DCIP) and cytochrome c(3+) was investigated. The wave-shaped log V versus pH profile revealed that optimal DCIP reduction occurred when a group, pK(a) of 7.6-7.8, was ionized. The (V/K)(NADPH) and (V/K)(DCIP) versus pH profiles increased with the protonation of a group with a pK(a) of 6.5 or 5.9 and the ionization of two groups with the same pK(a) of 7.5 or 7.0, respectively. (V/K)(DCIP) decreased with the ionization of a group, pK(a) of 9.0. Similar V, (V/K)(NADPH), and (V/K)(DCIP) versus pH profiles for DCIP reduction were obtained with and without CaM, indicating that CaM does not influence ionizable groups involved in catalysis or substrate binding. In contrast, CaM affected the pH dependence of cytochrome c(3+) reduction. The wave-shaped log V versus pH profile for basal cytochrome c(3+) reduction revealed that ionization of a group, pK(a) of 8.6, increased catalysis. Log V for CaM-stimulated cytochrome c(3+) reduction displayed a bell-shaped pH dependence with the protonation of a group with a pK(a) of 6.4 and the ionization of a group with a pK(a) of 9.3, resulting in a loss of activity. The log(V/K)(cytc) versus pH profiles with and without CaM were bell-shaped with the ionization of a group at pK(a) of 7.1 or 7.6 (CaM) or pK(a) of 9.4 or 9.6 (CaM), increasing and decreasing (V/K)(cytc). These results suggest that CaM may change the nature of the rate-limiting catalytic steps or ionizable groups involved in cytochrome c(3+) reduction.

Reaction of neuronal nitric-oxide synthase with 2,6-dichloroindolphenol and cytochrome c3+: influence of the electron acceptor and binding of Ca2+-activated calmodulin on the kinetic mechanism

Binding of Ca(2+)-activated calmodulin (Ca(2+)-CaM) to neuronal nitric-oxide synthase (nNOS) increases the rate of 2,6-dichloroindolphenol (DCIP) reduction 2-3-fold and that of cytochrome c(3+) 10-20-fold. Parallel initial velocity patterns indicated that both substrates were reduced via two-half reactions in a ping-pong mechanism. Product and dead-end inhibition data with DCIP were consistent with an iso ping-pong bi-bi mechanism; however, product and dead-end inhibition studies with cytochrome c(3+) were consistent with the (two-site) ping-pong mechanism previously described for the NADPH-cytochrome P450 reductase-catalyzed reduction of cytochrome c(3+) [Sem, D., and Kasper, C. (1994) Biochemistry 33, 12012--12021]. Dead-end inhibition by 2'-adenosine monophosphate (2'AMP) was competitive versus NADPH for both electron acceptors, although the value of the slope inhibition constant, K(is), was 25-30-fold greater with DCIP as the substrate than with cytochrome c(3+). The difference in the apparent affinity of 2'AMP is proposed to result from a rapidly equilibrating isomerization step that occurs in both mechanisms prior to the binding of NADPH. Thus, initial velocity, product, and dead-end inhibition data were consistent with a di-iso ping-pong bi-bi and an iso (two-site) ping-pong mechanism for the reduction of DCIP and cytochrome c(3+), respectively. The presence Ca(2+)-CaM did not alter the proposed kinetic mechanisms. The activated cofactor had a negligible effect on (k(cat)/K(m))(NADPH), while it increased (k(cat)/K(m))(DCIP) and (k(cat)/K(m))(cytc) 4.5- and 23-fold, respectively.