Inhibition of pyridoxal kinase by methylxanthines

In the presence of saturating concentrations of adenosine triphosphate (ATP) and rate-limiting amounts of pyridoxal, theophylline was found to inhibit sheep brain pyridoxal kinase (EC 2.7.1.35) competitively. The apparent inhibition constant (Ki) of theophylline for pyridoxal kinase was determined as 8.7 mumol/l. Theophylline concentrations of up to 60 mumol/l did not affect pyridoxal phosphorylation in the presence of saturating amounts of pyridoxal and rate-limiting concentrations of ATP. Caffeine was less potent to inhibit pyridoxal kinase (Ki = 45 mumol/l) due to the presence of a methyl group on the 7 position of the xanthine ring structure. Theobromine showed only a weak inhibition of pyridoxal kinase (Ki = 453 mumol/l). The presence of a hydroxyethyl, hydroxypropyl or dihydroxypropyl group on the N7 position of theophylline completely abolished inhibition of pyridoxal kinase. Enprofylline (3-propylxanthine), a recently described bronchodilator, was also able to inhibit pyridoxal kinase with a Ki of 256 mumol/l.     

Human pyridoxal kinase: overexpression and properties of the recombinant enzyme

Pyridoxal kinase catalyses the phosphorylation of the vitamin B6. A human brain pyridoxal kinase cDNA was isolated, and the recombinant enzyme was overexpressed in E. coli as a fusion protein with maltose binding protein (MBP). Pure pyridoxal kinase exhibits a molecular mass of about 40 kDa when examined by SDS-PAGE and FPLC gel filtration. The recombinant enzyme is a monomer endowed with catalytic activity, indicating that the native quaternary structure of pyridoxal kinase is not a prerequisite for catalytic function. Zn2+ is the most effective divalent cation in the phosphorylation of pyridoxal, and the human enzyme has maximum catalytic activity in the narrow pH range of 5.5-6.0. The Km values for two substrates pyridoxal and ATP are 97 microM and 12 microM, respectively. In addition, the unfolding processes of the recombinant enzyme were monitored by circular dichroism. The values of the free energy change of unfolding (AGo = 1.2 kcal x mol(-1) x K(-1)) and the midpoint transition (1 M) suggested that the enzyme is more stable than ovine pyridoxal kinase against denaturation by guanidine hydrochloride. Intrinsic fluorescence spectra of the human enzyme from red-edge excitation and fluorescence quenching experiments showed that the tryptophanyl residues are not completely exposed and more accessible to neutral acrylamide than to the negatively charged iodide. The first complete set of catalytic and structural properties of human pyridoxal kinase provide valuable information for further biochemical studies on this enzyme.     

Proteolytic cleavage of pyridoxal kinase into two structural domains

Chymotryptic digestion of sheep brain pyridoxal kinase, a dimer of identical subunits each of 40 kDa, yields 2 fragments of 24 and 16 kDa with concomitant loss of catalytic activity. These fragments were separated by chromatographic techniques and analyzed for interaction with the ATP analogue, trinitrophenyl-ATP, using fluorescence spectroscopy. The absorption and fluorescence properties of trinitrophenyl-ATP bound to the fragment of 24 kDa (emission maximum, 540 nm, emission anisotropy at 460 nm, 0.30, and fluorescence lifetime, gamma = lns) are indistinguishable from those of the ATP analogue bound to the native enzyme. The fragment of 16 kDa does not bind trinitrophenyl-ATP. The results are consistent with the hypothesis that monomeric pyridoxal kinase is folded into 2 domains connected by a single polypeptide chain sensitive to proteolytic cleavage.     

Crystal structure of brain pyridoxal kinase,a novel member of the ribokinase superfamily

The three-dimensional structures of brain pyridoxal kinase and its complex with the nucleotide ATP have been elucidated in the dimeric form at 2.1 and 2.6 A, respectively. Results have shown that pyridoxal kinase, as an enzyme obeying random sequential kinetics in catalysis, does not possess a lid shape structure common to all kinases in the ribokinase superfamily. This finding has been shown to be in line with the condition that pyridoxal kinase binds substrates with variable sizes of chemical groups at position 4 of vitamin B(6) and its derivatives. In addition, the enzyme contains a 12-residue peptide loop in the active site for the prevention of premature hydrolysis of ATP. Conserved amino acid residues Asp(118) and Tyr(127) in the peptide loop could be moved to a position covering the nucleotide after its binding so that its chance to hydrolyze in the aqueous environment of the active site was reduced. With respect to the evolutionary trend of kinase enzymes, the existence of this loop in pyridoxal kinase could be classified as an independent category in the ribokinase superfamily according to the structural feature found and mechanism followed in catalysis.     

Brain pyridoxal kinase. Purification and characterization

Pyridoxal kinase has been purified 9000-fold from sheep brain. The purification procedure involves ammonium sulphate fractionation, DEAE-cellulose chromatography, affinity chromatography and Sephadex G-100 gel filtration. The final chromatography step yields a homogeneous preparation of high specific activity with a pI of 5. The molecular mass of the native enzyme was estimated to be approximately 80 kDa by 10-25% gradient polyacrylamide gel electrophoresis and Sephadex G-200 gel filtration. The subunit molecular mass was determined by sodium dodecyl sulphate (SDS)/polyacrylamide gel electrophoresis to be 40 kDa compared with a series of molecular mass standards. This indicates that pyridoxal kinase is a dimeric enzyme. Further results obtained from electron microscopy, using a negative staining technique, provide evidence that pyridoxal kinase exists as a dispherical subunit structure.     

Interaction of various nucleotide-dependent enzymes with bifunctional analogs of ATP, derivatives of polymethylene diamines

The interaction of bifunctional ATP derivatives, Appp5'[NH-(CH2) n-NH]ppp5'A (n = 0 or 2-8) with tyrosyl-, valyl-, lysyl-, tryptophanyl-tRNA synthetases and creatine kinase was investigated. ATP derivatives don't inhibit the tRNA aminoacylation catalyzed by tyrosyl-tRNA synthetase. These derivatives behave as mixed-type inhibitors with respect to ATP in the case of valyl- and lysyl-tRNA-synthetases. In the case of the other enzymes all analogs of ATP manifest competitive inhibition towards ATP. The affinity of all ATP derivatives to tryptophanyl-tRNA synthetase does not differ significantly (Ki = 0.2 divided by 0.6 mM). The Ki values for these derivatives in the case of creatine kinase are also very similar with the exception of A5'ppp-NH-(CH2)3-NH-ppp5'A. The Ki value for this derivative is one order of magnitude lower than for other ones. The affinity reagents received by periodate oxidation of bifunctional ATP analogs derivatives of di-, tetra- and heptamethylenediamine modify non-identical subunits of creatine kinase with different velocities, but modification of M- and M'-subunits proceeds independently. An analogues derivative of trimethylenediamine interacts simultaneously with two centers of the dimeric form of kinase forming non-equivalent complexes. The covalent attachment of the reagent to one subunit of creatine kinase does not except the complex formation and covalent binding of bifunctional ATP analogs with the other subunit of the dimer, but results in a one order of magnitude decrease in affinity of the ATP derivative to the nonmodified centre of the enzyme. These data permit to evaluate the distance between ATP binding sites of creatine kinase in its dimeric form as 5-6 A approximately. Such a distance between active sites may be the reason for the higher activity of the M- and M'-creatine kinase subunits taken separately as compared to the enzyme dimeric form.     

Cloning and characterization of Arabidopsis thaliana pyridoxal kinase

Pyridoxal kinase (PK; EC 2.7.1.35), a key enzyme in vitamin B(6) metabolism, was cloned from Arabidopsis thaliana (L.) Heynh. and characterized. The amino acid sequence of the A. thaliana PK was found to be similar to the mammalian enzyme, with a homology of more than 40%. Characterization studies showed that the kinase is a dimeric molecule consisting of two identical subunits, each subunit having a molecular mass of approximately 35 kDa. The enzyme exhibited maximal activity at pH 6.0. Similar to the mammalian enzyme, the enzyme from A. thaliana preferred Zn(2+) instead of the commonly used Mg(2+) as the divalent cation for catalysis. Under optimal conditions, the V(max) of the enzyme was 604 nmol pyridoxal 5'-phosphate (PLP) mg(-1) min(-1), and the K(m) values for pyridoxal and ATP were 688 micro M and 98 micro M, respectively. Examination of levels of enzyme expression showed that leaves, stems, roots and flowers can generate PLP independently at similar levels. Furthermore, expression of the PK gene in A. thaliana seeds was found to start 60 h after imbibition. Results from the present study suggest that plant tissues depend on PK for the production of PLP.     

The crystal structure of an ADP complex of Bacillus subtilis pyridoxal kinase provides evidence for the parallel emergence of enzyme activity during evolution

Pyridoxal kinase catalyses the phosphorylation of pyridoxal, pyridoxine and pyridoxamine to their 5' phosphates and plays an important role in the pyridoxal 5' phosphate salvage pathway. The crystal structure of a dimeric pyridoxal kinase from Bacillus subtilis has been solved in complex with ADP to 2.8 A resolution. Analysis of the structure suggests that binding of the nucleotide induces the ordering of two loops, which operate independently to close a flap on the active site. Comparisons with other ribokinase superfamily members reveal that B. subtilis pyridoxal kinase is more closely related in both sequence and structure to the family of HMPP kinases than to other pyridoxal kinases, suggesting that this structure represents the first for a novel family of "HMPP kinase-like" pyridoxal kinases. Moreover this further suggests that this enzyme activity has evolved independently on multiple occasions from within the ribokinase superfamily.     

Modulation of the catalytic activity of pyridoxal kinase by metallothionein

Pyridoxal kinase displays high catalytic activity in the presence of metallothionein. The apoprotein of metallothionein as well as the peptide LYS-CYS-THR-CYS-CYS-ALA exert a strong inhibitory effect upon pyridoxal kinase by sequestering free Zn ions. Several steps intervene in the process of pyridoxal kinase activation, i.e. binding of Zn ions by ATP and interaction of Zn-ATP with the enzyme; but direct interaction between metallothionein and pyridoxal kinase (protein association) could not be detected by emission anisotropy measurements. Since the concentration of free Zn++ in mammalian tissues is lower than 10(-9)M, it is postulated that the concentration of metallothionein regulates the catalytic activity of pyridoxal kinase. The mechanism of reconstitution of the metalloenzyme yeast aldolase in the presence of metallothionein was also investigated.     

Brain pyridoxal kinase. Mechanism of substrate addition, binding of ATP, and rotational mobility of the inhibitor pyridoxaloxime

The inhibition kinetic patterns obtained when ATP and pyridoxal analogues are used as inhibitors of the reaction catalyzed by pyridoxal kinase are consistent with a rapid equilibrium random Bi Bi, in which binary complexes, i.e. enzyme . ATP and enzyme . pyridoxal, are formed in kinetically significant amounts. Protein fluorescence quenching was used to determine the dissociation constant (Kd = 25 microM) of ATP . Zn bound to the nucleotide site of the kinase. The binding of ATP to the kinase induces a conformational change which is transmitted to other areas of the macromolecule. Pyridoxaloxime, a competitive inhibitor of pyridoxal, was used as a probe of the pyridoxal-binding site. It binds to the kinase with Ki = 2 microM and displays a fluorescent decay time of 7.8 ns. Time emission anisotropy measurements yield a rotational correlation time for bound pyridoxaloxime of approximately 2 ns, which is considerably shorter than the rotational correlation time of the protein (phi = 38 ns). The fast rotation of pyridoxaloxime remains unaffected by the binding of ATP.     

Crystal Structure of human pyridoxal kinase: structural basis of M(+) and M(2+) activation

Pyridoxal kinase catalyzes the transfer of a phosphate group from ATP to the 5' alcohol of pyridoxine, pyridoxamine, and pyridoxal. In this work, kinetic studies were conducted to examine monovalent cation dependence of human pyridoxal kinase kinetic parameters. The results show that hPLK affinity for ATP and PL is increased manyfold in the presence of K(+) when compared to Na(+); however, the maximal activity of the Na(+) form of the enzyme is more than double the activity in the presence of K(+). Other monovalent cations, Li(+), Cs(+), and Rb(+) do not show significant activity. We have determined the crystal structure of hPLK in the unliganded form, and in complex with MgATP to 2.0 and 2.2 A resolution, respectively. Overall, the two structures show similar open conformation, and likely represent the catalytically idle state. The crystal structure of the MgATP complex also reveals Mg(2+) and Na(+) acting in tandem to anchor the ATP at the active site. Interestingly, the active site of hPLK acts as a sink to bind several molecules of MPD. The features of monovalent and divalent metal cation binding, active site structure, and vitamin B6 specificity are discussed in terms of the kinetic and structural studies, and are compared with those of the sheep and Escherichia coli enzymes.     

Activation of pyridoxal kinase by metallothionein

Brain pyridoxal kinase, which uses ATP complexed to either Zn(II) or Co(II) as substrates, displays high catalytic activity in the presence of Zn-thionein and Co-thionein. Several steps intervene in the process of pyridoxal kinase activation, i.e., binding of Zn ions to ATP and interaction between Zn-ATP and the enzyme. Equilibrium binding studies show that ATP mediates the release of Zn ions from the metal-thiolate clusters of the thioneins, whereas spectroscopic measurements conducted on Co-thionein reveal that the absorption transitions corresponding to the metal-thiolate of the protein are perturbed by ATP. The binding Zn-ATP to the kinase proceeds with a delta G = -6.3 kcal/mol as demonstrated by fluorometric titrations. Direct interaction between the kinase and derivatized-metallothionein could not be detected by emission anisotropy measurements, indicating that juxtaposition of the proteins does not influence the exchange of metal ions. Since the concentration of free Zn in several mammalian tissues is lower than 1 nM, it is postulated that under in vivo conditions the concentration of metallothionein regulates the catalytic activity of pyridoxal kinase.     

Preparation of functionally intact monomers by limited disulfide reduction of human plasma fibronectin dimers

Most (90 to 95%) human plasma fibronectin (PFn) molecules exist as 450-kDa disulfide-rich dimers comprised of two major types of subunits (A, 220 kDa; B, 215 kDa) that are joined near the COOH terminus by two disulfide bonds. Smaller PFn species (Zone II; 190-235 kDa) consist mainly of monomers and/or a monomeric subunit joined covalently to a smaller peptide remnant presumably derived by proteolysis of a parent 450-kDa molecule. A relatively simple and selective method for preparing functionally active, partially reduced monomeric fibronectin subunits (PR-PFn) by limited and selective reduction of dimeric plasma fibronectin (PFn) has been developed. PR-PFn was prepared by incubating PFn in phosphate-buffered saline, pH 7.4, for 2 h at room temperature in the presence of 17 mM dithiothreitol (DTT). Following S-carboxymethylation or S-carboxyamidomethylation, the material was passed through a gelatin-Sepharose column and nonbinding material was discarded; gelatin-bound material was eluted using a 0 to 2 M KSCN gradient. Residual dimeric species (10-20%) could be separated from monomers in high yield by gel-sieving chromatography on a Sepharose 6B-Cl in the presence of a chaotropic salt, 0.3 M KSCN. Most new SH groups (74-81%) in that fraction of PR-PFn binding to gelatin were localized in proteolytic fragments containing the COOH terminus, thus suggesting that selective cleavage of the interchain disulfide bridges had taken place. The binding affinity of PR-PFn to gelatin- and fibrin-Sepharose was lower than that of dimeric PFn, but the same as that of Zone II PFn and other monomeric gelatin-binding proteolytic derivatives. PR-PFn also bound to heparin-Sepharose and promoted cell attachment and spreading. We conclude that PR-PFn monomers possess the same functional activities as those of the parent chains.     

Phosphagen kinase evolution. Expression in echinoderms

Arginine kinase and creatine kinase that catalyze the transfer of a phosphate group between ATP and arginine and creatine, respectively, play an important role in cellular energetics. In contrast to most animals which exhibit a single phosphagen kinase activity (creatine kinase in chordates and arginine kinase in protostomians), echinoderms exhibit both arginine kinase and creatine kinase activities, sometimes in the same tissue. In contrast to chordates in which creatine kinases are dimers (consisting of two subunits of 40 kDa) and protostomians in which arginine kinases are usually monomers (40 kDa), echinoids contain specific phosphagen kinases: a dimeric arginine kinase (consisting of two subunits of 42 kDa) in eggs and a monomeric creatine kinase (145 kDa) in sperm. We have examined echinoderms from the five existing classes (echinoids, asteroids, ophiuroids, holothurians and crinoids) for the expression of these specific phosphagen kinases in different tissues. Gel filtration was used to determine the molecular masses of the native enzymes. Antibodies specific for arginine kinase or for creatine kinase were used to characterize the subunit composition of arginine kinase and creatine kinase after SDS/PAGE and transfer. In all echinoderms analyzed, arginine kinase always occurred as an enzyme of about 81 kDa consisting of two subunits of 42 kDa and creatine kinase as a monomeric enzyme of 140-155 kDa. The occurrence in echinoderms of both phosphagen kinases with distinct specificities and specific molecular structures is discussed from both a developmental and evolutionary point of view.     

Purification and characterization of cAMP dependent protein kinase from Microsporum gypseum

A cyclic AMP dependent protein kinase (PKA), its regulatory (R) and catalytic (C) subunits were purified to homogeneity from soluble extract of Microsporum gypseum. Purified enzyme showed a final specific activity of 277.9 nmol phosphate transferred min(-1) mg protein(-1) with kemptide as substrate. The enzyme preparation showed two bands with molecular masses of 76 kDa and 45 kDa on sodium dodecyl polyacrylamide gel electrophoresis. The 76 kDa subunit was found to be the regulatory (R) subunit of PKA holoenzyme as determined by its immunoreactivity and the isoelectric point of this subunit was 3.98. The 45 kDa subunit was found to be the catalytic (C) subunit by its immunoreactivity and phosphotransferase activity. Gel filtration using Sepharose CL-6B revealed the molecular mass of PKA holoenzyme to be 240 kDa, compatible with its tetrameric structure, consisting of two regulatory subunits (76 kDa) and two catalytic subunits (45 kDa). The specificity of enzyme towards protein acceptors in decreasing order of phosphorylation was found to be kemptide, casein, syntide and histone IIs. Purified enzyme had apparent K(m) values of 71 microM and 25 microM for ATP and kemptide, respectively. Phosphorylation was strongly inhibited by mammalian PKA inhibitor (PKI) but not by inhibitors of other protein kinases. The PKA showed maximum activity at pH 7.0 and enzyme activity was inhibited in the presence of N-ethylmaleimide (NEM) which shows the involvement of sulfhydryl groups for the activity of PKA. PKA phosphorylated a number of endogenous proteins suggesting the multifunctional role of cAMP dependent protein kinase in M. gypseum. Further work is under progress to identify the natural substrates of this enzyme through which it may regulate the enzymes involved in phospholipid metabolism.     

DNA Polymerase III holoenzyme of Escherichia coli. IV. The holoenzyme is an asymmetric dimer with twin active sites

Pol III, a subassembly of Escherichia coli DNA polymerase III holoenzyme lacking only the auxiliary beta subunit, was purified to homogeneity by an improved procedure. This assembly consists of nine different polypeptides, likely in a 1:1 stoichiometry: a catalytic core (pol III) of alpha (132 kDa), epsilon (27 kDa), and theta (10 kDa), and six auxiliary subunits: tau (71 kDa), gamma (52 kDa), delta (35 kDa), delta' (33 kDa), chi (15 kDa), and psi (12 kDa). The assembly behaves on gel filtration as a particle of about 800 kDa, indicating a content of two each of the subunits. A new procedure for purifying the core yielded a novel dimeric form which may provide the foundation for the dimeric nature of the more complex pol III and holoenzyme forms. Pol III readily dissociates into several subassemblies including pol III', likely a dimeric core with two tau subunits. The holoenzyme, purified by a similar procedure with ATP and Mg2+ present throughout, retained the beta subunit (37 kDa) as well as all the subunits present in pol III; the mass of the holoenzyme was estimated to be 900 kDa. The isolated initiation complex of holoenzyme with a primed template DNA and the elongation complex (formed in the presence of three deoxynucleoside triphosphates) had the same composition and stoichiometry as observed for pol III with two beta dimers in addition. An initiation complex assembled from a mixture of monomeric pol III core, gamma 2 delta delta' chi psi complex (gamma complex), beta, and tau retained the core, one beta dimer, and two tau subunits but was deficient in the gamma complex. When tau was omitted from the assembly mixture, the initiation complex contained one or two gamma complexes instead of the tau subunit. Based on these data, pol III holoenzyme is judged to be an asymmetric dimeric particle with twin pol III core active sites and two different sets of auxiliary units designed to achieve essentially concurrent replication of both leading and lagging strand templates.     

Binding of a photoaffinity analogue of pyridoxal to pyridoxal kinase

The binding of pyridoxal analogues to the structural domains of pyridoxal kinase was studied by fluorescence spectroscopy and chromatographic techniques. Two fragments of 24 and 16 kDa, arising from limited proteolysis of the native enzyme, were separated by ion-exchange chromatography and used for binding studies with pyridoxal oxime. Fluorometric titrations yielded dissociation constants of 6 and 12.4 MicroM for pyridoxal oxime bound to the native enzyme and 24-kDa fragment, respectively. 4-(4-Azido-2-nitrophenyl)-pyridoxamine, a new photolabeling reagent, binds irreversibly to the kinase with concomitant loss of catalytic activity. The modified kinase (2.1 mol label/mol dimer) yields two fragments upon limited proteolysis with chymotrypsin. The two fragments were separated by reverse-phase HPLC and SDS/polyacrylamide gel electrophoresis. Radiolabeled ligand was detected only in the 24-kDa fragment. It is postulated that the pyridoxal binding site is located in the 24-kDa structural domain.     

Brain pyridoxal kinase. Mobility of the substrate pyridoxal and binding of inhibitors to the nucleotide site.

Pyridoxal kinase has been purified 2000-fold from pig brain. The enzyme preparation migrates as a single protein and activity band on analytical gel electrophoresis. The interactions of the substrate pyridoxal and the inhibitor N-dansyl-2-oxopyrrolidine (dansyl = 5-dimethylaminonaphthalene-1-sulfonyl) with the catalytic site were examined by means of fluorescence spectroscopy. The increase in emission anisotropy that follows the binding of pyridoxal to the kinase was used to determine the equilibrium dissociation constant. Pyridoxal kinase binds one molecule of substrate with a Kd = 11 microns at pH 6. The emission anisotropy spectrum of bound pyridoxal reveals that the substrate is not rigidly trapped by the protein matrix. N-Dansyl-2-oxopyrrolidine is a competitive inhibitor with respect to ATP at saturating concentrations of pyridoxal. It binds to the enzyme with a dissociation constant of 6 microns. N-Dansyl-2-oxopyrrolidine is immobilized by strong interactions with the enzyme, but it is displaced from the catalytic site by ATP. The results are consistent with the hypothesis that N-dansyl-2-oxopyrrolidine binds at the nucleotide binding site of pyridoxal kinase.     

Mitochondrial aspartate aminotransferase-independent function of the catalytic binding sites.

The enzyme mitochondrial aspartate aminotransferase from beef liver is a dimer of identical subunits. The enzymatic activity of the resolved enzyme is restored upon addition of the cofactor pyridoxal 5-phosphate. The binding of 1 molecule of cofactor restores 50% of the original enzymatic activity, whereas the binding of a 2nd molecule of cofactor brings about more than 95% recovery of the catalytic activity. Following addition of 1 mol of pyridoxal-5-P per dimer, three forms of the enzyme may exist in solution: apoenzyme-2 pyridoxal 5'-phosphate, apoenzyme-1 pyridoxal 5'-phosphate, and apoenzyme. The enzyme species are separated by affinity chromatography and the following distribution was found: apoenzyme-2 pyridoxal 5'-phosphate/apoenzyme-1 pytidoxal 5'-phosphate/apoenzyme, 2/6/2. Similar distribution was observed after reduction with NaBH4 of the mixture containing apoenzyme and pyridoxal-5-P at a mixing ratio of 1:1. Fluorometric titrations conducted on samples of apoenzyme and apoenzyme-1 pyridoxal 5'-phosphate reveal that the enzyme species display identical affinity towards the inhibitor 4-pyridoxic-5-P (KD equals 1.1 times 10- minus 6 M). It is concluded that the binding of the cofactor to one of the catalytic sites does not affect the affinity of the second site for the inhibitor. These results, obtained by two independent methods, lend strong support to the hypothesis that the two subunits of the enzyme function independently.     

Stoichiometric arginine binding in the oxygenase domain of inducible nitric oxide synthase requires a single molecule of tetrahydrobiopterin per dimer

In addition to its catalytic roles, the nitric oxide synthase (NOS) cofactor tetrahydrobiopterin (H4B) is required for substrate binding and for stabilization of the dimeric structure. We expressed and purified the core of the iNOS oxygenase domain consisting of residues 75-500 (CODiNOS) in the presence (H4B+) and absence (H4B-) of this cofactor. Both forms bound stoichiometric amounts of heme (>0.9 heme per protein subunit). H4B- CODiNOS was unable to bind arginine, gave an unstable ferrous carbonyl adduct, and was a mixture of monomer and dimer. H4B+ CODiNOS bound arginine, gave a stable ferrous carbonyl adduct, and was exclusively dimeric. The H4B cofactor content of this species was only one per dimer yet this was sufficient to form two competent arginine binding sites as determined by optical stoichiometric titrations.