Spectroscopic observation of the intramolecular electron transfer in the photoactivation processes of nitrile hydratase

Electronic structure, bonding, spectroscopy and energetics of Fe-dependent nitrile hydratase active-site models

Fe-type nitrile hydratase (NHase) is a non-heme Fe(III)-dependent enzyme that catalyzes the hydration of nitriles to the corresponding amides. Despite experimental studies of the enzyme and model Fe(III)-containing complexes, many questions concerning the electronic structure and spectroscopic transitions of the metal center remain unanswered. In addition, the catalytic mechanism of nitrile hydration has not yet been determined. We now report density functional theory (B3LYP/6-31G) calculations on three models of the Fe(III) center in the active site of NHase corresponding to hypothetical intermediates in the enzyme-catalyzed hydration of acetonitrile. Together with natural bond orbital (NBO) analysis of the chemical bonding in these active-site models and INDO/S CIS calculations of their electronic spectra, this theoretical investigation gives new insight into the molecular origin of the unusual low-spin preference and spectroscopic properties of the Fe(III) center. In addition, the low-energy electronic transition observed for the active form of NHase is assigned to a dd transition that is coupled with charge-transfer transitions involving the metal and its sulfur ligands. Calculations of isodesmic ligand-exchange reaction energies provide support for coordination of the Fe(III) center in free NHase by a water molecule rather than a hydroxide ion and suggest that the activation of the nitrile substrate by binding to the metal in the sixth coordination site during catalytic turnover cannot yet be definitively ruled out.

Theoretical investigations of the electronic structure and spectroscopy of mononuclear, non-heme [Fe-NO](6) complexes

The unusual metal coordination and spin-state of the Fe(III) center in nitrile hydratase (NHase) has stimulated the synthesis of numerous model complexes in efforts to understand the reactivity and spectroscopic properties of the enzyme. A particular problem has been the development of model Fe(III) complexes that exhibit reversible, photolabile binding to nitric oxide (NO) in a manner similar to that observed for the NHase metal center. We now report a detailed NBO analysis of the ground-state chemical bonding in three [Fe-NO](6) complexes that exhibit different responses to irradiation, together with investigations of their spectroscopic properties using semiempirical INDO/S CI singles calculations. Our computational studies reveal a correlation between the photolability of these complexes and the existence of low-energy transitions that promote an electron into the Fe-NO pi(*) antibonding molecular orbital. In addition to providing detailed insights into how the ligand field influences the spectroscopy of these mononuclear complexes, these studies strengthen our previous conclusions regarding the role of post-translational cysteine modification in modulating the photoreactivity of the inactive, NO complex of NHase.

Fe-type nitrile hydratase

The characteristic features of Fe-type nitrile hydratase (NHase) from Rhodococcus sp. N-771 are described. Through the biochemical analyses, we have found that nitric oxide (NO) regulates the photoreactivity of this enzyme by association with the non-heme iron center and photoinduced dissociation from it. The regulation is realized by a unique structure of the catalytic non-heme iron center composed of post-translationally modified cysteine-sulfinic (Cys-SO2H) and -sulfenic acids (Cys-SOH). To understand the biogenic mechanism and the functional role of these modifications, we constructed an over-expression system of whole NHase and individual subunits in Escherichia coli. The results of the studies on several recombinant NHases have shown that the Cys-SO2H oxidation of alphaC112 is indispensable for the catalytic activity of Fe-type NHase.

Arginine 56 mutation in the beta subunit of nitrile hydratase: importance of hydrogen bonding to the non-heme iron center

Arginine 56 in the beta subunit (betaArg56) of the iron-containing nitrile hydratase (NHase), one of the strongly conserved residues within the NHase family, is known to form hydrogen bonds to the sulfinyl (-SO2H) and sulfenyl (-SOH) groups of the post-translationally modified cysteine residues in the catalytic center. BetaArg56 was substituted by tyrosine, glutamate or lysine, respectively, and the respective mutant enzymes generated by reconstitution were characterized. The betaR56K mutant complex exhibited about 1% of the enzymatic activity of native NHase, while the others were totally inactive. The kinetic analysis of the betaR56K mutant complex exhibited a drastic decrease in turnover number and decreases in kinetic constants for substrate and inhibitors as compared to the native NHase. Changes in UV-visible absorption and light-induced Fourier transform infrared difference spectra suggest that betaArg56 is involved in the positioning of the -SO2H and -SOH groups of the modified Cys residues in the catalytic center so as to fine tune the electronic state of the iron center suitable for catalysis. Thus, betaArg56 is essential for catalysis.

An enzyme controlled by light: the molecular mechanism of photoreactivity in nitrile hydratase

Extensive studies have revealed the molecular mechanism of the photoreactivity of nitrile hydratase from Rhodococcus sp. N-771. In the inactive enzyme, nitric oxide is bound to the non-heme ferric iron at the catalytic center, stabilized by a claw-like structure formed by two post-translationally modified cysteines and a serine. The inactive nitrile hydratase is activated by the photoinduced release of the nitric oxide. This result might provide a means of designing novel photoreactive chemical compounds or proteins that would be applicable to biochips and light-controlled metabolic systems.

Metalloenzyme nitrile hydratase: structure, regulation, and application to biotechnology

Nitrile hydratase (NHase), which catalyzes the hydration of nitriles to amides, has been used in the industrial production of acrylamide and nicotinamide. Recent studies on NHases, which are roughly classified into iron and cobalt types according to the metal involved, have clarified the photoactivation mechanism, the novel ligand structure of the metal-binding sites, the unique mechanism of the enzyme hyper-induction, and the occurrence of an accessory gene involved in cobalt-containing NHase formation. These detailed analyses have led to the development of biotechnological applications of NHase, including biotransformation and bioremediation.

Highly efficient control of iron-containing nitrile hydratases by stoichiometric amounts of nitric oxide and light

The reaction of two iron-containing nitrile hydratases (NHase) with NO has been studied: NHase from Rhodococcus sp. R312, which is probably similar to the photosensitive N771 NHase, and the new NHase from Comamonas testosteroni NI1 whose aminoacid sequence is quite different from those of BR312 and N771 NHases. Both enzymes are equally inactivated after addition of stoichiometric amounts of NO added as an anaerobic solution or produced in situ under physiological conditions by a rat brain NO-synthase. Both enzymes are reactivated by photoirradiation, and two cycles of NO inactivation/photoactivation can be performed without significant loss of activity. Both iron-containing NHases have a high affinity for NO, similar to that of methemoglobin.

Screening of novel microbial enzymes for the production of biologically and chemically useful compounds

Enzymes have been generally accepted as superior catalysts in organic synthesis. Micro-organisms in particular have been regarded as treasure sources of useful enzymes. The synthetic technology using microbial enzymes or micro-organisms themselves is called microbial transformation. In designing a microbial transformation process, one of the most important points is to find a suitable enzyme for the reaction of interest. Various kinds of novel enzymes for specific transformations have been discovered in micro-organisms and their potential characteristics revealed. This article reviews our current results on the discovery of novel enzymes for the production of biologically and chemically useful compounds, and emphasizes the importance of screening enzymes in a diverse microbial world.

Key role of alkanoic acids on the spectral properties, activity, and active-site stability of iron-containing nitrile hydratase from Brevibacterium R312

Interaction of n-butyric acid with dialyzed nitrile hydratase from Brevibacterium R312, which is characterized by a charge-transfer band at 680 nm and EPR signals typical of a low-spin Fe(III) with delta g = 0.22, leads to a form displaying different spectral properties (lambda = 710 nm, delta g = 0.31). Butyric acid also acts as a competitive inhibitor of nitrile-hydratase-catalyzed hydration of acrylonitrile with a Ki value of 0.9 mM. Formation of the complex between the enzyme and butyric acid is highly dependent on the concentration of the latter and on pH. When stored with high levels of butyric acid, nitrile hydratase is completely inactive. The active uncomplexed enzyme is restored under the high dilution conditions used for the enzymatic assays, while the complexed form is favored at acidic pH and is not formed at pH above 8. Furthermore, the inhibitory potency of butyric acid decreases upon increasing pH (IC50 increases from 0.8 mM at pH 6.2 to 12 mM at pH 8.2). These data show that nitrile hydratase interacts with the acid form of butyric acid with a high affinity (Ki' approximately 4 microM at pH 7.2). At pH < 3, the visible spectrum of the enzyme disappears, presumably because of demetallation, whereas that of the complex exhibits a charge-transfer band shifted to 800 nm, the presence of butyric acid preventing nitrile hydratase from demetallation. Other linear carboxylic acids such as valeric and hexanoic acids behave similarly; they act as inhibitors of nitrile hydratase and protect the enzyme during storage. A structure of the nitrile hydratase active site interacting with butyric acid is tentatively proposed in which the latter is hydrogen-bonded to the Fe(III)-OH moiety. This interaction between butyric acid and nitrile hydratase should be considered when deducing the nature of nitrile hydratase active site and mechanisms, from spectral and enzymatic data, since most results published previously have been obtained on nitrile hydratase containing large amounts of butyric acid and interpreted without taking into account the presence of this acid in the active site.

Photosensitive nitrile hydratase intrinsically possesses nitric oxide bound to the non-heme iron center: evidence by Fourier transform infrared spectroscopy

Nitrile hydratase (NHase) from Rhodococcus sp. N-771 is a photosensitive enzyme that catalyzes hydration of nitriles to the corresponding amides. Light-induced Fourier transform infrared difference spectra between the inactive and active forms of NHase were measured with both the natural (14N) and 15N-labeled NHases. The results showed, for the first time, that NHase intrinsically possesses nitric oxide (NO) molecules bound to the non-heme iron center. The possible role of NO in the photoactivation process of NHase is discussed.