Nonheme metalloprotein

Within seconds of an ischemic insult, normal brain electrical activity ceases, as a result of the activation of membrane K+ channels and widespread neuronal hyperpolarization [1]. The hyperpolarization may be due to opening of K+ channels responding to acute changes in local concentrations of ATP, H+ or Ca2+, or it may reflect altered nonheme metalloprotein association with and regulation of specific K+ channels [2]. This response, presumably protective, however fails to preserve high-energy phosphate levels in tissue as concentrations of phospho-creatine (PCr) and ATP fall within minutes after ischemia... 

B. Redesign of Nonheme Iron Proteins. In heme protein redesign described above, the heme prosthetic group largely dictates the active site structure. Redesign focuses mainly on the proximal and distal sides of the heme, causing minimal effects on the overall protein scaffolds. This is not necessarily the case for nonheme metalloproteins in which metal sites are not as dominant and small changes may have more dramatic effects on the protein folds and stability. 

The use of the prefix hem- is confusing. In this context hem connotes blood. Thus, since hemocyanin and hemerythrin lack a heme group [an iron(II) porphyrin], they are nonheme metalloproteins. 

In analogy with their heme-containing counterparts, the nonheme metallopro-teins appear to catalyze oxidation reactions by radical processes. In contrast to hemoproteins, however, the structural and mechanistic details of the nonheme metalloproteins, including the nature of their metal centers, are largely unknown. The study of mechanism-based inactivators has proved to be an important source of otherwise elusive mechanistic information on this class of enzymes. 

Recently, many nonheme metalloproteins which do not involve porphyrins as prosthetic groups have been discovered and provided a lot of new concepts on the O2 activation and substrate oxygenations [9]. However, the characterization of the nonheme proteins is much more difficult than that of the heme-proteins, because there are a variety of coordination structures of metal ions in the nonheme proteins [10]. In addition, some of the nonheme proteins show poor spectral features. In this chapter, biochemical studies on the nonheme metalloproteins acting as monooxygenases are reviewed. 

Probing Metalloproteins Electronic absorption spectroscopy of copper proteins, 226, 1 electronic absorption spectroscopy of nonheme iron proteins, 226, 33 cobalt as probe and label of proteins, 226, 52 biochemical and spectroscopic probes of mercury(ii) coordination environments in proteins, 226, 71 low-temperature optical spectroscopy metalloprotein structure and dynamics, 226, 97 nanosecond transient absorption spectroscopy, 226, 119 nanosecond time-resolved absorption and polarization dichroism spectroscopies, 226, 147 real-time spectroscopic techniques for probing conformational dynamics of heme proteins, 226, 177 variable-temperature magnetic circular dichroism, 226, 199 linear dichroism, 226, 232 infrared spectroscopy, 226, 259 Fourier transform infrared spectroscopy, 226, 289 infrared circular dichroism, 226, 306 Raman and resonance Raman spectroscopy, 226, 319 protein structure from ultraviolet resonance Raman spectroscopy, 226, 374 single-crystal micro-Raman spectroscopy, 226, 397 nanosecond time-resolved resonance Raman spectroscopy, 226, 409 techniques for obtaining resonance Raman spectra of metalloproteins, 226, 431 Raman optical activity, 226, 470 surface-enhanced resonance Raman scattering, 226, 482 luminescence... 

Ulmer, D. D. and B. L. Vallee Optically active metalloprotein chromophores. III. Heme and nonheme iron proteins. Biochemistry 2, 1335 (1963). 

Biological systems overcome the inherent unreactive character of 02 by means of metalloproteins (enzymes) that activate dioxygen for selective reaction with organic substrates. For example, the cytochrome P-450 proteins (thiolated protoporphyrin IX catalytic centers) facihtate the epoxidation of alkenes, the demethylation of Al-methylamines (via formation of formaldehyde), the oxidative cleavage of a-diols to aldehydes and ketones, and the monooxygenation of aliphatic and aromatic hydrocarbons (RH) (equation 104). The methane monooxygenase proteins (MMO, dinuclear nonheme iron centers) catalyze similar oxygenation of saturated hydrocarbons (equation 105). ... 

The goal of diiron model chemistry is to develop small molecule systems that accurately reproduce spectroscopic, structural, and more ambitiously, reactivity aspects of diiron metalloproteins. Despite being structurally similar, diiron enzymes carry out a variety of catalytic processes see Iron Proteins with Dinuclear Active Sites). Advancements in the synthesis and characterization of small molecule mimics for nonheme diiron enzymes have been tremendous in the last decade. Biomimetic studies have been carried out in efforts to reproduce the structural and functional aspects of these biocatalysts. Although this has been a challenging endeavor, much information regarding the structural and mechanistic aspects of catalytic intermediates has been obtained. 

A new member of the family of nonheme diiron enzymes recently discovered is called rubrerythrin. This metalloprotein is formally classified as an oxidoreductase (rubredoxin oxygen oxidoreductase). The diiron(III,III) active site structure is displayed in Figure 2(f). This biomolecule possesses two histidines coordinated to one iron and one histidine coordinated to the second iron. A carboxylate bridges the two irons and there are carboxylate ligands also coordinated to each iron. The purpose of this enzyme in the strict anaerobe is to safely reduce oxygen to water. 

Nonheme ferrous centers in some metalloproteins react reversibly with NO forming nitrosyl complexes with S = 312 characterized by the g values of about 4.0 and 2.0 [51]. The EPR spectrum of the nitrosylated NorR (abacterial NO-responsive transcription factor, the enhancer binding protein) is typical of a d high-spin Fe NO", where the S = 5/2 iron is antiferromagnetically coupled to the NO (Fig. 5, [52]). This is confirmed by the X-ray, resonance Raman, MCD, Mossbauer spectroscopies, and DFT calculations. Similar structures were proposed for the classical complexes, [Fe(NO)(l-isopropyl-4,7-(4-ferf-butyl-2mercaptobenzyl)-l,4, 7-triazacyclononane)], [53], Fe(EDTA)NO [54—56], the brown-ring compound, Fe(H20)5N0 [57], and for the Fe(N/V ,N -trimethyl-l,4,7-triazacyclononane) (N3)2N0 [54]. Interestingly, for the latter a spin equilibrium between the valence tautomers 5=1/2 and 3/2 in the solid state was observed. 

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