Heme proteins model complexes

These minimalistic peptide scaffolds potentially provide a biologically relevant laboratory in which to explore the details of heme-peptide interactions and, with development, perhaps approach the observed range of natural heme protein fimction. These heme-peptide systems are more complex than typical small molecule bioinorganic porphyrin model compoimds, and yet are seemingly not as enigmatic as even the smallest natural heme proteins. Thus, in the continuum of heme protein model complexes these heme-peptide systems lie closer to, but certainly not at, the small molecule limit which allows for the effects of single amino acid changes to be directly elucidated. 

Because MCD signals can be either positive or negative in sign, considerably more fine structure is seen in MCD spectra than in the corresponding electronic absorption spectra. Furthermore, MCD is a property of the molecular electronic structure of a chromophore, and so the only structural changes that will influence the MCD spectrum are those that modify the electronic structure. Furthermore, the MCD spectrum is relatively insensitive to the environment in which the chromophore is located, whether it is the protein microenvironment for a heme protein center or the solvent for a model complex. Thus, comparisons of the MCD spectra of synthetic heme iron model complexes with those of heme protein active sites are possible and have been shown to be of considerable utility in assigning the coordination structures of the heme protein active sites (I). 

The four-coordinate sqnare planar iron(n) porphyrins discussed above are not only of great valne in heme protein model chemistry, but also in chemical applications, since they undergo a wealth of ligand addition reactions. For example it has been shown that TPPFe complexes are active catalysts for important carbon transfer reactions in organic chemistry and are found to catalyze the stereoselective cyclopropanation of aUcenes, olefin formation from diazoalkanes, and the efficient and selective olefination of aldehydes and other carbonyl compounds. The active species in these carbon transfer reactions are presumably iron porphyrin carbene complexes. " It was also found that ferrous hemin anchored to Ti02 thin films reduce organic halides, which can pose serious health problems and are of considerable environmental concern because of their prevalence in groundwater. ... 

These enzymes are homodimeric periplasmic proteins (encoded by nirB) that contain two different heme groups per monomer. One is a c-type heme bound via a Cys-X-X-Cys-His motif and has a role in electron transfer the other, a noncovalently linked d heme, is an unusual ferric-dioxoisobacteriochlorin, restricted to this class of enzyme and forms the active site. The biosynthetic pathway of heme di is unique among the tetrapyrroles in having 0x0, methyl, and acrylate side chains, involving some seven nir genes. The Em values of heme d model complexes are 200mV lower than iron porphyrins. 

The kinetics of reactions of NO with ferri- and ferro-heme proteins and models under ambient conditions have been studied by time-resolved spectroscopic techniques. Representative results are summarized in Table I (22-28). Equilibrium constants determined for the formation of nitrosyl complexes of met-myoglobin (metMb), ferri-cytochrome-c (Cyt111) and catalase (Cat) are in reasonable agreement when measured both by flash photolysis techniques (K= konlkQff) and by spectroscopic titration in aqueous media (22). Table I summarizes the several orders of magnitude range of kon and kQs values obtained for ferri- and ferro-heme proteins. Many k0f[ values were too small to determine by flash photolysis methods and were determined by other means. The small values of kQ result in very large equilibrium constants K for the... 

The low reactivity of both Cyt111 and Cyt11 toward NO can be attributed to occupation of the heme iron axial coordination sites by an imidazole nitrogen and by a methionine sulfur of the protein (28). Thus, unlike other heme proteins where one axial site is empty or occupied by H20, formation of the nitrosyl complex not only involves ligand displacement but also significant protein conformational changes which inhibit the reaction with NO. However, the protein does not always inhibit reactivity given that Cat and nNOS are more reactive toward NO than is the model complex Fem(TPPS)(H20)2 (Table II). Conversely, the koS values... 

Earlier kinetics studies (20c) of ferro-heme proteins and model compounds have led to the proposal of a mechanism in which an encounter complex, Fen(Por) L, is formed prior to ligand bond formation according to Eq. (16). 

Flash photolysis techniques were unsuitable for measuring the slow off reactions for the iron(II) model complexes such as Fen(TPPS)(NO), since the experimental uncertainties in the extrapolated intercepts of kohs vs. [NO] plots were larger than the values of the intercepts themselves. When trapping methods were used to evaluate NO labilization from FeII(TPPS)(NO), k(,n values were found to be quite small but were sensitive to the nature of the trapping agents used. Lewis bases that could coordinate the metal center appeared to accelerate NO loss. More reliable estimates for the uncatalyzed off reaction were obtained by using Ru(edta)- as an NO scavenger, and the koS values listed in Table I were obtained in this manner (21c). The small kQ values found for Fe(II) models are consistent with the trend observed for the ferro-heme proteins discussed above. 

It is quite evident that the ferrous complexes of porphyrins, both natural and synthetic, have extremely high affinities towards NO. A series of iron (II) porphyrin nitrosyls have been synthesized and their structural data [11, 27] revealed non-axial symmetry and the bent form of the Fe-N=0 moiety [112-116]. It has been found that the structure of the Fe-N-O unit in model porphyrin complexes is different from those observed in heme proteins [117]. The heme prosthetic group is chemically very similar, hence the conformational diversity was thought to arise from the steric and electronic interaction of NO with the protein residue. In order to resolve this issue femtosecond infrared polarization spectroscopy was used [118]. The results also provided evidence for the first time that a significant fraction (35%) of NO recombines with the heme-Fe(II) within the first 5 ps after the photolysis, making myoglobin an efficient N O scavenger. 

Simonneaux G, Le Maux P (2006) Carbene Complexes of Heme Proteins and Iron Porphyrin Models. 17 83-122... 

While these complex model heme proteins have a large potential for functionalization, an interesting approach that is very different has been taken by other workers in that the heme itself functions as the template in the formation of folded peptides. In these models peptide-peptide interactions are minimized and the driving force for folding appears to be the interactions between porphyrin and the hydrophobic faces of the amphiphiUc peptides. The amino acid sequences are too small to permit peptide-peptide contacts as they are separated by the tetrapyrrole residue. These peptide heme conjugates show well-re-solved NMR spectra and thus well-defined folds and the relationship between structure and function can probably be determined in great detail when functions have been demonstrated [22,23,77]. They are therefore important model systems that complement the more complex proteins described above. 

The rich spectroscopy and electrochemistry of the heme moiety yields a wealth of opportunities for the denovo heme protein design to evaluate the success of the heme binding site design. Combinations of these spectroscopic and electrochemical methods are elucidating the structure and function of de novo heme proteins and illustrating that they serve as excellent bioinorganic model complexes for simple cytochromes. 

Fe "-OOH (ES) complex, 43 95-97 heme-bound CO, 43 115 lock-and-key model, 43 106-107 mutation in proximal heme cavity, 43 98 residue location, 43 101-102 van der Waals surfaces, 43 112-113 Velcro model, 43 107 zinc-substituted, 43 110-111 plastocyanin, cross-linked, cyclic voltammogram, 36 357-358 promoters, 36 345-346 protein-electrode complex, 36 345, 347 redox potential, 36 349 self-exchange rate constants, 36 402 stability at electrode/electrolyle interface, 36 349-350... 

Mott transition, 25 170-172 paramagnetic states, 25 148-161, 165-169 continuum model, 25 159-161 ESR. studies, 25 152-157 multistate model, 25 159 optical spectra, 25 157-159 and solvated electrons, 25 138-142 quantitative theory, 25 138-142 spin-equilibria complexes, 32 2-3, see also specific complex four-coordinated d type, 32 2 implications, 32 43-44 excited states, 32 47-48 porphyrins and heme proteins, 32 48-49 electron transfer, 32 45-46 race-mization and isomerization, 32 44—45 substitution, 32 46 in solid state, 32 36-39 lifetime limits, 32 37-38 measured rates, 32 38-39 in solution, 32 22-36 static properties electronic spectra, 32 12-13 geometric structure, 32 6-11 magnetic susceptibility, 32 4-6 vibrational spectra, 32 13 summary and interpretation... 

Nitrosyl complexes of both S = and S = are common. The S = nitrosyl complexes or iron and copper have slightly anisotropic g tensors (Ag/g < 2) with the anisotropy provided primarily by contributions of orbital angular momentum from the metal d orbitals. As an example, we consider the model proposed by Kon and Katakoa (1969) for ferroprotoheme-NO complexes, which is also applicable to the ferrous-nitrosyl complexes of heme proteins. In the complexes studied by these workers, the axial ligands are a nitrogenous base and NO. They proposed that the unpaired electron resides primarily in the metal d z orbital. The spin-orbit coupling would then mix contributions from the d, and dy orbitals. 

At the present time we have no certain knowledge of the state of the heme in these 450 nm species. We do not know if there are heme aggregates although they are unlikely. It is therefore reasonable to look at systems where the haem is aggregated as well as those where it is not in order to see how the absorption spectra can be mimic-ed. It seems reasonable to assume that the iron is low-spin in the carbon monoxide, isocyanide, and nitric oxide complexes as no high-spin iron complexes of this type are known. In the high-spin or low-spin state it may be that the thiol is weakly bound, if at all, for Fe(II) heme in models or in hemoglobin does not bind to thiols. In an attempt to understand these spectra we shall use a semi-empirical approach based on the theoretical discussion in the previous article (52) and elaborated in what follows immediately. Only Fe(II) complexes will be analysed as the Fe(III) proteins have been previously examined (52). 

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