Heme-acetate complex

Catalase reacts reversibly with some weak acids forming spectroscopically and magnetically distinct noncovalent derivatives. Of these, catalase-cyanide, -azide, -fluoride, -formate, and -acetate complexes have been extensively studied (37, 135, 136) and reviewed in some detail (16-18). Briefly, there is a consensus that such reactions do not involve heme-heme interaction and, with the possible exception of carboxylate ligands (102), all presumably result in replacement of the proximal aquo ligand at Ls in a stoichiometric reaction shown in Eq. (11) ... 

Peroxynitrite reacts with heme proteins such as prostacycline synthase (PGI2), microperoxidase, and the heme thiolate protein P450 to form a ferryl nitrogen dioxide complex as an intermediate [120]. Peroxynitrite also reacts with acetaldehyde with the rate constant of 680 1 mol 1 s" 1 forming a hypothetical adduct, which is decomposed into acetate, formate, and methyl radicals [121]. The oxidation of NADH and NADPH by peroxynitrite most certainly occurs by free radical mechanism [122,123], Kirsch and de Groot [122] concluded that peroxynitrite oxidized NADH by a one-electron transfer mechanism to form NAD and superoxide ... 

This emphasizes that iron(II) complexes bearing more bulky bis(pyrazol-l-yl)acetate ligands should be good structural models to mimic mononuclear non-heme iron dependent enzymes. 

Using Mossbauer spectroscopy to monitor the formation of p-hematin under in vitro reaction conditions, Adams et al. have demonstrated that the reaction is a psuedo-zero-order process [109]. Such a process is consistent with a mechanism whereby a small concentration of heme is kept soluble via acetate, functioning as a phase-transfer catalyst, in a heme-saturated solution. In the rate limiting step, the soluble heme aggregates to P-hematin, which in turn grows until it precipitates from solution. There are clearly complicated heterogeneous reaction equilibria involved in the aqueous chemical formation of p-hematin. Consequently, it should be emphasized that the detailed mechanistic analysis of the complex solubilization of the species involved in the chemical synthesis... 

By contrast, the catalytic site responsible for the halogenation, hydroxylation, and other (two-electron) oxygenation reactions has been better, although not completely, characterized by X-ray crystallography of CPO complexed with several substrates (such as iodide/bromide and cyclopentane-1,3-dione) and other compounds (such as carbon monoxide, thiocyanate, nitrate, acetate, formate and, in a ternary complex, with dimethylsulfoxide and cyanide) [88, 90]. The above substrates bind at the distal side of heme, and the corresponding structures were also useful to establish the mechanism of Compound I formation as discussed above [90]. 

Dienyl acetate (256) (readily prepared from l-hepten-6-one and lacking the disulfone moiety) was also easily transfoimed to ( )-l,4 ene (259 87%) ( heme 53). This legiochemistiy parallels that of stoichiometric additions of allylpalla um complexes to noihomadiene, but was subject to uncertainty in view of the relatively fast 1,3-Pd migration within the allyl component. Therefore the conversion (256) - (259) demonstrates a new stereocontroUed access to exocyclic tiisubstituted alkenes which implies C—C bond formation at the less-substituted allylpalladium terminal and thus shows a regio- and stereo-chemistry both opposite to the type n magnesium-ene process ([Pg.56]

The role of siroheme in the reduction of nitrite is supported by the observation that nitrite perturbs the spectrum of the oxidized or reduced enzyme. Carbon monoxide reaction with the reduced siroheme can be followed spec-trophotometrically. The complex can be dissociated with oxygen. The kinetics of CO-complex formation and dissociation correspond with those for CO inhibition of nitrite reductase. The inhibition of nitrite reductase by p-chloromercuribenzoate, phenyl mercury acetate, and sodium mersalyl demonstrate the involvement of SH groups presumably associated with cysteine residues found in nitrite reductase. Epr studies and spectral data indicate the oxidation/reduction of iron sulfur centers and the formation of a NO-heme complex however, other intermediates have not been confirmed (Aparicio et al., 1975 Vega and Kamin, 1977 Hucklesby et al., 1979). 

The biosynthetic chain of chlorophyll begins with the small building blocks, acetate and glycine molecules, which are part of the basic metabolic milieu. These small molecules are condensed in a series of n steps to form the complex molecule protoporphyrin. From protoporphyrin two classes of compounds are formed namely, the iron porphyrins or hemes and the magnesium porphyrins which give rise eventually to chlorophyll. According to this scheme, heme and chlorophyll are related to each other biochemically, since both arise from the same precursor molecule, protoporphyrin. 

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