Enzymatic ferredoxins

Dorner E, M Boll (2002) Properties of 2-oxoglutarate ferredoxin oxidoreductase from Thauera aromatica and its role in enzymatic reduction of the aromatic ring. J Bacterial 184 3975-3983. 

Fig. 16. Ferredoxin (Fd)/ferredoxin-NADP+-reductase (FNR) mediated enzymatic carboxylation of pyruvic acid to form malic acid catalyzed by the NADPH-dependent malic enzyme (ME)...

It has been pointed out 33) that synthesis of [Fe2S2(Ss)2] from [Fe(SPh)4] and dibenzyl trisulfide would have implications regarding the enzymatic biosynthesis of the metal clusters in Fe2 ferredoxins, since trisulfides seem to be present in biological systems. 

This enzyme [EC 1.1.3.22] catalyzes the reaction of xanthine with dioxygen and water to produce urate and hydrogen peroxide. Enzymatic activity requires iron, FAD, and molybdenum. Hypoxanthine and some other purines and pterins can act as substrates. Under some conditions, the product is mainly superoxide rather than hydrogen peroxide thus, R—H reacts with two dioxygen and water to produce R—OH, two H+, and two 02 molecules. The Micrococcus enzyme can use ferredoxin as the acceptor substrate. The mammalian enzyme can be interconverted to xanthine dehydrogenase [EC 1.1.1.204]. See Xanthine Dehydrogenase... 

The active forms of the D vitamins are la,25-dihydroxy-vitamin Dj and 25-hydroxy-vitamin Dj. They are formed by enzymatic hydroxylation in the liver microsomes and then in the kidney mitochondria by a ferredoxin flavoprotein and cytochrome P-450. The 1,25-dihydroxy vitamin is then transported to the bone, intestine, and other target organs (kidneys, parathyroid gland). Consequently, it can be considered a hormone since it is produced in one organ but used elsewhere. It mobilizes calcium and phosphate and also influences the absorption of these ions in the intestine, thus promoting bone mineralization. The hormone is also active in relieving hypoparathyroidism and postmenopausal osteoporosis, which, for example, results in the brittle bones of elderly women. 

Although electron transfer as such is not considered as catalysis, most enzymatic redox reactions require the presence of electron-transfer proteins for fast and efficiently directed electron transfer to the active sites. The ferredoxins, azurins, and cytochromes are most well known in this respect. Variations of over 15 A in distance may occur, and as a consequence, the electron-transfer rate may vary over 10 orders of magnitude [35], Exciting developments are ongoing in this field, and are highly relevant for the bioinorganic catalytic subject. 

The spectra of adrenodoxin and testis iron protein reduced either by dithionite or by NADPH plus adrenodoxin reductase (flavoprotein) exhibit a distinct feature (Fig. 2) the absorption maxima at 455 mp, 414 mp, and 320 mp graetly decrease, although they do not disappear completely. A new distinct maximum appears at 540 mp. However, adrenodoxin can not be reduced by ascorbate or borohydride. The chemical reduction by an excess amount of dithionite gives an 86% decrease in the absorbance, whereas the enzymatic reduction produces a 65% decrease. This difference can be interpreted as gradual loss of iron from the chromophore upon reduction by dithionite. When the reduced form is reoxidized by air, the original spectrum can be regained. The identical characteristic is observed in preparations of testis iron protein. Therefore, adrenal and testis non-heme iron proteins are autoxidizable as well as bacterial and spinach ferredoxins are. 

The first evidence for cobalamin involvement in the conversion of methanol to methane was provided by Blaylock and Stadtman [196,216-218] with extracts of methanol-grown M. barkeri they demonstrated enzymatic formation of methylcobalamin from methanol, and subsequent reduction of methylcobalamin to methane. Later Blaylock [196] showed that conversion of methanol to methylcobalamin requires a heat-stable cofactor and at least three proteins, a 100-200 kDa Bi2-enzyme (methyltransferase), a ferredoxin, and an unidentified protein. Blaylock speculated that the role of hydrogen and ferredoxin in the conversion of methanol to methylcobalamin was in the reduction of the Bi2-protein. This work led to the proposal that methylcobalamin was the direct precursor of methane in methanogenesis from various substrates [196,218]. 

In all photoautotrophs, reduction of NOj" to NH4 is achieved in two distinct enzymatic steps (Campbell, 2001). First, assimilatory nitrate reductase (NR) catalyzes the two electron reduction from NOj" to NO2. NR is a large soluble cytoplasmic enzyme with FAD (flavin adinine dinucleotide), an iron-containing cytochrome and molybdopterin prosthetic groups, and requires NADH and/or NADPH as an electron donor (Guerrero et al, 1981). Functional NR is in the form of a homodimer and therefore requires two atoms of iron per enzyme. Following transport into the chloroplast, NO2 undergoes a 6 e reduction to NH4 via assimilatory nitrite reductase (NiR). NiR, a soluble chloroplastic enzyme, contains five iron atoms per active enzyme molecule, and requires photosynthetically reduced ferredoxin as an electron donor (Guerrero et al., 1981). 

The simplest example of such reactions is the decarboxylation of pyruvate. Both model and enzyme studies have shown the intermediacy of covalent complexes formed between the cofactor and the substrate. Kluger and coworkers have studied extensively the chemical and enzymatic behavior of the pyruvate and acetaldehyde complexes of ThDP (2-lactyl or LThDP, and 2-hydroxyethylThDP or HEThDP, respectively) . As Scheme 1 indicates, the coenzyme catalyzes both nonoxidative and oxidative pathways of pyruvate decarboxylation. The latter reactions are of immense consequence in human physiology. While the oxidation is a complex process, requiring an oxidizing agent (lipoic acid in the a-keto acid dehydrogenases , or flavin adenine dinucleotide, FAD or nicotinamide adenine dinucleotide , NAD " in the a-keto acid oxidases and Fe4.S4 in the pyruvate-ferredoxin oxidoreductase ) in addition to ThDP, it is generally accepted that the enamine is the substrate for the oxidation reactions. 

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