Oxygen transfer reactions, enzymes

Kinetic Aspects of Oxygen Transfer Reactions -Enzymes vs. Models... 

Ichinose, H. Wariishi, H., and Tanaka, H., Effective oxygen transfer reaction catalyzed by microperoxidase-11 during sulfur oxidation of dibenzothiophene. Enzyme and Microbial Technology, 2002. 30 pp. 334—339. 

The bottom line is that for substrates larger than one or two atoms in size, steric restrictions imposed on the peroxidase active site prevent oxygen transfer reactions and confine substrate-enzyme interactions to the heme edge. 

N—Fe(IV)Por complexes. Oxo iron(IV) porphyrin cation radical complexes, [O—Fe(IV)Por ], are important intermediates in oxygen atom transfer reactions. Compound I of the enzymes catalase and peroxidase have this formulation, as does the active intermediate in the catalytic cycle of cytochrome P Q. Similar intermediates are invoked in the extensively investigated hydroxylations and epoxidations of hydrocarbon substrates cataly2ed by iron porphyrins in the presence of such oxidizing agents as iodosylbenzene, NaOCl, peroxides, and air. 

The most conspicuous use of iron in biological systems is in our blood, where the erythrocytes are filled with the oxygen-binding protein hemoglobin. The red color of blood is due to the iron atom bound to the heme group in hemoglobin. Similar heme-bound iron atoms are present in a number of proteins involved in electron-transfer reactions, notably cytochromes. A chemically more sophisticated use of iron is found in an enzyme, ribo nucleotide reductase, that catalyzes the conversion of ribonucleotides to deoxyribonucleotides, an important step in the synthesis of the building blocks of DNA. 

The majority of the enzyme-catalyzed reactions discussed so far are oxidative ones. However, reductive electron transfer reactions take place as well. Diaphorase, xanteneoxidase, and other enzymes as well as intestinal flora, aquatic, and skin bacteria—all of them can act as electron donors. Another source of an electron is the superoxide ion. It arises after detoxification of xenobiotics, which are involved in the metabolic chain. Under the neutralizing influence of redox proteins, xenobiotics yield anion-radicals. Oxygen, which is inhaled with air, strips unpaired electrons from these anion-radicals and gives the superoxide ions (Mason and Chignell 1982). 

Achromobacter xylosoxydans has been used to cany out the selective hydroxylation in high yield, using the enzyme which catalyses the first step in nicotinic acid degradation. The whole-cell biotransformation process has been scaled-up to 12 m, which is sufficient to produce high purity 6-hydroxynicotinic acid for the subsequent chemical reactions. The hydroxylation is oxygen requiring, so that oxygen transfer rate-limits the reaction. 

Flavohydroquinone bound to apoproteins plays a very important role in flavo-protein-catalysis, either in the electron-transfer to substrates or other enzymes or in the oxygen activation reaction. The chemical reactivity of 1,5-dihydroflavin bound to apoproteins can differ drastically from that of free flavin. The reactivity is likely governed by factors such as the conformation of the bound flavohydroquinone and the ionization state (cf. below). 

Thermodynamic considerations indicate that the oxygen atom transfer reactions between Mo02(R2dtc)2 and enzyme substrates should be largely irreversible.175 For example, the hypothetical reaction of equation (17) has AH = — 28.5 5.5 kcalmol-1 (= —119 23.0 kJ mol1). The reaction should have a small AS and hence should have a negative AG and proceed spontaneously.175 There are, however, complicating factors. 

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