ion step follows the decarboxylation, which is consistent with the deuterium isotopic effects observed for thymine 7-hydroxylase which indicate that an irreversible step (or steps) occurs prior to the C-H bond breaking.395 It has also been shown for prolyl 4-hydroxylase that a substrate-derived radical is generated in the reaction, which is consistent with a rebound mechanism.437 It is important to point out that no oxygen intermediate (i.e., bridged superoxo or oxo-ferryl) has been observed for any R-KGdependent enzyme. This warrants future theoretical and experimental study. A detailed molecular mechanism has been proposed for IPNS based on spectroscopic and crystallographic studies.422 Resting IPNS/FeII is also 6C and thus relatively stable toward dioxygen. Substrate ACV binds directly to FeII IPNS through its thiolate group, providing an open coordination position at the FeII. O2 can then react to form an FeIII-superoxo intermediate. This intermediate is suggested422 to perform the first hydrogen-atom abstraction step and close the â-lactam ring, resulting in the formation of the first water molecule and generating an FeIVdO-II intermediate, which completes the second ringclosure process by hydrogen-atom abstraction forming a thiazolidine ring. Previously proposed mechanisms of ACCO involved direct binding of cosubstrate ascorbate to the iron before O2 as part of the oxygen activation process.438,439 The EPR and ESEEM studies of the NO complex of ACCO suggested a quite different molecular mechanism for ACCO.435 An FeIII-superoxo intermediate is proposed. Whether it is preceded by a 6C f 5C process with substrate binding is presently under study.440 This intermediate is thought to initiate a radical process by single hydrogen-atom abstraction or electron-coupled proton transfer (PT)ion or electron-coupled proton transfer (PT) from the bound amino group. The resulting substrate radical may undergo spontaneous conversion into products. The role of cosubstrate ascorbate is proposed to reduce the toxic peroxo byproduct to water. Alternatively, the two-electron reduction of FeIIIsuperoxo by the cosubstrate ascorbate could result in an FeIVdO-II intermediate which initiates the radical reaction.435 4. Rieske-Type Dioxygenases Biochemical Characterization. The Rieske ironsulfur center is a two iron-two sulfur cluster ([2Fe2S]) which has a 2His (on one iron), 2Cys (on the other iron) coordination environment, instead of the 4Cys present in plant ferredoxins. It plays a key role in the electron transport pathway in membranebound cytochrome complexes as well as in some dioxygenases.441 The latter are mainly comprised of two protein components: a reductase containing flavin and a ferredoxin [2Fe-2S], and a terminal oxygenase containing a Rieske [2Fe-2S] cluster and a non-heme iron active site.442 Except for the recently reported alkene monooxygenase that has a binuclear iron site in its terminal oxygenase,10 most of the Rieske-type oxygenases have a mononuclear iron site, which is believed to be the site of dioxygen activation and substrate oxygenation.442,443 The majority of the Rieske-type mononuclear non-heme oxygenases form a family of enzymes which are aromatic-ring-hydroxylating dioxygenases. These catalyze the regioand stereospecific cis-dihydroxylation of an aromatic ring using dioxygen and NAD(P)H (Table 1). Examples include benzene dioxygenase (BDO, EC 1.14.12.3),444 phthalate dioxygenase (PDO, EC 1.14.12.7),445 toluene dioxygenase (EC 1.14.12.11),446 and naphthalene 1,2-dioxygenase (NDO, EC 1.14.12.12),447 which initiate the aerobic degradation of aromatic compounds in the soil bacteria and are targets for bioengineering in bioremediation. This step is the first step in the pathway that ultimately leads to ring cleavage by the intraand extradiol dioxygenases (sections II.B.2 and II.C.1).443 Besides these bacterial dioxygenases, other Rieske-type mononuclear non-heme oxygenases include anthranilate 1,2-dioxygenase (EC 1.14.12.1),448 which deaminates and decarboxylates the substrate to produce catechol; chlorophenylacetate 3,4-dioxygenase (EC 1.14.2.13),449 which converts substrate to catechol with chloride elimination; and 4-methoxybenzoate O-demethylase (putidamonooxin),450 which catalyzes the conversion of 4-methoxybenzoic acid to 4-hydroxybenzoic acid and formaldehyde. The reductase component is usually a monomer (MW ) 12-15 kDa) and utilizes flavin to mediate ET from the two-electron donor NAD(P)H to the oneelectron acceptor [2Fe-2S] cluster and is specific to each terminal oxygenase; other electron donors do not support efficient oxygenation.442 The crystal structure of phthalate dioxygenase reductase is available.451 The terminal oxygenases are large protein aggregates (MW ) 150-200 kDa) containing either multiples of R subunits (BDO R2, PDO R4) or an equimolar combination of R and â subunits (toluene dioxygenase R2â2, NDO R3â3). The R subunits contain a Rieske [2Fe-2S] cluster and a catalytic non-heme FeII center. â subunits do not seem to be involved in the catalytic function (vide infra). Kinetics. Steady-state kinetic studies coupled with various rapid reaction studies of the partial reactions of PDO allowed Ballou et al. to propose a kinetic scheme (Scheme 15).443 On the basis of steady state 278 Chemical Reviews, 2000, Vol. 100, No. 1 Solomon et al.

Geometric and Electronic Structure/Function Correlations in Non-Heme Iron Enzymes

Dioxygen Activation by Enzymes with Mononuclear Non-Heme Iron Active Sites

Zinc Metabolism in the Brain: Relevance to Human Neurodegenerative Disorders ☆

Evidence for a peroxide-initiated free radical mechanism of prostaglandin biosynthesis.

Mechanism of action of aspartate aminotransferase proposed on the basis of its spatial structure.

Interaction of S-100 protein with cations and liposomes

The Interaction of Aspartate Aminotransferase with α-Methylaspartic Acid*

A temperature jump study of aspartate aminotransferase. a reinvestigation.

1
The conversion of the native iron-containing lipoxygenase-1 into yellow and purple ferric enzyme species by 13-L-hydroperoxy-9-cis, 11-trans-octadecadienoic acid (R-13-OOH) was studied by measuring the absorbance changes at 330 nm and 580 nm, respectively in the stopped-flow apparatus. The pseudo-first-order rate constant for this reaction was found to be 50 s−1 at 4.4°C, either in the presence or absence of O2. This rate constant is lower than the one for R-13-OOH formation from linoleic acid and O2 catalysed by lipoxygenase-1: kcat= 232 s−1 at 4.4°C.

2
The coloured ferric enzymes are formed only in the presence of R-13-OOH; therefore the formation of the coloured ferric enzymes from oxygen, linoleic acid and the native enzyme indicates that the latter is active even before it is converted into the ferric forms.

3
Under anaerobic conditions the yellow and purple enzyme species are rapidly converted into colourless ferrous enzyme species by linoleic acid at a rate which is faster than the formation of the coloured enzymes from R-13-OOH and the native ferrous enzyme.

4
Both the aerobic formation of the coloured ferric enzymes and the anaerobic bleaching of these enzymes occur more slowly when [11-2H2]linoleic acid instead of unlabeled linoleic acid is used. The kinetic deuterium isotope effect as estimated from the rates of aerobic formation of R-13-OOH is approximately 9, whereas in the anaerobic conversion of R-13-OOH and deuterated linoleic acid a value of 1.1 is found. This demonstrates that H abstraction from linoleic acid determines the rate of the aerobic formation but not of the anaerobic conversion of R-13-OOH.

5
The rate constant for the anaerobic conversion of [11-2H2]linoleic acid by the yellow ferric enzyme species, k(2H) = 30 s−1 at 25°C, is close to the rate constant for the overall oxygenation reaction of the deuterated substrate, kcat(2H) = 32 s−1. For unlabelled linoleic acid the rate constants of the anaerobic H-abstraction step and the overall oxygenation reaction were found to be 111 s-1 and 290 s−1, respectively at 25°C. The former rate constant is probably affected by substrate inhibition.

6
Since the native ferrous enzyme is capable of catalysing H abstraction from linoleic acid under aerobic but not under anaerobic conditions, while the yellow ferric enzyme is active either in the absence or presence of O2, it is proposed that O2 reversibly converts the ferrous enzyme species into a ferric state.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1977.tb11617.x

On the mechanism of action of soybean lipoxygenase-1. A stopped-flow kinetic study of the formation and conversion of yellow and purple enzyme species

: Standard rectangular wave guides, loaded by the use of a ridge, were designed to make parallel slot antenna arrays capable of being scanned =45 degrees off the broadside position. The array design is proposed as a means of using the same aperture for multiple-frequency bands. Radiation patterns of 36-slot and 72-slot planar arrays are included. (Author)

Paolo Fasella

Papers

Interaction of S-100 protein with cations and liposomes

The Interaction of Aspartate Aminotransferase with α-Methylaspartic Acid*

A temperature jump study of aspartate aminotransferase. a reinvestigation.

On the mechanism of action of soybean lipoxygenase-1. A stopped-flow kinetic study of the formation and conversion of yellow and purple enzyme species

A Kinetic Study of Glutamic-Aspartic Transaminase