Biochemistry of Tungstoenzymes from Pyrococcus furiosus

Biochemistry of Tungstoenzymes from Pyrococcus furiosus

Bol, E.

Hagen, W.R. (promotor)

Applied Sciences

2007-10-09

The cell uses a variety of transition metals to provide greater catalytic diversity than could be achieved using only the functional groups of amino acids. The biochemistry of molybdenum and tungsten is unusual: they are the only 4d and 5d metal ions with established biological role(s). This thesis reports studies on the identification of tungstoenzymes using functional proteomics and on the physiological substrate, function and catalytic mechanism of one of the abundantly present tungstoenzymes of Pyrococcus furiosus; formaldehyde ferredoxin oxidoreductase (FOR). A new method of native-native two dimensional gel electrophoresis (2DGE) was developed in which no denaturants such as urea or SDS were applied (Chapter 2). Radioactively labeled tungsten (187WO42-) was added to the growth medium of P. furiosus to detect tungsten proteins for subsequent identification with mass spectrometry. The radiograms of gels containing radioactively labeled tungsten revealed six spots with tungsten-associated proteins. These spots were excised and analyzed by mass spectrometry. Two tungstoenzymes were identified as aldehyde ferredoxin oxidoreductase and formaldehyde ferredoxin oxidoreductase. No tungsten containing proteins could be identified from the other spots. A new tungsten containing oxidoreductase (WOR5) was discovered. It is the fifth and last member of the family of tungsten containing oxidoreductases from the hyperthermophilic archaeon P. furiosus. WOR5 was purified and characterized with EPR spectroscopy and electrochemistry (Chapter 3). It was found to be a homo-dimeric protein (subunit: 65 kDa) that contains one [4Fe-4S] cluster and one tungstobispterin cofactor per subunit. The enzyme has a broad substrate specificity with a high affinity for several substituted and non-substituted aliphatic and aromatic aldehydes with variable chain lengths, and it can use ferredoxin as electron acceptor in catalysis. The redox chemistry of the tungsten and iron-sulfur prosthetic groups in P. furiosus formaldehyde ferredoxin oxidoreductase (Chapter 4) and steady and pre-steady state kinetics (Chapter 5) for this enzyme were studied using formaldehyde as substrate and ferredoxin as electron acceptor. The intermediate, paramagnetic W(V) state could be trapped only by reduction of FOR with substrate, with consecutive one-electron intraprotein electron transfer to the single [4Fe-4S](2+;1+) cluster and partial comproportionation of the tungsten over W(IV, V, VI); this is a stable state in the absence of an external electron acceptor. Due to the very unfavorable hydratation equilibrium of the formaldehyde/methylene glycol couple no W(V) was found in dye mediated equilibrium redox titrations. The development of this intermediate was slow even at elevated temperatures and with a excess of substrate. The free formaldehyde, and not the methylene glycol, is the enzyme's substrate, implying that the KM for formaldehyde is three orders-of-magnitude less than the value previously reported in literature. The steady state kinetics of FOR is consistent with a substrate substituted-enzyme mechanism for three substrates (formaldehyde plus two ferredoxin molecules). The determined KM of ferredoxin (14 μM) was an order of magnitude less than previously reported values, due to the fact that at high concentrations of substrate the enzyme is inhibited and denatured. Pre-steady state difference spectra revealed peak shifts and a lack of isosbestic points, an indication that several out-of-phase processes happen simultaneously in the first seconds of the reaction. The binding and the oxidation of the substrate are both fast processes. The release of the product and the electron shuffling over the tungsten and iron-sulfur center in the absence of an external electron acceptor are slower. Based on these results, in combination with results from previous EPR studies two alternatives for a catalytic redox cycle are proposed.

biochemistry
tungstoenzymes
pyrococcus furiosus

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978-90-9022172-4

Institutional Repository

doctoral thesis

(c) 2007 E. Bol