Robert Huber

Aerobic and Anaerobic Life on Carbon Monoxide (CO)


Abstract

CO is a colorless, odorless gas, which is highly toxic to most forms of life. Despite its toxicity CO can be used by several bacteria and archaea as a chemolithoautotrophic growth substrate, providing these microbes with energy and a carbon source.CO dehydrogenases are the key enzymes in this process and catalyse the formal reaction: CO + H2O  CO2 + 2H+ + 2e-. Two structurally unrelated principal types of CO dehydrogenases have been described.

The CO dehydrogenase from the aerobic CO-oxidizing bacterium Oligotropha carboxidovorans is a 277-kDa Mo- and Cu-containing iron-sulfur flavoprotein .The enzyme´s active site in the oxidized or reduced state and after inactivation with potassium cyanide or n-butylisocyanide has been reinvestigated by multiple wavelength anomalous dispersion measurements up to 1.09 Å resolution. We gained evidence for a binuclear heterometal [CuSMoOOH] cluster in the active site of the oxidized or reduced enzyme, in which both metals are bridged by a µ-sulfido ligand. The cluster is coordinated through interactions of the Mo with the dithiolate pyran ring of molybdopterin cytosine dinucleotide and of the Cu with the S of cysteine388. The structure of the enzyme with the inhibitor n-butylisocyanide bound has led to a model for the catalytic mechanism of CO oxidation which involves a thiocarbonate-like intermediate state. The homodimeric nickel-containing CO dehydrogenase from the anaerobic bacterium Carboxydothermus hydrogenoformans catalyses the oxidation of CO to CO2. A crystal structure of the reduced enzyme has been solved at 1.1 angstrom resolution. This structure represents the prototype for the Ni-containing CO dehydrogenases from anaerobic bacteria and archaea. It contains five metal clusters of which clusters B, B´ and a subunit-bridging, surface-exposed cluster D are cubane-type [4Fe-4S] clusters. The active site clusters C and C´ are novel, asymmetric [Ni-4Fe-5S] clusters. Their integral Ni-ion is the likely site of CO oxidation .

The knowledge obtained from these two representative structures allows us now to gain further insight into the function and evolutionary relationships of CO dehydrogenases and related metalloproteins.




References:

Crystal structure of a carbon monoxide dehydrogenase reveals a [Ni-4Fe-5S] cluster.
Dobbek, H., Svetlitchnyi, V., Gremer, L., Huber, R., Meyer, O. (2001) Science 293, 1281-1285.

Catalysis at a dinuclear [CuSMo(=O)OH] cluster in a CO dehydrogenase resolved at 1.1Å resolution.
Dobbek, H., Gremer, L., Kiefersauer, R., Huber, R., Meyer, O. (2002), Proc. Natl. Acad. Sci. USA 99, 15971-15976.

A functional Ni-Ni-[4Fe-4S] cluster in the monomeric acetyl-CoA synthase from Carboxydothermus hydrogenoformans.
Svetlitchnyi, V., Dobbek, H., Meyer-Klaucke, W., Meins, T., Thiele, B., Römer, P., Huber, R. and Meyer, O. (2004), Proc. Natl. Acad. Sci. USA 101, 446-451.


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