The A-cluster of acetyl-CoA synthase (ACS) catalyzes the formation of acetyl-
CoA from CO, coenzyme-A, and a methyl group donated by a corrinoid iron-sulfur
protein. Recent crystal structures have exhibited three different metals, Zn, Cu, and Ni,
in the proximal site, which bridges a square-planar nickel site and a [Fe4S4] cubane.
Contradicting reports supported both the nickel and copper containing forms as
representing active enzyme. The results presented here indicate that copper is not
necessary or sufficient for catalysis and that copper addition to ACS is deleterious.
Several proposed mechanisms exist for the synthesis of acetyl-CoA, the two most
prominent are the Âparamagnetic and Âdiamagnetic mechanisms. The ÂdiamagneticÂ
mechanism proposes a two electron activation that precedes methylation to produce an
EPR silent Ni2+-CH3 species. This then reacts with CO and coenzyme-A to form acetyl-
CoA and regenerate the starting species. The Âparamagnetic mechanism assumes a one
electron activation prior to the methylation of the paramagnetic Ni1+-CO state to form an
unstable Ni3+-acetyl species. This is immediately reduced by an electron shuttle.
Results are presented here that no shuttle or external redox mediator is necessary for
catalysis. This supports the Âdiamagnetic mechanism, specifically that a two-electron
reductive activation is necessary and that the Ni1+-CO species is not an intermediate.
The two-electron reductive activation required by the Âdiamagnetic mechanism
results in an unknown electronic state. Two proposals have been made to describe this
form of the A-cluster. The first hypothesis from Brunold et al involves a one-electron
reduction of the [Fe4S4]2+ cube and a one-electron reduction of the Nip
2+. This should
result in a spin-coupled state that is S = integer. The Ni0 hypothesis requires both
electrons to localize on the Nip
2+ forming a zero-valent proximal nickel. Mössbauer
spectroscopy has been used to probe the oxidation state and spin state of the [Fe4S4] cube
in the reduced active form. No integer spin system is found and this is interpreted as
supporting the Ni0 hypothesis. Additionally, spectra are presented that indicate the
heterogeneous nature of the A-cluster is not caused by the occupancy of the proximal
site.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/3233 |
| Date | 12 April 2006 |
| Creators | Bramlett, Matthew Richard |
| Contributors | Lindahl, Paul A. |
| Publisher | Texas A&M University |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | 1601797 bytes, electronic, application/pdf, born digital |
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