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  • Title: Identification of the 4-glutamyl radical as an intermediate in the carbon skeleton rearrangement catalyzed by coenzyme B12-dependent glutamate mutase from Clostridium cochlearium.
    Author: Bothe H, Darley DJ, Albracht SP, Gerfen GJ, Golding BT, Buckel W.
    Journal: Biochemistry; 1998 Mar 24; 37(12):4105-13. PubMed ID: 9521732.
    Abstract:
    A series of 2H- and 13C-labeled glutamates were used as substrates for coenzyme B12-dependent glutamate mutase, which equilibrates (S)-glutamate with (2S,3S)-3-methylaspartate. These compounds contained the isotopes at C-2, C-3, or C-4 of the carbon chain: [2-2H], [3,3-2H2], [4,4-2H2], [2,3,3,4,4-2H5], [2-13C], [3-13C], and [4-13C]glutamate. Each reaction was monitored by electron paramagnetic resonance (EPR) spectroscopy and revealed a similar signal characterized by g'xy = 2.1, g'z = 1.985, and A' = 5.0 mT. The interpretation of the spectral data was aided by simulations which gave close agreement with experiment. This approach underpinned the idea of the formation of a radical pair, consisting of cob(II)alamin interacting with an organic radical at a distance of 6.6 +/- 0.9 A. Comparison of the hyperfine couplings observed with unlabeled glutamate with those from the labeled glutamates enabled a principal contributor to the radical pair to be identified as the 4-glutamyl radical. These findings support the currently accepted mechanism for the glutamate mutase reaction, i.e., the process is initiated through hydrogen atom abstraction from C-4 of glutamate by the 5'-deoxyadenosyl radical, which is derived by homolysis of the Co-C sigma-bond of coenzyme B12.
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