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. 2015 Feb 27;5(1):604–636. doi: 10.3390/life5010604

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© 2015 by the authors; licensee MDPI, Basel, Switzerland.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/4.0/).

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General reaction mechanism for class I and II RNRs, with important side chains in the active site indicated. The radical cysteine (top of the active site) is also present in class III RNR, while the electron donating cysteine pair (bottom of the active site) is replaced in the majority of class III RNRs with a single cysteine. Additionally, in most class III RNRs no residue corresponding to the proton shuttling glutamate has been found, and instead formate has been proposed to fill this function. Figure adapted from reference [55].