Locked and loaded for apoptosis

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Science  23 Jun 2017:
Vol. 356, Issue 6344, pp. 1236
DOI: 10.1126/science.aan5587

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The concept of protein entatic states is a cornerstone of bioinorganic chemistry (1, 2). “Entatic” literally means stretched or under tension. In a metalloprotein entatic state, the metal active site is poised in a particular conformation, ready and waiting to carry out catalysis or electron transfer. The active site is maintained in this state by structural constraints imposed by the surrounding protein structure. But quantifying the energetic cost of entatic stabilization is fiendishly difficult and has not been accomplished previously. On page 1276 of this issue, Mara et al. (3) use ultrafast x-ray spectroscopy to quantify the energetic cost of entatic control in cytochrome c. It is surprisingly small, explaining how cytochrome c can switch between disparate functions in respiration and apoptosis.