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Notas:
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For deoxyribonucleotide synthesis during anaerobic
growth, Escherichia coli cells depend on an oxygen-sensitive
class III ribonucleotide reductase. The enzyme
system consists of two proteins: protein a, on which
ribonucleotides bind and are reduced, and protein b, of
which the function is to introduce a catalytically essential
glycyl radical on protein a. Protein b can assemble
one [4Fe-4S] center per polypeptide enjoying both the
[4Fe-4S]21 and [4Fe-4S]11 redox state, as shown by iron
and sulfide analysis, Mo¨ssbauer spectroscopy (d 5 0.43
mmzs21
, DEQ 5 1.0 mmzs21
, [4Fe-4S]21), and EPR spectroscopy
(g 5 2.03 and 1.93, [4Fe-4S]11). This iron center
is sensitive to oxygen and can decompose into stable
[2Fe-2S]21 centers during exposure to air. This degraded
form is nevertheless active, albeit to a lesser extent because
of the conversion of the cluster into [4Fe-4S] forms
during the strongly reductive conditions of the assay.
Furthermore, protein b has the potential to activate
several molecules of protein a, suggesting that protein b
is an activating enzyme rather than a component of an
a2b2 complex as previously claimed. |
