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Figure 2
Structures from the proposed nitrite reductase mechanism determined in the 190 K MSOX series. The blue arrows represent the catalytic cycle in which the resting-state enzyme has water bound to T2Cu(II) (ds50). Binding of nitrite and displacement of water forms the initial enzyme–substrate complex T2Cu(II)–NO2 (ds1), following which electron transfer from the reduced T1Cu site causes nitrite to switch into its side-on binding mode (ds2). Associated proton transfer produces the side-on NO product (ds18). The red arrow indicates an alternative branch where T2Cu is reduced prior to nitrite binding, resulting in a three-coordinate T2Cu(I) form with (His)3 ligation (ds69). This loss of the T2Cu-coordinated water results in loss of enzyme activity.

IUCrJ
Volume 5| Part 3| May 2018| Pages 283-292
ISSN: 2052-2525
https://doi.org/10.1107/S205225251800386X