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Figure 3
N terminus of Cu–CAII (T-1 site). (a) The T-1 copper is stabilized by the N terminus of Cu–CAII by residues Ser2, His3 and His4. The copper is also hydrogen bonded to solvent molecule facing His64 (presumably for electron transfer to the T-2 site). Interestingly, residue His3 adopts dual conformations, one away and one towards the copper (for electron density refer to Fig. S3). (b) Structure of an iron-containing porphyrin ring from P. aeruginosa nitrite reductase (PDB entry 1n15, Nurizzo et al., 1999BB54). (c) Superposition of Cu–CAII N terminus with the P. aeruginosa nitrite reductase heme with an RMSD of 0.27 Å. It is important to note that the N terminus T-1 site is less ordered in comparison to the rest of the structure. The occupancy and B factor of the T-1 site are 0.71 and 29.1 Å2 respectively, while for the T-2 site the occupancy and B factor are 1.00 and 11.4 Å2 respectively. This is because the N terminus needs to be transient, only forming when needed in the blood, effectively acting as an on/off switch. Also, this transient feature would allow rapid metal exchange, allowing trace metals in the blood to quickly bind and disassociate for electron transfer.

IUCrJ
Volume 7| Part 2| March 2020| Pages 287-293
ISSN: 2052-2525
https://doi.org/10.1107/S2052252520000986