3.D.4.3.3
Cbb3 cytochrome c oxidase (COX; Cbb3; CcoNOP).  The 3-d structure is known to 3.2 Å resolution (PDB# 3MK7; 5DJQ) (Buschmann et al. 2010; Lee et al., 2012).  The structure explains a proton-pumping mechanism and the high activity of family-C heme-copper oxidases compared to that of families A and B (Buschmann et al., 2010; Lee et al., 2012). A small subunit of 36 aas and 1 TMS, CcoM, was identified in the structure and plays a role in assembly and stability (Kohlstaedt et al. 2016; Carvalheda and Pisliakov 2017). CcoQ, another small protein of 62 aas (acc # F8H837) is an assembly factor for Cbb₃-1 and Cbb₃-2 (Kohlstaedt et al. 2017). The A-, B- and C-type oxygen reductases each have an active-site tyrosine that forms a unique cross-linked histidine-tyrosine cofactor. In the C-type oxygen reductases (also called cbb3 oxidases), this post-translationally generated co-factor occurs in a different TMS than for the A- and B-type reductases (Hemp et al. 2006).