Structural and functional properties of HCOs and NORs.

aThe amino acid residues that compose the D-channel are Asp124, Asn199, Asn113, Asn131, Tyr35, Ser134, Ser193 and Glu278 for A1 type enzymes (P. denitrificans numbering). In the D-channel of A2 Type enzymes a tyrosine replaces the glutamate residue [13].
bK-channel of A Type enzymes is constituted by Lys354, Thr351, Ser291 and Tyr280 (Tyr-I) residues. In the alternative K-channel of B Type enzymes besides Tyr-I, those residues are replaced by a Thr, a Ser and a Tyr residues. The alternative K-channel from C Type enzymes is constituted by Tyr-II and a Ser and a Tyr residue in the same sequence position of the Thr-351 and SerI-291 residues [13].