Structural groups of Ni-CODHs. The representative protein sequences of each structural group of Ni-CODHs and HCPs are represented by multiple sequence alignments. The alignment of the N- and C-terminal extensions and the insertions was manually modified for representation. The number of proteins in each structural group is shown in parentheses. The Cys residues of the metal clusters and catalytic residues are highlighted by different color backgrounds as follows: green, D-cluster; blue, B-cluster; magenta, C-cluster; yellow, E- and F-clusters; cyan, acid-base catalysts; and red, putative novel clusters. The His residues in the His-rich regions of structural groups B-1 and B-4 are shown in blue letters, and the Glu residues at the equivalent position of Glu494 in the catalytic hybrid cluster of D. vulgaris HCP are shown in yellow letters (Aragão et al., 2008). The NCBI accession numbers of the representative sequences are as follows: A-1, WP_011305243; A-2, WP_010878596; A-3, OGW06734; A-4, OIP92259; A-5, ODS42986; A-6, OIP30420; B-1, WP_026514536; B-2, WP_015485077; B-3, WP_012645460; B-4, WP_011393470; C-1, WP_039226206; C-2, WP_013237576; C-3, WP_010870233; C-4, WP_044921150; D-1, WP_011342982; D-2, WP_015926279; D-3, WP_079933214; D-4, WP_096205957; E-1, WP_012571978; E-2, WP_010939375; E-3, WP_088535808; F-1, WP_011343033; F-2, WP_011389181; G-1, OGP75751; and HCP, WP_010939296.