2.6.3. Superoxide Dismutase

The serum superoxide dismutases (SODs) could effectively eliminate reactive oxygen species (ROS) and maintain the redox balance [43], thereby playing a key role in protection against oxidative tissue injury in both prokaryotes and eukaryotes [44–48]. Three sequences were successfully scanned with their similarities identified at 51.8–75.3% in mucus-enriched proteins of *A. coerulea* (Table 2). A SOD (pdb ID: 1Q0E) was selected as the model sequence (Figure 8A) and structural template (Figure 8B) of the two putative Cu/Zn-SODs. The core is constituted of two groups of parallel β-sheets that are surrounded by scattered helixes, turns and coils. The discrepancies include an extra helix-turn-helix in 52DIYTRGC58 and an extended 130GTGAKRAGSQKTG<sup>142</sup> of TRINITY\_DN37490\_c0\_g1|m.12223. Phylogenetic analysis shows that TRINITY\_DN36380\_c0\_g1|m.11103 is close to ALQ81852 in *Cyanea capillata* while TRINITY\_DN37490\_c0\_g1|m.12223 is near XP\_022803558 in *Stylophora pistillata* (Figure 8C).

**Figure 8.** Sequence alignment, 3D modeling and phylogenetic analysis of the putative Cu/Zn superoxide dismutases (SODs) from *A. coerulea*. (**A**) Two putative sequences, TRINITY\_DN36380\_c0\_g1|m.11103 and TRINITY\_DN37490\_c0\_g1|m.12223, in mucus-enriched proteins are aligned with a model SOD (pdb ID: 1Q0E). At the bottom of columns, asterisks (\*) show conserved positions, colons (:) show conserved substitutions and points (.) show non-conserved substitutions. Grey lines, green bends, blue-banded arrowheads and red solenoids represent coils, turns, sheets and helices, respectively. Different sections are framed by red lines. (**B**) 3D modeling was simulated using the template SOD (pdb ID: 1Q0E) by SWISS-MODEL and viewed by Discovery Studio 4.5. The colors grey, green, blue and red represent coils, turns, sheets and helices, respectively. Different sections are indicated by red arrows. (**C**) Phylogenetic tree constructed by two putative SODs in mucus-enriched proteins and 13 representative sequences from different species by MEGA 7 with the Neighbor-Joining method.
