| Position in GST-σ | Residue in GST-σ | Position in S-crystallin (OCTS1) | Residue in S-crystallin | Status (Conserved/Substituted) | Functional Impact |
|---|---|---|---|---|---|
| 7 | Tyr | 8 | Tyr | Conserved | Catalytic Tyr retained; proposed general base that lowers the pKa of the GSH thiol, so catalytic loss is not explained by loss of this residue alone (pqac-00000009, pqac-00000013) |
| 13 | Arg | 14 | Arg | Conserved/perturbed | Arg retained, but in S-crystallin its catalytic role is weakened by nearby Asp101, which alters charge interactions and destabilizes optimal Meisenheimer-complex stabilization (pqac-00000008, pqac-00000010, pqac-00000013) |
| 38 | Trp | 39 | Trp | Conserved | Essential GSH-binding residue retained; supports preservation of G-site/GSH affinity despite loss of efficient transferase catalysis (pqac-00000009, pqac-00000011) |
| 42 | Lys | 43 | Arg | Conservative substitution | G-site residue remains basic; little predicted effect compared with major H-site changes (pqac-00000011) |
| 48 | Asn | 49 | Ser | Substituted | G-site altered at one contact position; contributes modestly to changed GSH-binding geometry but is not the main cause of activity loss (pqac-00000011) |
| 62/64 | Asn | 64 | Asn | Conserved | Conserved GSH-binding residue; consistent with strong GSH affinity in S-crystallin (pqac-00000011, pqac-00000014) |
| 63 | Ser | 65 | Ser | Conserved | Conserved GSH-binding residue; supports intact G-site chemistry for ligand binding (pqac-00000011, pqac-00000014) |
| 50 | Met | 51 | Met | Conserved | Conserved G-site contact; supports preservation of glutathione binding rather than efficient xenobiotic conjugation (pqac-00000011, pqac-00000014) |
| 98 | Phe | 100 | Leu | Substituted | Aromatic H-site residue replaced by smaller aliphatic residue, helping collapse/reshape the hydrophobic substrate-binding pocket and weakening binding of aromatic electrophiles such as CDNB (pqac-00000010, pqac-00000015) |
| 99 | Asn | 101 | Asp | **Critical substitution** | Charge reversal near Arg14; alters positive environment needed to stabilize the negatively charged Meisenheimer intermediate and is a key mechanistic explanation for reduced GST catalysis (pqac-00000008, pqac-00000010, pqac-00000012, pqac-00000013) |
| 102 | Val | 104 | Met | Substituted | Alters H-site hydrophobic pocket geometry and contributes to poor electrophilic substrate accommodation (pqac-00000008, pqac-00000010, pqac-00000015) |
| 106 | Phe | 108 | Gln/His | **Critical substitution** | Aromatic hydrophobic residue replaced by polar residue in native S-crystallin; strongly disrupts H-site architecture. Q108F back-mutation partially restores GST-like activity, showing this site is functionally decisive (pqac-00000010, pqac-00000012, pqac-00000014, pqac-00000015) |
| α4-α5 region | No long insertion | 112-122 | 11-residue insertion including Cys112 | Insertional divergence | Long loop occludes access to the active site and promotes high-affinity GSH binding; Cys112 can form a disulfide with bound GSH. This favors stabilization/structural function over catalysis (pqac-00000008, pqac-00000010, pqac-00000012, pqac-00000014) |
| Summary | G-site largely intact | OCTS1 active site overall | G-site conserved, H-site remodeled | Qualifying summary | OCTS1 preserves many canonical GSH-binding residues, explaining strong GSH binding, but has multiple H-site substitutions plus the α4-α5 loop insertion that block/reshape substrate access and collapse catalytic competence; this pattern is consistent with a GST-derived pseudo-enzyme adapted as a lens crystallin (pqac-00000000, pqac-00000003, pqac-00000010, pqac-00000011, pqac-00000013) |


*Table: This table compares key catalytic and binding residues between authentic octopus GST-σ and OCTS1 S-crystallin. It highlights that the glutathione-binding G-site is mostly retained, whereas H-site substitutions and a loop insertion explain the major loss of glutathione transferase activity.*