> Wild-type octopus S-crystallin shows **Km,GSH ≈ 0.03 mM, Km,CDNB ≈ 3.9 mM, and kcat ≈ 0.24 s⁻¹**, whereas authentic octopus **GST-σ** shows **Km,GSH ≈ 1.3 mM, Km,CDNB ≈ 0.47 mM, and kcat ≈ 173.6 s⁻¹**; thus S-crystallin retains very tight glutathione binding but has drastically impaired catalysis toward CDNB-like electrophilic substrates (pqac-00000000, pqac-00000010).
>
> Relative to GST-σ, S-crystallin therefore has an approximately **700-fold lower kcat** and an approximately **6000-fold lower kcat/Km,CDNB**, indicating that the GST-derived fold is retained but catalytic efficiency for transferase chemistry is largely lost (pqac-00000000, pqac-00000001).
>
> Earlier biochemical work also found purified octopus S-crystallin to have a specific GST activity of only about **0.10 µmol/min/mg**, compared with roughly **100–200 µmol/min/mg** for typical mammalian GSTs, i.e. about a **~1000-fold reduction** in specific activity (pqac-00000004, pqac-00000017).
>
> Taken together, these data support the conclusion that OCTS1/S-crystallin is best understood as a **GST-derived lens crystallin that binds GSH tightly for stability/protection, while having essentially lost canonical glutathione transferase catalytic function toward electrophilic substrates** (pqac-00000001, pqac-00000003, pqac-00000016, pqac-00000018).


*Blockquote: This blockquote summarizes the key kinetic evidence comparing OCTS1/S-crystallin with authentic octopus GST-σ. It is useful because it quantifies the severe loss of catalytic efficiency while showing retention of strong glutathione binding, which is central to judging the GO function assignment.*