Archaeal Aldehyde Dehydrogenase ST0064 from Sulfolobus tokodaii, a Paralog of Non-Phosphorylating Glyceraldehyde-3-phosphate Dehydrogenase, Is a Succinate Semialdehyde Dehydrogenase

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Aldehyde dehydrogenase ST0064, the closest paralog of previously characterized allosteric non-phosphorylating glyceraldehyde-3-phosphate (GAP) dehydrogenase (GAPN, ST2477) from a thermoacidophilic archaeon, Sulfolobus tokodaii, was expressed heterologously and characterized in detail. ST0064 showed remarkable activity toward succinate semialdehyde (SSA) (K_m of 0.0029 mM and k_cat of 30.0 s⁻¹) with no allosteric regulation. Activity toward GAP was lower (K_m of 4.6 mM and k_cat of 4.77 s⁻¹), and previously predicted succinyl-CoA reductase activity was not detected, suggesting that the enzyme functions practically as succinate semialdehyde dehydrogenase (SSADH). Phylogenetic analysis indicated that archaeal SSADHs and GAPNs are closely related within the aldehyde dehydrogenase superfamily, suggesting that they are of the same origin.