ジャーナル: Acta Crystallogr D Struct Biol / 年: 2022 タイトル: Unravelling the regulation pathway of photosynthetic AB-GAPDH. 著者: Roberto Marotta / Alessandra Del Giudice / Libero Gurrieri / Silvia Fanti / Paolo Swuec / Luciano Galantini / Giuseppe Falini / Paolo Trost / Simona Fermani / Francesca Sparla / 要旨: Oxygenic phototrophs perform carbon fixation through the Calvin-Benson cycle. Different mechanisms adjust the cycle and the light-harvesting reactions to rapid environmental changes. Photosynthetic ...Oxygenic phototrophs perform carbon fixation through the Calvin-Benson cycle. Different mechanisms adjust the cycle and the light-harvesting reactions to rapid environmental changes. Photosynthetic glyceraldehyde 3-phosphate dehydrogenase (GAPDH) is a key enzyme in the cycle. In land plants, different photosynthetic GAPDHs exist: the most abundant isoform is formed by AB heterotetramers and the least abundant by A homotetramers. Regardless of the subunit composition, GAPDH is the major consumer of photosynthetic NADPH and its activity is strictly regulated. While A-GAPDH is regulated by CP12, AB-GAPDH is autonomously regulated through the C-terminal extension (CTE) of its B subunits. Reversible inhibition of AB-GAPDH occurs via the oxidation of a cysteine pair located in the CTE and the substitution of NADP(H) with NAD(H) in the cofactor-binding site. These combined conditions lead to a change in the oligomerization state and enzyme inhibition. SEC-SAXS and single-particle cryo-EM analysis were applied to reveal the structural basis of this regulatory mechanism. Both approaches revealed that spinach (AB)-GAPDH oligomers with n = 1, 2, 4 and 5 co-exist in a dynamic system. B subunits mediate the contacts between adjacent tetramers in AB and AB oligomers. The CTE of each B subunit penetrates into the active site of a B subunit of the adjacent tetramer, which in turn moves its CTE in the opposite direction, effectively preventing the binding of the substrate 1,3-bisphosphoglycerate in the B subunits. The whole mechanism is made possible, and eventually controlled, by pyridine nucleotides. In fact, NAD(H), by removing NADP(H) from A subunits, allows the entrance of the CTE into the active site of the B subunit, hence stabilizing inhibited oligomers.