On the chemical mechanism of succinic semialdehyde dehydrogenase (GabD1) from Mycobacterium tuberculosis

Succinic semialdehyde dehydrogenases (SSADHs) are ubiquitous enzymes that catalyze the NAD(P)⁺-coupled oxidation of succinic semialdehyde (SSA) to succinate, the last step of the γ-aminobutyrate shunt. Mycobacterium tuberculosis encodes two paralogous SSADHs (gabD1 and gabD2). Here, we describe the first mechanistic characterization of GabD1, using steady-state kinetics, pH-rate profiles, ¹H NMR, and kinetic isotope effects. Our results confirmed SSA and NADP⁺ as substrates and demonstrated that a divalent metal, such as Mg²⁺, linearizes the time course. pH-rate studies failed to identify any ionizable groups with pK_a between 5.5 and 10 involved in substrate binding or rate-limiting chemistry. Primary deuterium, solvent and multiple kinetic isotope effects revealed that nucleophilic addition to SSA is very fast, followed by a modestly rate-limiting hydride transfer and fast thioester hydrolysis. Proton inventory studies revealed that a single proton is associated with the solvent-sensitive rate-limiting step. Together, these results suggest that product dissociation and/or conformational changes linked to it are rate-limiting. Using structural information for the human homolog enzyme and ¹H NMR, we further established that nucleophilic attack takes place at the Si face of SSA, generating a thiohemiacetal with S stereochemistry. Deuteride transfer to the Pro-R position in NADP⁺ generates the thioester intermediate and 4A-²H, 4B-¹H] NADPH. A chemical mechanism based on these data and the structural information available is proposed.