Human succinic semialdehyde dehydrogenase: an extensive functional characterization and its implication in pathogenic variants

Succinic semialdehyde dehydrogenase deficiency (SSADH-D) is a rare genetic disorder, which is characterized by a disruption in the catabolic pathway of γ-aminobutyric acid (GABA). The impaired metabolism leads to an accumulation of neurotoxic metabolites (e.g. GABA; succinic semialdehyde or SSA; and γ-hydroxybutyric acid), which are considered central to the disease’s pathology. The lack of functional activity in the NAD+-dependent succinic semialdehyde dehydrogenase (SSADH) is considered the primary cause of the disease, but the limited knowledge of the human enzyme prevents a comprehensive understanding of the molecular mechanism that governs the disease’s manifestations. Therefore, this study aims to gain insights into the crucial role played by the wild-type (WT) enzyme providing the first extensive investigation of its biochemical and biophysical features. Several artificial protein variants have been also employed to address the importance of key amino acidic residues in the active site. All of the enzymatic forms were cloned, expressed, and purified using a recombinant E. coli expression system. The WT enzyme showed a folded tetrameric structure in a wide range of concentrations (0.1-5 mg·mL-1), which is stabilised by the binding of NAD+ (KD of 3.8 ± 0.4 μM). This is a biphasic process in which the catalytic C340 is involved. Our results cast doubt on the role of this residue in the formation of the protein·cofactor complex and its characteristic spectroscopic signal. Additionally, the functional characterization allowed the determination of the kcat for the enzyme (115 ± 9 s-1) and the Michaelis-Menten constants for the two co-substrates (31.5 ± 6 and 1.3 ± 0.2 μM for NAD+ and SSA, respectively). Notably, SSA exhibits an uncompetitive inhibition of catalysis with a physiologically relevant Ki in the low micromolar range. This is also a key concept for the complete understanding of SSADH kinetic mechanism, which involves the formation of a ternary complex in a Bi-Bi ordered fashion, at sub-inhibitory concentration of SSA. These findings are further enhanced by elucidating the molecular basis of some pathogenic variants and by preliminary results obtained through the application of more complex cellular disease models (e.g., full-brain organoids). Moreover, an in silico study, combined with patients’ genotypes and patients’ clinical manifestations allowed the definition of a first genotype-to-phenotype correlation for SSADH-D. This dissertation aims to deepen the knowledge of SSADH in the attempt to find a molecular rationale for the flaw mechanism of the pathology. This may not only lead towards the developing of a curative treatment, but also aid in unravelling features of neuropsychological disorders characterized by unbalanced neurotransmission.