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succinate-semialdehyde dehydrogenase Succinic semialdehyde dehydrogenase is one of three enzymes constituting 4-aminobutyrate (GABA) degradation in both prokaryotes and eukaryotes, catalyzing the (NAD(P)+)-dependent catabolism reaction of succinic semialdehyde to succinate for metabolism by the citric acid cycle. The EC number depends on the cofactor: 1.2.1.24 for NAD only, 1.2.1.79 for NADP only, and 1.2.1.16 if both can be used. In Escherichia coli, succinic semialdehyde dehydrogenase is located in an unidirectionally transcribed gene cluster encoding enzymes for GABA degradation and is suggested to be cotranscribed with succinic semialdehyde transaminase from a common promoter upstream of SSADH. Similar gene arrangements can be found in characterized Ralstonia eutropha and the genome analysis of Bacillus subtilis. Prokaryotic succinic semialdehyde dehydrogenases (1.2.1.16) share high sequence homology to characterized succinic semialdehyde dehydrogenases from rat and human (1.2.1.24), exhibiting conservation of proposed cofactor binding residues, and putative active sites (G-237 & G-242, C-293 & G-259 respectively of rat SSADH). Eukaryotic SSADH enzymes exclusively utilize NAD+ as a cofactor, exhibiting little to no NADP+ activity. While a NADP+ preference has been detected in prokaryotes in addition to both NADP+- and NAD+-dependencies as in E.coli, Pseudomonas, and Klebsiella pneumoniae. The function of this alternative SSADH currently is unknown, but has been suggested to play a possible role in 4-hydroxyphenylacetic degradation. Just outside the scope of this model, are several sequences belonging to clades scoring between trusted and noise. These sequences may be actual SSADH enzymes, but lack sufficiently close characterized homologs to make a definitive assignment at this time. SSADH enzyme belongs to the aldehyde dehydrogenase family (pfam00171), sharing a common evolutionary origin and enzymatic mechanism with lactaldehyde dehydrogenase. Like in lactaldehyde dehydrogenase and succinate semialdehyde dehydrogenase, the mammalian catalytic glutamic acid and cysteine residues are conserved in all the enzymes of this family (PS00687, PS00070). [Central intermediary metabolism, Other] |
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