Structural basis for cofactor and substrate selection by cyanobacterium succinic semialdehyde dehydrogenase

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Aldehyde dehydrogenase (ALDH) enzymes are crucial for oxidizing aldehydes to carboxylic acids.
• Cyanobacterium Synechococcus utilizes a unique ALDH (Sp2771) in a non-canonical tricarboxylic acid cycle.
• The molecular basis for Sp2771's substrate specificity and cofactor preference remains largely unelucidated.

Purpose of the Study:

• To elucidate the molecular mechanisms underlying substrate selection and cofactor preference in cyanobacterial ALDH (Sp2771).
• To provide structural insights into the function of Sp2771 and compare it with other ALDH enzymes.

Main Methods:

• X-ray crystallography was employed to determine the structures of wild-type Sp2771, Sp2771 mutants (S419A, C262A), and a complex with NADP(+) and SSA.
• Structural comparison was performed between Sp2771 and Gluconobacter oxydans aldehyde dehydrogenase (Gox0499).
• Site-directed mutagenesis was utilized to investigate the roles of specific residues.

Main Results:

• Crystal structures revealed wild-type Sp2771, Sp2771 mutants, and ternary complexes with NADP(+) and SSA.
• Structural comparison identified Ser157 in Sp2771 and Pro159 in Gox0499 as critical for NADP(+) and NAD(+) preference, respectively.
• The substrate binding funnel's characteristics, influenced by hydrophobic residues, dictate substrate selection.

Conclusions:

• The study provides key structural insights into how Sp2771 selects its cofactor (NADP+) and substrates.
• Specific amino acid residues and the architecture of the substrate-binding site are critical determinants of ALDH enzyme function.
• This research enhances our understanding of metabolic pathways involving ALDH in cyanobacteria.