Evolutionary relationship between initial enzymes of tetrapyrrole biosynthesis

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Glutamate-1-semialdehyde 2,1-aminomutase (GSAM) is crucial for tetrapyrrole biosynthesis in bacteria, archaea, and plants.
• GSAM catalyzes the pyridoxal 5'-phosphate (PLP)-dependent conversion of glutamate-1-semialdehyde to 5-aminolevulinic acid (ALA).
• Understanding GSAM structure is key to elucidating tetrapyrrole pathway regulation and enzyme interactions.

Purpose of the Study:

• To determine the crystal structure of GSAM from Thermosynechococcus elongatus (GSAM(Tel)) in its PLP-bound form.
• To investigate the quaternary structure and potential flexibility of GSAM relevant to its interaction with glutamyl-tRNA reductase (GluTR).
• To explore the evolutionary relationship between GSAM and other ALA-producing enzymes like 5-aminolevulinate synthase (ALAS).

Main Methods:

• X-ray crystallography was employed to resolve the structure of GSAM(Tel) at 2.85A resolution.
• Comparative structural analysis was performed between GSAM(Tel), GSAM from Synechococcus (GSAM(Syn)), and ALAS.
• Bioinformatic analysis was used to infer evolutionary relationships between enzyme subfamilies.

Main Results:

• The crystal structure revealed GSAM(Tel) as a symmetric homodimer, contrasting with the previously described asymmetric GSAM(Syn).
• An extensive flexible region was identified at the GSAM(Tel)-GluTR interface, suggesting a role in enzyme complex formation.
• GSAM was found to be structurally related to ALAS, indicating a potential shared evolutionary origin.

Conclusions:

• The dimeric symmetry of GSAM(Tel) challenges the concept of negative cooperativity in this enzyme.
• The flexibility at the dimer interface may facilitate the interaction of GSAM with GluTR.
• GSAM and ALAS likely belong to closely related evolutionary subfamilies, with ALAS potentially evolving from a GSAM-like ancestor.