Structural Mimicry Without Glyoxalase I Functional Convergence: A Homogentisate 1,2-Dioxygenase From Acinetobacter

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Homogentisate 1,2-dioxygenase (HGD) is vital for amino acid metabolism.
• Acinetobacter-derived HGD (AcHGD) shares structural similarity with glyoxalase I (GLO1).

Purpose of the Study:

• To elucidate the structural basis for AcHGD's lack of GLO1 activity.
• To investigate the molecular differences between AcHGD and GLO1.

Main Methods:

• X-ray crystallography (1.5 Å resolution)
• Enzymatic assays
• Isothermal titration calorimetry (ITC)
• Site-directed mutagenesis

Main Results:

• AcHGD specifically binds Fe2+ and adopts a GLO1-like β-barrel fold coordinating Zn2+.
• AcHGD's active site tunnel is too narrow for GLO1's substrate, S-D-lactoylglutathione.
• Mutations to mimic GLO1 abolished AcHGD activity, while AcHGD binds homogentisate but not S-D-lactoylglutathione.

Conclusions:

• Structural and substrate specificity differences prevent AcHGD from exhibiting GLO1 activity.
• This study clarifies the structure-function relationship and evolutionary divergence between HGD and GLO1 enzymes.