Structural and functional basis for (S)-allantoin formation in the ureide pathway

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• The ureide pathway degrades uric acid to (S)-allantoin, a crucial late stage of purine catabolism.
• The complete ureide pathway and its enzymes were recently identified, but the mechanism for selective (S)-allantoin production remained unclear.

Purpose of the Study:

• To determine the molecular mechanism of 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline (OHCU) decarboxylase in producing (S)-allantoin.
• To elucidate the structural basis for the stereoselective decarboxylation reaction.

Main Methods:

• X-ray crystallography of OHCU decarboxylase complexed with (S)-allantoin at 2.5-Å resolution.
• Site-directed mutagenesis and structural analysis to propose a reaction mechanism.

Main Results:

• The crystal structure revealed a novel homodimeric helical motif for OHCU decarboxylase, with an active site lacking cofactors.
• A mechanism was proposed involving direct decarboxylation and a critical role for an invariant histidine residue in stereoselective proton transfer.
• This mechanism explains the exclusive production of (S)-allantoin.

Conclusions:

• The study provides the first molecular insights into the mechanism of OHCU decarboxylase.
• The findings reveal how organisms selectively produce (S)-allantoin through a cofactor-independent enzymatic reaction.
• This work addresses a fundamental question in purine metabolism.