Structural conservation, functional decline: The fate of human OHCU decarboxylase

The conversion of uric acid to (S)-allantoin is catalyzed by urate oxidase (uricase), 5-hydroxyisourate hydrolase (HIUase), and 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline decarboxylase (OHCUd). In hominids, including Homo sapiens, these enzymes are generally not expressed, making uric acid the final product of purine catabolism. In previous work, we detected transcription of the genes encoding uricase (UOX), HIUase (URAHP), OHCUd (URAD), and allantoicase (ALLC) in H. sapiens, with URAD producing protein-coding transcripts. Here, we characterized recombinant human OHCUd (rhOHCUd) to evaluate its structural and functional properties. Spectroscopic and structural analyses revealed a well-folded α-helical protein with high similarity to zebrafish OHCUd (52% sequence identity, RMSD <0.8 Å), but with reduced stability. Functional assays showed that rhOHCUd has markedly lower catalytic activity than its zebrafish counterpart, consistent with partial loss of function. Molecular dynamics simulations further indicated distinct ligand-binding behavior and altered conformational flexibility in the human enzyme, which may contribute to its reduced catalytic efficiency. Together, these findings indicate that URAD encodes an evolutionarily conserved enzyme, with impaired activity, reflecting the progressive relaxation of selective pressure following uricase loss in hominids.