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Structural and functional insights into (S)-ureidoglycine aminohydrolase, a key enzyme of nitrogen utilization
The ureide pathway has recently been identified as the metabolic route of purine catabolism in plants and some bacteria. In this pathway, uric acid, which is a major product of the early stage of purine catabolism, is degraded into glyoxylate and ammonia via stepwise reactions of seven different enzymes. Therefore, the pathway has a possible physiological role in mobilization of purine ring nitrogen for further assimilation. (S)-Ureidoglycine aminohydrolase enzyme converts (S)-ureidoglycine into (S)-ureidoglycolate and ammonia, providing the final substrate to the pathway. Here, we report a structural and functional analysis of this enzyme from Arabidopsis thaliana (AtUGlyAH). The crystal structure of AtUGlyAH in the apo-form shows a monomer structure in the bi-cupin fold of the β-barrel and an octameric functional unit, as well as an Mn2+ ion binding site. The structure of AtUGlyAH in complex with (S)-ureidoglycine revealed that the Mn2+ ion acts as a molecular anchor to bind (S)-ureidoglycine and its binding mode dictates the enantioselectivity of the reaction. Further kinetic analysis characterized the functional roles of the active site residues, including the Mn2+ ion binding site and residues in the vicinity of (S)-ureidoglycine. These analyses provide molecular insights into the structure of the enzyme and its possible catalytic mechanism.
