Structural constraints on protein self-processing in L-aspartate- -decarboxylase

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Abstract
Aspartate decarboxylase, which is translated as a pro‐protein, undergoes intramolecular self‐cleavage at Gly24–Ser25. We have determined the crystal structures of an unprocessed native precursor, in addition to Ala24 insertion, Ala26 insertion and Gly24→Ser, His11→Ala, Ser25→Ala, Ser25→Cys and Ser25→Thr mutants. Comparative analyses of the cleavage site reveal specific conformational constraints that govern self‐processing and demonstrate that considerable rearrangement must occur. We suggest that Thr57 Oγ and a water molecule form an ‘oxyanion hole’ that likely stabilizes the proposed oxyoxazolidine intermediate. Thr57 and this water molecule are probable catalytic residues able to support acid–base catalysis. The conformational freedom in the loop preceding the cleavage site appears to play a determining role in the reaction. The molecular mechanism of self‐processing, presented here, emphasizes the importance of stabilization of the oxyoxazolidine intermediate. Comparison of the structural features shows significant similarity to those in other self‐processing systems, and suggests that models of the cleavage site of such enzymes based on Ser→Ala or Ser→Thr mutants alone may lead to erroneous interpretations of the mechanism.