Escherichia coli l-aspartate-α-decarboxylase: preprotein processing and observation of reaction intermediates by electrospray mass spectrometry

Abstract

The Escherichia coli panD gene, encoding l-aspartate-α-decarboxylase, was cloned by PCR, and shown to complement apanD mutant defective in β-alanine biosynthesis. Aspartate decarboxylase is a pyruvoyl-dependent enzyme, and is synthesized initially as an inactive proenzyme (the π-protein), which is proteolytically cleaved at a specific X–Ser bond to produce a β-subunit with XOH at its C-terminus and an α-subunit with a pyruvoyl group at its N-terminus, derived from the serine. The recombinant enzyme, as purified, is a tetramer, and comprises principally the unprocessed π-subunit (of 13.8 kDa), with a small proportion of the α- and β-subunits (11 kDa and 2.8 kDa respectively). Incubation of the purified enzyme at elevated temperatures for several hours results in further processing. Using fluorescein thiosemicarbazide, the completely processed enzyme was shown to contain three pyruvoyl groups per tetrameric enzyme. The presence of unchanged serine at the N-terminus of some of the α-subunits was confirmed by electrospray mass spectrometry (ESMS) and N-terminal amino acid sequencing. A novel HPLC assay for aspartate decarboxylase was established and used to determine the Km and kcat for l-aspartate as 151±16 μM and 0.57 s-1 respectively. ESMS was also used to observe substrate and product adducts trapped on the pyruvoyl group by sodium cyanoborohydride treatment.