Proteolytic activation of human cathepsin A [Enzymology]

March 5th, 2014 by Kolli, N., Garman, S. C.

Galactosialidosis is a human lysosomal storage disease caused by deficiency in the multifunctional lysosomal protease cathepsin A (also known as Protective Protein/Cathepsin A, PPCA, catA, HPP, and CTSA; E.C. 3.4.16.5). Previous structural work on the inactive precursor human cathepsin A (zymogen) led to a two-stage model for activation, where proteolysis of a 1.6 kDa excision peptide is followed by a conformational change in a blocking peptide occluding the active site. Here we present evidence for an alternate model of activation of human cathepsin A, needing only cleavage of a 3.3 kDa excision peptide to yield full enzymatic activity, with no conformational change required. We present X-ray crystallographic, mass spectrometric, amino-acid sequencing, enzymatic, and cellular data to support the cleavage-only activation model. The results clarify a long-standing question about the mechanism of cathepsin A activation and point to new avenues for the design of mechanism-based inhibitors of the enzyme.