Characterisation of a β-N-acetylhexosaminidase from a commercial papaya latex preparation

A β-N-acetylhexosaminidase (β-NAHA) (EC 3.2.1.52) with molecular mass of 64.1 kDa and isoelectric point of 5.5 was purified from a commercial papaya latex preparation. The optimum pH for p-nitrophenyl-N-acetyl-β-d-glucosaminide (pNP-β-GlcNAc) hydrolysis was five; the optimum temperature was 50 °C; the Km was 0.18 mM, Vmax was 37.6 μmol min−1 mg−1 and activation energy (Ea) was 10.3 kcal/mol. The enzyme was thermally stable after holding at 30–45 °C for 40 min, but its activity decreased significantly when the temperature exceeded 50 °C. Heavy metal ions, Ag+ and Hg2+, at a concentration of 0.25 mM and Zn2+ and Cu2+, at a concentration of 0.5 mM, significantly inhibited enzyme activity. The β-NAHA had only one active site for binding both pNP-β-GlcNAc and p-nitrophenyl-N-acetyl-β-d-galactosaminide (pNP-β-GalNAc). A prototropic group with pKa value of about five on the enzyme may be involved in substrate binding and transformation, as examined by Dixon–Webb plots.