Rhamnogalacturonan -d-Galactopyranosyluronohydrolase¹
An Enzyme That Specifically Removes the Terminal Nonreducing Galacturonosyl Residue in Rhamnogalacturonan Regions of Pectin¹

Margien Mutter, Gerrit Beldman, Stuart M. Pitson, Henk A. Schols, and Alphons G.J. Voragen^*

Wageningen Agricultural University, Department of Food Chemistry, Bomenweg 2, 6703 HD Wageningen, The Netherlands (M.M., G.B., H.A.S., A.G.J.V.); and School of Biochemistry and Molecular Genetics, The University of New South Wales, Sydney 2052 Australia (S.M.P.)

A new enzyme, rhamnogalacturonan (RG) -d-galactopyranosyluronohydrolase (RG-galacturonohydrolase), able to release a galacturonic acid residue from the nonreducing end of RG chains but not from homogalacturonan, was purified from an Aspergillus aculeatus enzyme preparation. RG-galacturonohydrolase acted with inversion of anomeric configuration, initially releasing -d-galactopyranosyluronic acid. The enzyme cleaved smaller RG substrates with the highest catalytic efficiency. A Michaelis constant of 85 µm and a maximum reaction rate of 160 units mg¹ was found toward a linear RG fragment with a degree of polymerization of 6. RG-galacturonohydrolase had a molecular mass of 66 kD, an isoelectric point of 5.12, a pH optimum of 4.0, and a temperature optimum of 50°C. The enzyme was most stable between pH 3.0 and 6.0 (for 24 h at 40°C) and up to 60°C (for 3 h).

Plant Physiol. (1998) 117: 153-163
Copyright Clearance Center:   0032-0889/98/117/0153/11
© 1998 American Society of Plant Physiologists

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