Effect of pH on subunit association and heat protection of soybean alpha-galactosidase.
Soybeans contain the enzyme alpha-galactosidase, which hydrolyzes alpha-1, 6 linkages in stachyose and raffinose to give sucrose and galactose.  We have found that galactose, a competitive product inhibitor of alpha-galactosidase, strongly promotes the heat stability of the tetrameric form of the enzyme at pH 4.0 and at temperatures of up to 70 degrees C for 60 min.  Stachyose and raffinose also protect alpha-galactosidase from denaturation at pH 4.0 although to a lesser extent.  Glucose and mannose have little effect.  At pH 7.0 the enzyme is a monomer, and galactose has no effect on the heat stability of the enzyme.  In the absence of heat protection of the enzyme by added sugars, a series deactivation mechanism was found to describe the deactivation data.  In comparison, a unimolecular, non-first order deactivation model applies at pH 4.0, where heat protection effects were observed.  At a temperature above 60 degrees C, simple deactivation is a suitable model.  The results suggest that alpha-galactosidase conformation and heat stability are directly related.
Porter
J E
JE
Department of Agricultural Engineering, Purdue University, West Layfayette, Indiana, USA.
Sarikaya
A
A
Herrmann
K M
KM
Ladisch
M R
MR
D004795:Enzyme Stability; D005690:Galactose / Q000096:biosynthesis; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D008954:Models, Biological; D009844:Oligosaccharides / Q000378:metabolism; D010940:Plant Proteins / Q000302:isolation & purification* / Q000378:metabolism; D011887:Raffinose / Q000378:metabolism; D013025:Glycine max / Q000201:enzymology*; D013395:Sucrose / Q000378:metabolism; D000519:alpha-Galactosidase / Q000302:isolation & purification* / Q000378:metabolism*
D009844:Oligosaccharides; D010940:Plant Proteins; C005695:stachyose; D013395:Sucrose; D000519:alpha-Galactosidase; D011887:Raffinose; D005690:Galactose