Purification and characterization of a thermostable β-galactosidase with high transgalactosylation activity from Saccharopolyspora rectivirgula

Abstract

We purified an extracellular thermostable β-galactosidase of Saccharopolyspora rectivirgula strain V2-2, a thermophilic actinomycete, to homogeneity and characterized it to be a monomeric enzyme with a relative molecular mass of 145 000 and s°_20,w of 7.1 s. In addition to the hydrolytic activity of 1-O-substituted β-d-galactopyranosides such as lactose [a Michaelis constant K _m=0.75 mm and molecular activity (k _cat)= 63.1 s⁻¹ at pH 7.2 and 55° C] and p-nitrophenyl β-d-galactopyranoside (K _m=0.04 mm k _cat= 55.8 s⁻¹), the enzyme had a high transgalactosylation activity. The enzyme reacted with 1.75 m lactose at 70°C and pH 7.0 for 22 h to yield oligosaccharides in a maximum yield (other than lactose) of 41% (w/w). A general structure for the major transgalactosylic products could be expressed as (Gal)_c-Glc, where n is 1, 2, 3, and 4 with a glucose at a reducing terminal. These oligosaccharides could selectively promote the growth of the genus Bifidobacterium found in human intestines. S. rectivirgula β-galactosidase was stable at pH 7.2 up to 60°C (for 4 h in the presence of 10 μm MnCl₂) or 70°C (for 22 h in the presence of 1.75 m lactose and 10 μm MnCl₂). Thus the enzyme is applicable to an immobilized enzyme system at high temperatures (60°C <) for efficient production of the oligosaccharides from lactose.