Evaluation of Subsite Affinities of Isomalto-dextranase from Arthrobacter globiformis.

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The subsite affinities of isomalto-dextranase (EC 3.2.1.94) from a soil bacterium, Arthrobacter globiformis T6 were evaluated to be>7.3, <-7.2, 6.7, 0.74 and 0.18 kcal/mol for subsites 1, 2, 3, 4 and 5, respectively, from the rate parameters (Km and k0) for the hydrolyses of isomalto-triose, -tetraose, -pentaose, -hexaose, -heptaose and -octaose. The intrinsic rate constant (kint) was calculated to be 3.1×10<SUP>2</SUP> s<SUP>-1</SUP>. Subsites 1 and 3, showing large affinity values, were thought to attract the substrate and to form the productive binding. Isopanose and maltotriose without α-isomaltosyl unit in their non-reducing ends acted neither as substrate nor as inhibitor for the isomaltodextranase, while the enzyme hydrolyzed oligosaccharides with α-isomaltosyl unit in their nonreducing ends. From these results, it was suggested that the binding of the α-isomaltosyl unit in the non-reducing end to subsites 1 and 2 was essential for the formation of the productive bindings be tween the substrates and the enzyme.
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