Substrate preference of a Geobacillus maltogenic amylase: a kinetic and thermodynamic analysis

The gene encoding a maltogenic amylase (MAase) from a newly isolated strain of thermophilic Geobacillus has been isolated, cloned and expressed. Following purification, biochemical and structural characterization have been performed. The enzyme exhibited maximal activity at a broad temperature range between 55 and 65 °C, clearly higher than that of other dimeric MAses. The optimum pH was 6.0 and catalytic activity increased by of Li(+) and K(+). A noticeable preference was demonstrated for α-, β- and γ-cyclodextrin (CD) compared to polymeric substrates (amylose, amylopectin, glycogen and starch) possibly due to steric interference. The affinity for CD substrates increased in the order of γ-CD>β-CD>α-CD, but k(cat)/K(m) increased as α-CD>β-CD>γ-CD, implying that increased substrate specificities are mainly attribute to kcat. Thermodynamic analysis of the activation process showed that improved activity (decrease in ΔG(#)) is accompanied by increases in activation entropy (ΔS(#)) for aforementioned substrates. Molecular docking on the binding interactions between substrates and active site residues revealed a considerably higher accessible surface area for the active site residues in the presence of α-CD than β-CD, indicating that interactions in the transition state are stronger in the presence of α-CD. This result explains the increased ΔH(#) of the activation process and increased K(m) of the enzyme in the presence of α-CD, compared to that of β-CD. This study, which presents the first detailed comparative analysis on the substrate preference of dimeric MAases for different substrates, may shed some lights into the molecular mechanism of these enzymes.