| Summary: | The initial action of porcine pancreatic a-amylase (PPA, EC 3.2.1.1) on various native and hydrothermally treated starches was investigated, in order to understand how structural differences in the substrate influence starch digestion. Initial reaction rates at various starch concentrations were measured and fitted to the Michaelis-Menten equation to obtain enzyme kinetic parameters, KM, kcat and kcalKM. The KM is a measure of the accessibility of the enzyme towards its substrate and kcalKM is an index of catalytic efficiency by which the effectiveness towards different substrates can be compared. The kcst is the dissociation rate of the enzymesubstrate (ES) complex. Starches from common food sources were used as substrates. Physical and chemical data on the characteristics of the starches was obtained, using methods such as scanning electron microscopy and differential scanning calorimetry. The kcab kcalKM ratio increased and KM value decreased following hydrothermal treatment of all starches at 100°C, but the effect was variable, depending on the source. Potato starch was the most favoured substrate for PPA in the heat-treated state and heat-treated rice starches were the worst substrate for PPA. The kinetic data, coupled with the results of physico-chemical investigations, indicated that the polymorph type, amylose content, the amount of amorphous material and granule size are important determinants of the amylolysis rate of native starches. Heat treatment at 100°C of the starches decreased the KM for all starches, which suggested that the enzyme attack is facilitated by the induced disorder (i.e. increase in the amount of amorphous material). In addition, the increase in kcat parameter suggests that the dissociation of the ES complex occurs more readily for hydrothermally treated starches. The effects of a wider range of hydrothermal treatments have been studied using wild-type and mutant pea starches. The kClIlKM ratio increased as the pre-treatment temperature increased, but each starch showed a distinct trend in this relationship and this was reflective of their gelatinisation behaviour.
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