Multiple attack mechanism in the porcine pancreatic alpha-amylase hydrolysis of amylose and amylopectin.

The action pattern of the porcine pancreatic alpha-amylase (PPA) is studied with amylose and amylopectin used as substrates. For initial stages of enzyme hydrolysis, the shape of the product chain length distribution is estimated and compared with predictions of the theory of multiple attack. The theory assumes sliding of the enzyme along the substrate polysaccharide chain without dissociation between successive cleavages. Very different initial distributions of oligomers are observed for amylose and amylopectin. In both cases, however, the theory describes the chain length distributions well. The temperature dependence of the shape of the multiple attack product distribution indicates that with rising temperature the probability of sliding increases with respect to that of dissociation. This dependence can be expected if the sliding occurs in a loose complex where enzyme and substrate are bound mainly due to hydrophobic interactions between their two continuous hydrophobic surfaces. The possible physiological role of the multiple attack in the mechanism of pancreatic amylases of mammals is discussed. The correspondence between the average length of the sliding of PPA and characteristic chain lengths of alpha-1,4-bound glucose fragments in amylopectin suggests that the degree of multiple attack of PPA is close to the optimum level required for efficient digestion of starch in the small intestine.