The subsite structure of an active component of porcine pancreatic α-amylase, called PPA II, and modified PPA II (M₂) obtained with 5, 5'-dithiobis-(2-nitrobenzoic acid) (DTNB), were estimated from their mode of action and rate parameters of hydrolyses of maltooligosaccharides. These results indicated that the native and modified PPA II have five subsites with the catalytic site located between the third and fourth subsites from the nonreducing end. The subsite structure calculated for M₂ differed from that of PPA II; subsite affinities of PPA II were calculated to be 1.6, 3.9 and 2.8 kcal/mol for subsites 1, 2 and. 5, respectively, whereas corresponding values for M₂ were 1.5, 1.4 and 2.6 kcal/mol, respectively. The sum of affinities of the third and fourth subsites was calculated to be -3.6 kcal/mol for PPA II and -2.1 kcal/mol for M₂, respectively. Evidently the mode of action of PPA II was changed by the introduction of bulky TNB^- ion at or near subsite 2. The difference of the mode of action between PPA II and M₂ was interpreted in terms of the structure of each subsite.

This article cannot obtain the latest cited-by information.