THESIS
THESIS
THESIS
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Structure and properties: The pullulanases appear to be composed of<br />
a single polypeptide chain (A. aerogenes, molecular weight 143,000), but nothing is<br />
yet known of the three dimensional structure adopted by the latter. Certain tryptophan<br />
groups may be involved in the catalytic action of pullulanase (Jensen and Norman,<br />
1984).<br />
The pH and temperature optimum: Pullulanases and isoamylases<br />
have pH optima for activity between 5 to 7, and temperature optimum between 45 to<br />
55 o C. The presence of a Ca 2+ ion seems to improve the activity of these enzymes,<br />
whereas heavy metals such as Hg 2+ , Zn 2+ , Cu 2+ , Ag 2+ and Co 2+ exert a more or less<br />
marked inhibitory action (Jensen and Norman, 1984).<br />
Action pattern: The pullulanases hydrolyze the α-(1,6) linkages of<br />
starch, glycogen, pullulan, and limit dextrins. However, the precise location of these<br />
α-(1,6) linkages is relatively important, since it strongly influences the ability of the<br />
enzyme to act on this substrates. In particular, the presence of two α-(1,4) linkages<br />
adjacent to the α-(1,6) linkages to be hydrolyzed (6 2 -O-α- maltotriose) is necessary<br />
Kennedy et al. (1987). The isoamylases show some differences when compared to the<br />
pullulanases. They are unable to hydrolyze pullulan and cannot cleaveα-(1,6) linkages<br />
in molecules containing less than three α-(1,4) linkages (Table 9).<br />
Table 9 Characteristics of pullulanase and isoamylases enzymes<br />
Characteristics<br />
Pullulanase<br />
Type of enzymes<br />
A. aerogenes B. cereus<br />
Isoamylase<br />
Cytophaga sp.<br />
Molecular weight 143,000 112,000 120,000<br />
pH optimum 6.5 6.0-6.5 5.0-6.0<br />
Temperature optimum ( o C ) 50 55 45<br />
Pullulan hydrolysis + + -<br />
Source: Kennedy et al. (1987)<br />
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