Electrical Potentials in Trypsin Isozymes

Kizhake Soman, An Suei Yang, Barry Honig, Robert Fletterick

Research output: Contribution to journalArticlepeer-review

66 Scopus citations


Cow and rat trypsin differ in net charge by 12.5 units yet have the same enzymatic mechanism. The role of electrical potentials in the catalytic mechanism of these trypsin isozymes is investigated by using the finite difference Poisson-Boltzmann method. The calculations reveal that the active sites are effectively shielded from surface charge, thus making it possible for the two enzymes to have essentially identical potentials in their catalytically important regions. The potentials in both active sites are dominated by local interactions arising both from partial charges and from the negative charge on Asp-102. The latter is found to stabilize the transition state by about 4 kcal/mol, a value that is consistent with the extent of reduced catalytic activity in the variant Asn-102 trypsin, in which the negative charge is absent. The calculations predict that Asp-102 is ionized and that His-57 is neutral in the resting state of the enzyme. In contrast to their negligible effect on catalytic activity, the cumulative effect of surface charges is found to raise the pK of the N-terminal α-amino group of Ile-16 in the rat enzyme by about 1.5 units relative to that of cow trypsin. This charged amino acid forms an ionic bond with Asp-194, which stabilizes the active conformation of the enzyme. An increase in pK of Ile-16 thus provides a possible explanation for the retention of activity of rat trypsin at high pH. The results of this study could not have been obtained from an electrostatic model based on Coulombic potentials.

Original languageEnglish (US)
Pages (from-to)9918-9926
Number of pages9
Issue number26
StatePublished - 1989
Externally publishedYes

ASJC Scopus subject areas

  • Biochemistry


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