On the mechanism of the reaction of tris(hydroxymethyl)aminomethane with activated carbonyl compounds: A model for the serine proteinases |
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Authors: | John de Jersey Ann K. Fihelly Burt Zerner |
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Affiliation: | Department of Biochemistry, University of Queensland, St. Lucia, Queensland, Australia 4067 |
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Abstract: | The pH dependence of the reaction of tris(hydroxymethyl)aminomethane (Tris) with the activated carbonyl compound 4-trans-benzylidene-2-phenyloxazolin-5-one (I) is given by the equation , where Ka is the dissociation constant of TrisH+. Spectrophotometric experiments show that the Tris ester of α-benzamido-trans-cinnamic acid is formed quantitatively over a range of pH values, regardless of the relative contribution of kb and ka terms to k′2. Hence, both terms refer to alcoholysis. While the mechanism of the reaction is not determined unequivocally in the present work, the magnitude of the kb term, together with its dependence on the basic form of Tris, suggests that ester formation is occurring by nucleophilic attack of a Tris hydroxyl group on the carbonyl carbon of the oxazolinone, with intramolecular catalysis by the Tris amino group. The rate enhancement due to this group is at least 102 and possibly of the order 106. This system is compared with other model systems for the acylation step of catalysis by serine esterases and proteinases. |
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