The substrate binding site of pepsin.

Abstract
Data are presented suggesting that pepsin has a primary binding site of high affinity for Z-His-Phe(NO2)-Pol [a synthetic pepsin substrate], and in addition binds the small molecule more weakly at secondary sites. That this primary binding site is located at the catalytic site of the enzyme is indicated by the finding that treatment of pepsin with the diazo ketone L-l-diazo-4-phenyl-3-tosylamido-butanone (DPTB) abolishes the high-affinity binding at the primary site. Earlier studies had shown that this diazo ketone specifically inhibits the catalytic activity of pepsin as a proteinase (substrate, hemoglobin), as a peptidase (substrate, Z-His-Phe(NO2)-Phe-OMe), and as an esterase (substrate, Z-His-Phe(NO2)-Pla-OMe); complete inactivation is achieved by the reaction of 1 molecule of the diazo ketone per molecule of pepsin.

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