Pepsin activation energy

Selected entries from Methods in Enzymology [vol, page(s)] Pepsin Activation energy, 63, 243-245 isotope exchange, 64, 10 kinetic constants, 63, 244 kinin-releasing enzyme, 80, 174 porcine, homology to cathepsin D, 80, 578 spectrokinetic probe,... 

Unless the catalytic surface of the enzyme is altered, an environmental change should theoretically have no effect on the activation energy Lipase, trypsin, and pepsin follow the Arrhenius equation, but below 0°... 

PAMPS, 107-9, 117, 118, 265, 266, 302, 303 activation energy, 307 electrical conductivity, 304-6 electrical resistance, 304 molar conductivity, 305-6 PAN, 247, 261 Particle-particle gap, 327 PDMS, 169 Pepsin, 139... 

Kinetic experiments demonstrate that the extended active site cleft of pepsins (Tang et al, 1978) accommodates at least seven amino acids and that increasing chain length from two to six residues has a major effect on kcatvalues and not on Km(Allen et al., 1990). Energies of activation have been estimated to be at the range of 7-9 kcal/mol for good substrates and 13-14 for lesser ones, in a series of blocked peptides where the lysine-nitrophenylalanine peptide bond is cleaved. 

In our glycine 76 study (Okoniewska et al., 2000), this position was substituted with alanine, valine, and serine. These amino acids differ in their van der Waals volumes, accessible surface areas, polarities, and allowable energy levels on Ramachandran plots for individual amino acids. Rate constants for the activation process were calculated for the mutants and the wild-type enzymes at pH 1.1, 2.0, and 3.0. Samples were taken at different activation times, quenched, and the amount of pepsin formed was determined with synthetic substrate I. Activation rate constants presented in Table 15.5 corresponded to first-order reaction constants. All the mutants activated at rates slower than those for the wild-type, regardless of amino acid size and polarity. At all pH conditions, the activation reactions were slowest for valine and serine mutants, which had comparable reaction rates. The alanine mutant activated more slowly than did the wild-type but was faster than the other two mutants. 

If this should be the case, either apparent acyl transfer or amine transfer would be possible by a direct condensation of the preferentially retained product and an acceptor that can readily displace the product that leaves more easily. That such condensation reactions are catalyzed by pepsin in the case of oligopeptides was demonstrated many years ago (54), and is consistent with the neglible free energy decrease in the hydrolysis of interior peptide bonds (55). For transpeptidation reactions in which an apparent E-Tyr amino-enzyme has been postulated, the free energy change in the condensation of an acceptor such as Ac-Phe with tyrosine would be more unfavorable in free solution, but the possibility must be considered that the ammonium pKa of the tyrosine retained at the active site may be lower than that of tyrosine in free solution, perhaps by virtue of the interaction of the carboxylate group of the amino acid with a complementary cationic group of the active site. 

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