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J. Biol. Chem., Vol. 255, Issue 5, 2036-2038, Mar, 1980

Direct observation of peptide exchange by stable isotope enrichment

CH Niu, H Shindo, S Matsuura and JS Cohen

A method for the kinetic determination of peptide exchange using stable isotope enrichment is described. Synthetic 90% enriched (epsilon- 13C)His 12 ribonuclease (RNase) (1-15) peptide was used as a probe to follow peptide exchange in the RNase S system by 13C nuclear magnetic resonance spectroscopy. The rate constant, k1, for dissociation of the RNase S complex containing the synthetic (1-15) peptide was found to be (4.1 +/- 0.3) x 10(-4) s-1 and its dissociation constant Kd, (0.2 +/- 0.8) x 10(-7) M, was greater than that of RNase S with natural S- peptide (residues 1 to 20) by a factor of five at 4 degrees C. This differences corresponds to the difference of the enthalpy of binding between the (1-15) and (1-20) peptides, which we determined to be 1.7 +/- 0.4 kcal/mol. This small enthalpy difference may originate from hydrogen bonding between Ser 16 and His 48 in the RNase S complex.
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