title: Study of the interaction of 4-bromomercuriocinnamic acid with alpha-chymotrypsin by 79 Br and 81Br pulsed nuclear-magnetic resonance.
abstract: Br- has been used as a nuclear magnetic resonance (NMR) probe to study the reversible association of alpha-chymotrypsin and an Hg-labelled substrate (4-bromomercuriocinnamic acid, BrHgCin) which rapidly exchanges Br-. T1 was measured for 79Br and 81Br, using a pulse spectrometer. Values of the parameters that determine T1, Obs in aqueous solutions of KBr (pH=5.5) containing alpha-chymotrypsin and BrHgCin are reported. It is found that the rate of Br exchange is diffusion-limited and faster than the rate of reorientation of the BrHgCin-alpha-chymotrypsin complex. The rate constant for the formation of the covalent BrHgCin-alpha-chymotrypsin complex determined by this technique agrees well with previously published data. The rapid rate of Br exchange with the complex, however, is incompatible with the side chain of BrHgCin being entirely buried in a nonpolar pocket on the enzyme but compatible with the side chain being exposed to the solution. The contribution to the NMR signal from the non-covalent complex is negligible.
mesh_terms: D001665:Binding Sites; D001966:Bromine; D002918:Chymotrypsin / Q000378:metabolism; D002934:Cinnamates; D007554:Isotopes; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008433:Mathematics; D008968:Molecular Conformation; D009941:Organomercury Compounds; D009942:Organometallic Compounds / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation
chemical_list: C012794:4-bromomercuriocinnamic acid; D002934:Cinnamates; D007554:Isotopes; D009941:Organomercury Compounds; D009942:Organometallic Compounds; D002918:Chymotrypsin; D001966:Bromine