title: On the relative location of the inhibitor-and calcium-binding sites in bovine trypsin as determined by nuclear magnetic resonance. Possible ambiguities in paramagnetic probe mapping studies.
abstract: Possible pitfalls in mapping studies utilizing the nuclear relaxation rates induced by paramagnetic probes are pointed out. In cases in which a distance is sought between a paramagnetic ion and a small molecule (e.g. substrate, inhibitor, etc.), both bound non-covalently to a macromolecule, heterogeneity in the system with respect to the binding of either of them may result in ambiguous conclusions. It is shown that the trypsin-gadolinium (III)-inhibitor system is heterogeneous, as revealed in the dependence of the water and inhibitor proton line-widths upon both the Gd3+ and the enzyme concentrations and in the effects of added Ca2+ on the line-widths. The results imply that in published work (Abbott et al. (1975) Biochemistry 14, 4935) the distance from a weak rather than from the strong metal ion binding site of trypsin (EC 3.4.21.4) may have been determined.
mesh_terms: D000578:Amidines / Q000378:metabolism; D000818:Animals; D001550:Benzamidines / Q000378:metabolism; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002417:Cattle; D005682:Gadolinium / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D014357:Trypsin / Q000378:metabolism*
chemical_list: D000578:Amidines; D001550:Benzamidines; D005682:Gadolinium; D014357:Trypsin; D002118:Calcium