Proton NMR spectroscopic studies of calcium-binding proteins. 1. Stepwise proteolysis of the C-terminal .alpha.-helix of a helix-loop-helix metal-binding domain

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Abstract
A series of modified parvalbumins, differing only in length of .alpha.-helix F at the C-terminus, was prepared by carboxypeptidase-mediated digestions of the .beta.-lineage parvalbumin (pI = 4.25) from carp (N; 108 residues). Removal of Ala-108 to form the N-1 derivative (des-Ala108-parvalbumin) or removal of Ala-108 and Lys-107 to form the N-2 derivative (des-Ala108,Lys107-parvalbumin) only slightly alters the protein''s ability to chelate Ca(II) or lanthanides(III). Analysis of the kinetics of their Yb(III) off-rates by optical stopped-flow techniques, determination of their Lu(III)-binding constants by high-resolution 1H NMR methods, and inspection of their solution structures by Yb(III)-shifted 1H NMR techniques indicate N-1 and N-2 are very similar to (0.1-0.2 M KCl; pH 6-7; 23-55.degree. C). However, removal of the next one or two residues, Val-106 or Val-106/Leu-105, to generate the N-3 and N-4 derivatives severely alters the metal ion binding characteristics of the protein. Although two Yb(III) off-rates are observed for N-3, both are faster than that for the unmodified protein: kCD by a factor of 2 and kEF by a factor of 2200. Removal of Ala-104 and Ala-104/Thr-103 to give a mixture of N-5 and N-6 derivatives eliminate the slow-release site altogehter, the single observable koff being 20-30 times faster than release of Yb(III) from the CD site of native parvalbumin. Removal of the C-terminal .alpha.-helix by digestion through Phe-102 to give N-7 destabilizes the entire protein structure as judged both by the random-coil appearance of its 1H NMR spectrum and by its aberrant kinetics. Although one abnormally fast koff is still observed at micromolar concentrations, Ln(III) chelation tends to precipitate N-7 at higher parvalbumin concentrations (1-3 mM). In contrast to the critical instability of the N-3 through N-7 derivatives, the remarkable stability of the N-1 and N-2 forms of carp parvalbumin may be attributed to the maintainance to two key structural features: an ion pair bond between the negatively charged C-terminal carboxyl function and the protonated .epsilon.NH3+ of Lys-27 and hydophobic interactions of the inner side of helix F with residues in the protein''s core.