Davis JP, Rall JA, Reiser PJ, Smillie LB, Tikunova SB. Engineering competitive magnesium binding into the first EF-hand of skeletal troponin C.
J Biol Chem 2002;
277:49716-26. [PMID:
12397067 DOI:
10.1074/jbc.m208488200]
[Citation(s) in RCA: 31] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The goal of this study was to examine the mechanism of magnesium binding to the regulatory domain of skeletal troponin C (TnC). The fluorescence of Trp(29), immediately preceding the first calcium-binding loop in TnC(F29W), was unchanged by addition of magnesium, but increased upon calcium binding with an affinity of 3.3 microm. However, the calcium-dependent increase in TnC(F29W) fluorescence could be reversed by addition of magnesium, with a calculated competitive magnesium affinity of 2.2 mm. When a Z acid pair was introduced into the first EF-hand of TnC(F29W), the fluorescence of G34DTnC(F29W) increased upon addition of magnesium or calcium with affinities of 295 and 1.9 microm, respectively. Addition of 3 mm magnesium decreased the calcium sensitivity of TnC(F29W) and G34DTnC(F29W) approximately 2- and 6-fold, respectively. Exchange of G34DTnC(F29W) into skinned psoas muscle fibers decreased fiber calcium sensitivity approximately 1.7-fold compared with TnC(F29W) at 1 mm [magnesium](free) and approximately 3.2-fold at 3 mm [magnesium](free). Thus, incorporation of a Z acid pair into the first EF-hand allows it to bind magnesium with high affinity. Furthermore, the data suggests that the second EF-hand, but not the first, of TnC is responsible for the competitive magnesium binding to the regulatory domain.
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