When size is important. Accommodation of magnesium in a calcium binding regulatory domain.
The accommodation of Mg2+ in the N-terminal domain of calmodulin was followed through amide 1H and 15N chemical shifts and line widths in heteronuclear single-quantum coherence spectroscopy NMR spectra. Mg2+ binds sequentially to the two Ca2+-binding loops in this domain, with affinities such that nearly half of the loops would be occupied by Mg2+ in resting eukaryotic cells. Mg2+ binding seems to occur without ligation to the residue in the 12th loop position, previously proven largely responsible for the major rearrangements induced by binding of the larger Ca2+. Consequently, smaller Mg2+-induced structural changes are indicated throughout the protein. The two Ca2+-binding loops have different Mg2+ binding characteristics. Ligands in the N-terminal loop I are better positioned for cation binding, resulting in higher affinity and slower binding kinetics compared with the C-terminal loop II (koff = 380 +/- 40 s-1 compared with approximately 10,000 s-1 at 25 degreesC). The Mg2+-saturated loop II undergoes conformational exchange on the 100-microseconds time scale. Available data suggest that this exchange occurs between a conformation providing a ligand geometry optimized for Mg2+ binding and a conformation more similar to that of the empty loop.
Malmendal
A
A
Physical Chemistry 2, Lund University, P.O. Box 124, S-22100 Lund, Sweden. anders@bor.fkem2.lth.se
Evenäs
J
J
Thulin
E
E
Gippert
G P
GP
Drakenberg
T
T
Forsén
S
S
D000595:Amino Acid Sequence; D002118:Calcium / Q000378:metabolism*; D002147:Calmodulin / Q000737:chemistry / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D008969:Molecular Sequence Data; D011485:Protein Binding; D017433:Protein Structure, Secondary
D002147:Calmodulin; D008274:Magnesium; D002118:Calcium