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Title
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Refined structure of chicken skeletal muscle troponin C in the two-calcium state at 2-A resolution.
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Authors
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K.A.Satyshur,
S.T.Rao,
D.Pyzalska,
W.Drendel,
M.Greaser,
M.Sundaralingam.
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Ref.
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J Biol Chem, 1988,
263,
1628-1647.
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PubMed id
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Abstract
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The structure of troponin C has been refined at 2A resolution to an R value of
0.172 using a total of 8,100 reflections. Troponin C has an unusual dumbbell
shape with only the two C-domain high affinity sites III and IV occupied with
metals, while the pair of N-domain low affinity sites I and II are devoid of
metals. The coordination of the Ca2+ approaches seven with the last glutamic
acid residue in each site forming an asymmetric bidentate ligand. The flanking
helices in the metal-bound EF hands are in similar orientation (both 113
degrees) while in the apo sites they are more obtuse (134 and 149 degrees). The
EF hands of holo sites III and IV are similar while the apo sites I and II are
less similar (rms for backbone atoms, 0.78 and 1.44). The half-loops of the
12-residue holo and apo sites show better agreement than the full loops
themselves, suggesting a hinge motion at the midpoint of the loops. The long
central helix is stabilized by electrostatic interactions and salt bridges
between charged side chains spaced at 3 or 4 residues along the helix. A cluster
of water molecules encircle the long helix and hydrogen bond to the backbone
carbonyls. At the beginning of the B-helix, a water molecule is interposed at
each of two consecutive backbone NH...OC hydrogen bonds. The terminal pair of
helices A/D (apo) match with E/H (holo), and the internal pair of helices B/C
(apo) match with F/G (holo). Thus, muscle contraction may be triggered by Ca2+
binding to loops I and II which results in a concerted rearrangement of residues
in the loops, including the essential Gly at position 6 in each loop. This
rearrangement than causes a reorientation of helices B and C along with the BC
linker.
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