Structure of a new polymorphic monoclinic form of human transthyretin at 3angstrom resolution reveals a mixed complex between unliganded and T-4-bound tetramers of TTR
A. Wojtczak et al., Structure of a new polymorphic monoclinic form of human transthyretin at 3angstrom resolution reveals a mixed complex between unliganded and T-4-bound tetramers of TTR, ACT CRYST D, 57, 2001, pp. 957-967
The crystal structure of a new polymorphic form of human transthyretin (hTT
R) with a lattice containing a unique assembly of apo hTTR and TTR-T-4 comp
lex has been determined to 3 Angstrom resolution. The monoclinic form of hu
man TTR reported here crystallizes in space group P2(1), with unit-cell par
ameters a = 76.7 (6), b = 96.7 (8), c = 81.7 (4) Angstrom, beta = 106.8 (4)
degrees. The asymmetric unit contains two tetramers of transthyretin relate
d by the non-crystallographic symmetry (NCS) operation of a 90.28 degrees r
otation between two hTTR molecules around an axis close to crystallographic
z. The r.m.s. difference between the two tetramers calculated from their C
-alpha positions is 0.48 Angstrom. The structure was refined using 15.0-3.0
Angstrom resolution data to R = 22.9% and R-free = 28.9% for reflections F
>0.0 sigma (F), and R = 19.7% and R-free = 25.8% for reflections F >3.0 sig
ma (F). The intermolecular interactions involve the tips of alpha -helices
and loops around Arg21, Glu61 and Ser100 of all monomers. The electron-dens
ity maps revealed residual thyroxine (T-4) bound in only one of the two uni
que tetrameric TTR molecules, with an occupancy of 53%, while the second te
tramer is unliganded. One thyroxine ligand is bound in a way similar to the
orientations described for the orthorhombic form of the hTTR-T-4 complex.
The T-4 bound in the second site is positioned similar to 3',5'-dinitro-N-a
cetyl-L-thyronine in its hTTR complex. Differences in the size of the centr
al channel defined by the D, A, G and H beta -strands of two monomeric subu
nits are observed between the apo TTR and T-4-bound tetramer. The averaged
distances between Ala108 C-alpha and its equivalent measured across each bi
nding site are 12.34 Angstrom for the T-4-bound and 10.96 Angstrom for the
unliganded TTR tetramer, respectively. The observed differences might refle
ct the mechanics of the ligand binding in the channel and possibly explain
the observed negative cooperativity effect for ligand binding.