The conformation of deamino-oxytocin: X-ray analysis of the 'dry' and 'wet' forms.
Two crystal structures of (1 beta-mercaptopropionic acid) deamino-oxytocin are reported. The 'dry form' in space group C2 has cell dimensions a = 27.08 +/- 0.03, b = 9.06 +/- 0.01, c = 22.98 +/- 0.02 A, beta = 102.06 +/- 0.03 with one deamino-oxytocin and six water molecules per asymmetric unit. The 'wet form' in space group P2(1) has cell dimensions a = 27.27 +/- 0.02, b = 9.04 +/- 0.01, c = 23.04 +/- 0.02 A, beta = 102.24 +/- 0.02, with two deamino-oxytocin and 13 water molecules per asymmetric unit. A local twofold parallel to the monoclinic axis gives a pseudo C2 packing. Initial phases of the 'dry form' were calculated by the heavy-atom method from the isomorphous and anomalous difference Pattersons and anomalous difference Fouier synthesis. The structure was refined by using restrained least-squares at 1.2 A resolution to a crystallographic R = 0.10. The molecular replacement method yielded the P2(1) structure that was refined with geometric restraints to R less than 0.09, by using all data to 1.09 A resolution. Deamino-oxytocin consists of a cyclic tocin ring formed by six amino acids, closed by a disulphide bridge, S1-S6, and held by two trans-annular hydrogen bonds N2-O5 and N5-O2 with a type II turn at residues 3 and 4. A flexible tripeptide tail has a loosely hydrogen-bonded type I beta-turn between N9 and O6. The sulphur of cysteine at position 1 is disordered in all the molecules leading to alternative hands of disulphide. The conformational flexibility of Ile 3, Asn 5, Pro 7 side chains and the disulphide bridge is consistent with previous models of oxytocin in which flexibility is necessary for biological activity.
Husain
J
J
Department of Crystallography, Birbeck College, University of London, U.K.
Blundell
T L
TL
Cooper
S
S
Pitts
J E
JE
Tickle
I J
IJ
Wood
S P
SP
Hruby
V J
VJ
Buku
A
A
Fischman
A J
AJ
Wyssbrod
H R
HR
D000595:Amino Acid Sequence; D003460:Crystallization; D004220:Disulfides; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010121:Oxytocin / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000302:isolation & purification; D011487:Protein Conformation; D014961:X-Ray Diffraction
D004220:Disulfides; C004748:deaminooxytocin; D010121:Oxytocin