Structural studies of binding site tryptophan mutants in the high-affinity streptavidin-biotin complex

被引:76
作者
Freitag, S
Le Trong, I
Chilkoti, A
Klumb, LA
Stayton, PS
Stenkamp, RE
机构
[1] Univ Washington, Dept Biol Struct, Seattle, WA 98195 USA
[2] Univ Washington, Biomol Struct Ctr, Seattle, WA 98195 USA
[3] Univ Washington, Dept Bioengn, Seattle, WA 98195 USA
关键词
molecular recognition; streptavidin; biotin; protein-ligand interactions; X-ray structure;
D O I
10.1006/jmbi.1998.1735
中图分类号
Q5 [生物化学]; Q7 [分子生物学];
学科分类号
071010 ; 081704 ;
摘要
Previous thermodynamic and computational studies have pointed to the important energetic role of aromatic contacts in generating the exceptional binding free energy of streptavidin-biotin association. We report here the crystallographic characterization of single site tryptophan mutants in investigating structural consequences of alterations in these aromatic contacts. Four tryptophan residues, Trp79, Trp92, Trp108 and Trp120, play an important role in the hydrophobic binding contributions, which along with a hydrogen bonding network curd a flexible binding loop give rise to tight ligand binding (K-a similar to 10(13) M-1). The crystal structures of ligand-free and biotin-bound mutants, W79F, W108F, W120F and W120A, in the resolution range from 1.9 to 2.3 Angstrom were determined. Nine data sets for these four different mutants were collected, and structural models were refined to X-values ranging from 0.15 to 0.20. The major question addressed here is how these mutations influence the streptavidin binding site and in particular how they affect the binding mode of biotin in the complex. The overall folding of streptavidin was not significantly altered in any of the tryptophan mutants. With one exception, only minor deviations in the unbound structures were observed. In one crystal form of unbound W79F, there is a coupled shift in the side-chains of Phe29 and Tyr43 toward the mutation site, although in a different crystal form these shifts are not observed. In the bound structures, the orientation of biotin in the binding pocket was not significantly altered in the mutant complex. Compared with the wild-type streptavidin-biotin complex, there were no additional crystallographic water molecules observed for any of the mutants in the binding pocket. These structural studies thus suggest that the thermodynamic alterations can be attributed to the local alterations in binding residue composition, rather than a rearrangement of binding site architectures. (C) 1998 Academic Press Limited.
引用
收藏
页码:211 / 221
页数:11
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