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Aminotryptophan-containing barstar: structure--function tradeoff in protein design and engineering with an expanded
Marina Rubini1, Sandra Lepthien, Ralph Golbik
1Max-Planck-Institut für Biochemie, Am Klopferspitz 18, D-82152 Martinsried, Germany.
Biochimica Et Biophysica Acta
|June 20, 2006
Summary
Substituting tryptophan with aminotryptophan in proteins creates variants with unique pH-sensitive charge transfer properties. However, this novel function comes at the cost of significantly reduced protein stability and altered folding dynamics.
Area of Science:
- Biochemistry
- Molecular Biology
- Protein Engineering
Background:
- Tryptophan (Trp) has unique properties due to its indole ring.
- Aminotryptophan analogs (4-NH2)Trp and (5-NH2)Trp possess distinct hydrophobicity and spectral characteristics.
- These analogs resemble DNA purine bases and exhibit pH-sensitive charge transfer.
Purpose of the Study:
- To investigate the functional and structural consequences of incorporating aminotryptophans into proteins.
- To explore the protein engineering potential of an expanded genetic code.
- To analyze the structure-function trade-offs associated with novel amino acid substitutions.
Main Methods:
- Site-directed mutagenesis and ribosomal incorporation of (4-NH2)Trp and (5-NH2)Trp into barstar.
- X-ray crystallography to determine the structure of substituted barstar.
- Spectroscopic and thermodynamic analyses (e.g., Tm determination, unfolding cooperativity) to assess protein stability and folding.
- Kinetic studies to analyze protein folding pathways.
Main Results:
- Crystal structure of (4-NH2)Trp-barstar is similar to wild-type barstar.
- (4-NH2)Trp- and (5-NH2)Trp-barstar show significantly altered spectral properties and thermodynamic stability.
- A decrease in Tm by ~20°C, reduced unfolding cooperativity, and loss of folding free energy were observed.
- Folding kinetics indicated a preference for the denatured state in (4-NH2)Trp-barstar.
Conclusions:
- Incorporating aminotryptophans into barstar introduces pH-sensitive charge transfer functionality at the expense of protein stability.
- This demonstrates a clear structure-function trade-off in protein engineering.
- Findings offer insights into amino acid repertoire evolution and challenges in expanding the genetic code for protein design.