Tryptophan fluorescence of mitochondrial uncoupling protein

Area of Science:

• Biophysics
• Biochemistry
• Membrane Protein Dynamics

Background:

• Mitochondrial uncoupling protein (UcP) features two key tryptophans (Trp-173, Trp-280) within transmembrane alpha-helices.
• Isolated UcP's tryptophan emission shows properties atypical for alpha-helices, including proximity to hydrophilic residues and potential water interfaces.

Purpose of the Study:

• To investigate the structural and dynamic properties of tryptophans within mitochondrial uncoupling protein (UcP).
• To analyze the impact of conformational changes on UcP's tryptophan emission characteristics.

Main Methods:

• Steady-state and time-correlated single-photon counting (TCSPC) fluorescence spectroscopy.
• Analysis of tryptophan emission spectra, quantum yield, and decay kinetics.
• Investigating the effects of N-ethylmaleimide modification on UcP structure and dynamics.

Main Results:

• UcP tryptophans display emission properties suggesting water accessibility and dynamic "breathing" of transmembrane segments.
• Fluorescence decay analysis reveals components similar to free tryptophan in water, indicating an "anionic" conformation.
• N-ethylmaleimide modification altered UcP dynamics and reduced quantum yield, without changing emission spectra, but increased tyrosine exposure.

Conclusions:

• The unusual tryptophan emission in UcP points to non-ideal alpha-helical structures and significant dynamic motion.
• An "anionic" tryptophan conformation likely exists within UcP, potentially stabilized by charge-transfer complexes.
• UcP's conformational flexibility and water interactions are crucial for its function and are modulated by chemical modifications.