Insights on Structural, Environmental and Dynamical Behaviour of Multi-Tryptophan Proteins by Analysis of Their Low Temperature Phosphorescence Spectra: A Review
Author(s): Priyanka Mukherjee, Sanjukta Chatterjee, Sourav Banerjee, Sudeshna Shyam Chowdhury, Rina Ghosh
The individual amino acids of proteins can experience a variation in their local environments because of the folded tertiary/quaternary structure of the former. These microenvironments are regions that differ from one another with respect to polarizability, polarity and lability of the residues. The three intrinsic fluorescent probes are Tyrosine (Tyr), Tryptophan (Trp) and Phenylalanine (Phe) which form a part of protein backbone. Fluorescence of Phe is weak and rarely used in protein studies. On the other hand, Trp fluorescence is strong. Therefore, most studies of intrinsic protein fluorescence focus on Trp residues, and it provides specific and sensitive information on the structure of proteins and their interaction with probes. In steady-state fluorescence spectra, the shift in λmax of Tryptophan to blue or red regions of the spectrum indicates the solvent accessibility of the former – a red-shifted band indicating significant solvent exposure. Though Trp fluorescence in proteins is an important tool to understand the structural and dynamical evolution of a protein, it is difficult to ascertain the contribution of individual Trp to the total fluorescence of the former. Usually steady-state fluorescence spectra are broad and even a protein with a single Trp residue can have a multi-exponential fluorescence decay upon time resolution. So, low temperature phosphorescence studies (LTP) at 77K in a suitable cryosolvent is an important tool from structural and biological aspects. LTP gives a structured spectra with definite (0-0) band characteristics of the Trp environment. In phosphorescence, a blue-shifted maximum is an indication of solvent exposed Trp whereas a red-shifted maximum indicates a buried Trp. Several proteins containing more than one Trp residues are known to exhibit multiple (0,0) bands. Usually, the no. of bands should correspond to the no. of Trp residues present in the protein. Non-correspondance between the two (no of Trp residues and no of bands) may be attributed to energy transfer to another residue, interaction with neighbouring residues or electron transfer from excited state. This review is an attempt to focus and analyze the low temperature phosphorescence spectra of multi-tryptophan proteins which will eventually throw light on the structure and function of such proteins or protein-substrate complexes. LTP of mutated proteins have also been reported. We have also documented the theoretical modeling studies which help us to understand the assignment of multiple (0-0) bands corresponding to different Trps in multi-Trp proteins.