Difference Raman spectroscopic studies of 1igandprotein interactions

We summarize here our recent advancements in application of Raman spectroscopy to studies of protein-ligand interactions. Two main experimental strategies are shown to yield a wealth of information regarding the specific binding 'handles' which contribute to protein-ligand affinity and selectivity. Non-resonance Raman difference spectroscopy, in which the vibrational spectrum of the bound ligand is obtained by subtraction of the apo-protein spectrum from that of the complex is applied to chymotrypsin and aspartate aminotransferase. In both cases, vibrational information derived from previous resonance Raman measurements is shown to be misleading due to photcchemically-induced changes in substrate conformation. Isotope editing extends the application of Raman difference spectroscopy to systems where measurements of the apo-protein spectrum is not a viable option because the bound ligand induces too many protein conformational changes that show up in the difference spectrum or in cases where the apoprotein is unstable. A difference spectrum is formed between two protein-ligand complexes, one of which is specifically labeled with a stable isotope. Vibrational modes which are associated with the isotopic tag show as spectral shifts in the difference spectrum, while all other bands cancel. The power of this method is shown by studies the binding of phosphate of GDP to EF-Tu, the elongation factor from E. Coll and of glucose-phosphate to phosphoglucomutase, PGM, where ¹⁸O labeling of phosphate reveals binding-induced changes in the vibrational modes of the phosphate moiety.