Lack of Negative Charge in the E46Q Mutant of Photoactive Yellow Protein Prevents Partial Unfolding of the Blue-Shifted Intermediate

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Abstract
The long-lived light-induced intermediate (pB) of the E46Q mutant (glutamic acid is replaced by glutamine at position 46) of photoactive yellow protein (PYP) has been investigated by NMR spectroscopy. The ground state of this mutant is very similar to that of wild-type PYP (WT), whereas the pB state, formed upon illumination, appears to be much more structured in E46Q than in WT. The differences are most striking in the N-terminal domain of the protein. In WT, the side-chain carboxylic group of E46 is known to donate its proton to the chromophore upon illumination. The absence of the carboxylic group near the chromophore in the E46Q mutant prohibits the formation of a negative charge at this position upon formation of pB. This prevents the partial unfolding of the mutant, as evidenced from NMR chemical shift comparison and proton/deuterium (H/D) exchange studies.