Chemical‐modification rescue assessed by mass spectrometry demonstrates that γ‐thia‐lysine yields the same activity as lysine in aldolase

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Abstract
The role of active site residues in fructose 1,6‐bisphosphate aldolase is investigated by chemical‐modification rescue. An active‐site mutation, K107C, is constructed in a background where the four solvent‐accessible cysteine residues are converted to alanine. The resulting mutant, tetK107C, when reacted with bromoethylamine (BrEA), shows a 40‐fold increase in activity (to 80% that of wild type). Determination of the sites and their degree of modification using electrospray ionization Fourier transform mass spectrometry (ESI‐FTMS) is developed, allowing correlation of activity after chemical modification rescue to the degree of modification. The stoichiometry of the reaction is 2.5 aminoethylations per subunit, as measured by ESI‐FTMS. Protein modification with a double‐labeled mix (1:1) of natural abundance isotope (d0‐BrEA) and 2‐bromoethyl‐1,1,2,2‐d4‐amine hydrobromide (d4‐BrEA), followed by dialysis and trypsin digestion, shows aminoethylated peptides as “twin peptides” separated by four mass units in ESI‐FTMS analysis. Using this detection procedure under nondenaturing (native) conditions, C107 is aminoethylated, whereas the four buried thiols remain unlabeled. Aminoethylation of other residues is observed, and correlates with those peptides containing histidine, methionine, and/or the amino terminus. Quantification of the aminoethylation reaction is achieved by labeling with nondeuterated d0‐BrEA under denaturing conditions following double labeling under native conditions. In addition to complete labeling all five thiols, the intensity of the d0‐BrEA peak for C107 containing peptides increases, and the change in the d0/d4 ratio between native and denaturing conditions shows 82 ± 4.5% aminoethylation at C107. This correlation of modification with the recovered activity, indicates that γ‐thia‐lysine replaces lysine in the catalytic mechanism. Kinetic constants measured for the rescued K107C mutant enzyme with the substrates fructose 1‐phosphate and fructose 1,6‐bisphosphate are consistent with the role of the positively charged lysine binding to the C6‐phosphate. ESI‐FTMS, combined with this double‐labeling procedure, allows precise identification of sites and measurement of degree of protein modification.

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