Figure 4.

Evidence for noncovalent association for amino acid substrates retained throughout protein purification. Shown is the region of the spectra containing 13⁺ ions (though all charge states were combined for quantitation) for the CNBr digestion peptides of PheATE after being treated with 0 (A) and 2 mM (B) ATP for 30 min. The region of the spectrum containing the 14⁺ charge states of the CNBr digest peptides of ProCAT following treatment with 0 (C) and 1 mM (D) ATP for 30 min; * indicates adduct peak.