Figure 8.

(A) PheATE loading competition experiment between l-Leu, l-Tyr, and l-Trp. Each amino acid was added at a final concentration of 50 μM with 2 mM ATP and 1 nM SrfA-D. The lines indicate the peak corresponding to each amino acid (shown is the region containing the 14⁺ charge states of the CNBr digestion product). The percentage of each form for the entire spectrum is as follows: holo (20%), Leu (10%), Tyr (5%), and Trp (65%). An * indicates an adduct peak. (B) Dipeptide competition experiment. A quadrupole-enhanced region of the spectrum in which the 16⁺ charge states of ProCAT peptides and 15⁺ charge states for PheATE peptides are observed. d-Leu, d-Tyr, and d-Trp were added to PheATE in equimolar concentration and then allowed to condense onto Ala-S-ProCAT for 10 min.