FIG 8.

Mass spectrometry of PenI and the C69F variant with ceftazidime (error, ±3 amu). (A) Ceftazidime (10 μM) and PenI (10 μM) at a 1:1 ratio revealed only apo-enzyme (28,348 amu) after 1 h. However, at a 100:1 ratio of ceftazidime to PenI, the acyl-enzyme (28,348 amu + 467 amu = 28,815 amu) was observed within 1 min. (B) Ceftazidime (10 μM) and the C69F variant (10 μM) at a 1:1 ratio over a time course of 15 s to 15 min revealed acyl-enzyme formation (28,857 amu) within 15 s and returned to complete apo-enzyme (28,390 amu) by 15 min.