The β-lactam-recognizing enzymes (BLRE) make up a superfamily of largely bacterial proteins that include, principally, the dd-peptidases and β-lactamases. The former enzymes catalyze the final step in bacterial cell wall biosynthesis and are inhibited by β-lactam antibiotics, while the latter enzymes catalyze the hydrolytic destruction of β-lactams and represent a major source of bacterial resistance to these antibiotics. The active site of this superfamily of enzymes includes a Ser1/Ser2(Tyr)/Lys1(His)/Lys2 tetrad in which Ser1 is a nucleophilic catalyst that becomes acylated in the formation of an acyl-enzyme intermediate. An oxyanion hole is also present. The amidase signature (AS) enzymes represent another serine amidohydrolase superfamily with no overall structural resemblance to the BLRE. The active site is characterized by a Ser1/Ser2/Lys1/NH tetrad and an oxyanion hole. We point out that there is a close spatial overlap between the two tetrads and speculate that this has arisen from a process of convergent evolution driven by a mechanistic imperative. Conversion of the backbone NH group of the AS tetrad into Lys2 of the BLRE is rationalized and leads to another mechanistic possibility that may dominate BLRE catalysis. The active site triads of other serine amidohydrolases are also briefly and comparatively discussed.