Orally administered penicillins are absorbed by a dipeptide transport system in the small intestine. Most penicillins are actively secreted by the renal tubules because of the presence of the C3 carboxylic acid. Amoxicillin is absorbed to a greater extent than is ampicillin because amoxicillin is more acid stable than is ampicillin. Part of the reason that the penicillin binds in the active site of the transpeptidase is that the thiazolidine ring mimics the terminal D-alanine of the peptide chain in the normal substrate for this enzyme. Stereochemical inversion of valine during the biosynthesis of Pen G is required so that the C3 position on the penicillin will mimic the stereochemistry of the terminal D-alanine of the peptide chain in the normal substrate for this enzyme.
Penicillinases are B-lactamases.
Transpeptidases were probably evolutionarily derived from penicillinases. The use of penicillinase to terminate an allergic reaction to penicillin x is used therapeutically based on the logical assumption that, if the person is reacting to the penicillin molecule, it would be logical to terminate the reaction by destroying the penicillin. While simple C3-esters are hydrolyzed by esterases, these more lipophilic esters are there simply to enhance absorption. While simple C3-esters are hydrolyzed by esterases, these more lipophilic esters are there simply to enhance tissue penetration. While simple C3-esters are hydrolyzed by esterases, these more lipophilic esters are there simply to enhance solubility.