It can travel through bodily fluids, targeting and inactivating enzymes responsible for cell-wall synthesis in gram-positive bacteria, meanwhile avoiding the surrounding non-targets. Penicillin can protect itself from spontaneous hydrolysis in the body in its anionic form while storing its potential as a strong acylating agent, activated only upon approach to the target transpeptidase enzyme and protonated in the active centre. This targeted protonation neutralizes the carboxylic acid moiety, which is weakening of the β-lactam ring N–C(=O) bond, resulting in a self-activation. Specific structural requirements are equated to constructing the perfect mousetrap for catching targeted prey.Penicillin kills bacteria by inhibiting the completion of the synthesis of peptidoglycans, the structural component of bacterial cell wall. It specifically inhibits the activity of enzymes that are needed for the cross-linking of peptidoglycans during the final step in cell wall biosynthesis. It does this by binding to penicillin binding proteins with the β-lactam ring, a structure found on penicillin molecules. This causes the cell wall to weaken due to fewer cross-links and means water uncontrollably flows into the cell because it cannot maintain the correct osmotic gradient. This results in cell lysis and death. Bacteria constantly remodel their peptidoglycan cell walls, simultaneously building and breaking down portions of the cell wall as they grow and divide. During the last stages of peptidoglycan biosynthesis, uridine diphosphate-N-acetylmuramic acid pentapeptide (UDP-MurNAc) is formed in which the fourth and fifth amino acids are both D-alanyl-D-alanine. The transfer of D-alanine is done (catalysed) by the enzyme DD-transpeptidase (penicillin-binding proteins are such type).
The reason is that the US FDA incorrectly assumed that the potency of penicillin V is the same mole-for-mole as penicillin G. In fact, penicillin V is less potent than penicillin G, and the current penicillin V unit reflects that fact. First international unit of penicillin V (1959). One unit of penicillin V was defined as 0.590 micrograms of a reference standard held at Mill Hill in London. This unit is now obsolete.A similar standard was also established for penicillin K.

Types
Natural penicillins
Penicillin G (benzylpenicillin) was first produced from a penicillium fungus that occurs in nature. The strain of fungus used today for the manufacture of penicillin G was created by genetic engineering to improve the yield in the manufacturing process. None of the other natural penicillins (F, K, N, X, O, U1 or U6) are currently in clinical use. Semi-synthetic penicillin
Penicillin V (phenoxymethylpenicillin) is produced by adding the precursor phenoxyacetic acid to the medium in which a genetically modified strain of the penicillium fungus is being cultured. Antibiotics created from 6-APA
There are three major groups of other semi-synthetic antibiotics related to the penicillins. They are synthesised by adding various side-chains to the precursor 6-APA, which is isolated from penicillin G. These are the antistaphylococcal antibiotics, broad-spectrum antibiotics, and antipseudomonal antibiotics. Antistaphylococcal antibiotics
Cloxacillin (by mouth or by injection)
Dicloxacillin (by mouth or by injection)
Flucloxacillin (by mouth or by injection)
Methicillin (injection only)
Nafcillin (injection only)
Oxacillin (by mouth or by injection)Antistaphylococcal antibiotics are so-called because they are resistant to being broken down by staphylococcal penicillinase.