As shown in the above diagram, penicillin is not a single compound but a group of
closely related compounds, all with the same basic ring-like structure (a β-lactam)
derived from two amino acids (valine and cysteine) via a tripeptide intermediate. The
third amino acid of this tripeptide is replaced by an acyl group (R in the diagram
below) and the nature of this acyl group produces specific properties on different
types of penicillin.