In Gram-negative bacteria, e.g. Escherichia coli, PDC consists of a central cubic core made up from 24 molecules of dihydrolipoyl transacetylase (E2).
Up to 16 homodimers of pyruvate dehydrogenase (E1) and 8 homodimers  of dihydrolipoyl dehydrogenase (E3) bind to the E2 PSBD around the E2 core.
In Gammaproteobacteria, the specificity of PSBD for binding either E1 or E3 is determined by the oligomeric state of PSBD. In each E2 homotrimer, two of the three PSBDs dimerize. While two E1 homodimers cooperatively bind dimeric PSBD, the remaining, unpaired PSBD specifically interacts with one E3 homodimer. PSBD dimerization thus determines the subunit composition of the pyruvate dehydrogenase complex when fully saturated with the peripheral subunits E1 and E3, which has a stoichiometry of E1:E2:E3 (monomers) = 32:24:16