Some key features about this enzyme is its structure. The protein has a beta-alpha-beta structure, yielding parallel beta-sheets. These domains together provide a shape and structure for the thiamine pyrophosphate to bind to it. The dimer of pyruvate decarboxylase is stabilized by the two TPP and two Mg++ that holds them together.
The two active sites of the protein consist of the thiamine pyrophosphate group and 20 amino acids. Although the amino acids are not highly conserved for the entire protein from species to species, the amino acids at the active site are highly conserved. The two glutamic acids (477 and 51) are vital to the reaction mechanism. Glu 477 stabilizes the thiazolium ring of TPP when it binds with pyruvate. Likewise, the Glu 51 is essential for cofactor binding and catalytic activity. Studies have shown that a mutation at this residue yields little to no catalytic activity.
For regulating the enzyme, the residue that seems to trigger the substrate cascade reaction is Cysteine 221. By looking at Cys 221 in the 3D picture, you would see it lies more than 20 Angstroms away from the active site. One would wonder why something away from the active site could affect its catalytic ability. However, studies have shown that the reaction mechanism is either triggered by or inhibited by substrates that bind to Cys 221. Cys 221 also seems to have a link to Histidine 92 in sending along the substrate activation message.