Density functional calculations are employed to explore the mechanisms of all elementary reaction steps involved in the catalytic cycle of pyruvate decarboxylase (PDC). Different models are constructed for mimicking the involvement of some key residues in a certain step. The effect of the protein framework on the potential energy profiles of active site models is approximately modeled by fixing some freedoms, based on the crystal structure of the PDC enzyme from Saccharomyces cerevisiae (ScPDC). Our calculations confirm that Glu51 is the most important residue in the formation of the ylide and the release of acetaldehyde via the proton relay between Glu51, N-1',N- and the 4'-amino group of thiamine diphosphate. The presence of Glu477 and Asp28 residues makes the decarboxylation of lactylthiamin diphosphate (LThDP) an endothermic process with a significant free energy barrier. The protonation of the alpha-carbanion to form 2-(1-hydroxyethyl)-thiamin diphosphate is found to go through a concerted double proton transfer transition state involving both Asp28 and His 115 residues. The final step, acetaldehyde release, is likely to proceed through a concerted transition state involving carbon-carbon bond-breaking and the deprotonation of the alpha-hydroxyl group. The decarboxylation of LThDP and the protonation of the alpha-carbanion are two rate-limiting steps, relative to the facile occurrence of the ylide formation and acetaldehyde release. The catalytic roles of residues Glu51, Glu477, Asp28, and Gly417 in the active site of ScPDC in individual steps elucidated from the present study are in good agreement with those derived from site-directed mutagenesis.