The mechanism of the unusual epimerization and desaturation reactions catalyzed by carbapenem synthase was investigated using the hybrid density functional method B3LYP. Several different models have been used in the calculations to study five component reactions. Both protonated and deprotonated models for the substrate have been explored so that the effects of hydrogen bonds could be characterized. Besides the iron site, it is proposed that a some tyrosine residue, possibly Tyr67, is involved in the hydrogen abstraction step. The calculated energetics and barrier heights support this hypothesis, and are consistent with the known experimental data concerning CarC and other 2-oxoglutarate dependent dioxygenases.