Quantum chemistry studies of the catalysis mechanism differences between the two isoforms of glutamic acid decarboxylase.

The production of gamma-aminobutyric acid (GABA) is catalyzed by two isoforms of glutamic acid decarboxylase (GAD), using pyridoxal 5'-phosphate (PLP) as the cofactor. Between the two enzymes, GAD67 accounts for normal GABA requirement, while GAD65 stays inactive until emergent demand for GABA. Recent crystal structure findings revealed that the distinct conformation of a common catalytic loop of the enzymes may account for their different functions (Fenalti et al Nat Struct Mol Biol, 14:280-286, 2007). Enlightened by their inferences, we studied the underlying reaction mechanism of the two GAD isoforms using density functional theory (DFT). A rather complete reaction pathway is identified, including nine transition state (TS) structures and 14 intermediate (IM) structures. The rate limiting step occurs early during the reaction and involves a proton transfer. In the late stage, there are two pathways that involve C(4') and C(α) protonation by Tyr or Lys. Our calculations show that the reaction barriers corroborate the conjecture made by Fenalti et al.