Aspartate aminotransferase (AATase) and aminocyclopropane carboxylate synthase (ACC synthase) are pyridoxal phosphate (PLP)-dependent enzymes whose common junction of mechanistic divergence is after the formation of a C a carbanion from the amino acid substrate bound to PLP as a Schiff base (aldimine). AATase catalyzes the reversible interconversion of a-amino acids and a-keto acids, while ACC synthase effects the irreversible decomposition of S-adenosylmethionine (SAM) to 1-aminocyclopropane-1-carboxylate (ACC) and 5'methylthioadenosine (MTA). ACC is subsequently converted to ethylene, the plant ripening and senescence hormone, by ACC oxidase, the next enzyme in the pathway. AATase and ACC synthase exhibit many similar phenomenological characteristics that result from different detailed mechanistic origins. The k cat /K M versus pH profiles for both enzymes are similar (AATase, acidic pK a = 6.9, basic pK a = 9.6; ACC synthase, acidic pK a = 7.5, basic pK a = 8.9); however the acidic pK a of AATase reflects the ionization of an enzyme proton from the internal Schiff base, and the basic one is that of the a-amino group of the substrate, while the opposite situation obtains for ACC synthase, i.e. the apparent pK a of 7.4 is due to the a-amino group of SAM, whereas that of 9 reflects the Schiff base pK a . The mechanistic imperative underlying this reversal is dictated by the reaction mechanism and the low pK a of the a-amino group of SAM. The low pK a of SAM requires that the enzyme pK a be moved upward in order to have sufficient quantities of the reacting species at neutral pH. It is shown by viscosity variation experiments with wildtype and active site mutant controls of both enzymes that the reaction of SAM with ACC synthase is 100% diffusion controlled (k cat /K M = 1.2 Γ 10 6 l mol Γ1 s Γ1 ) while the corresponding reaction for the combination of L-aspartate with AATase is insensitive to viscosity, and is therefore chemically not diffusion limited. Tyr225 (AATase) or Tyr233 (ACC synthase) forms a hydrogen bond with the PLP in both enzymes, but that formed with the former enzyme is stronger and accounts for the lower pK a of the Schiff base.