Pyridoxal phosphate transimination

According to O Leary the kl2/k13 KIE (for loss of C02) varies between 1.01 and 1.03 for a variety of pyridoxal phosphate-dependent decarboxylases141. Assuming (based on models) that the intrinsic KIE is ca 1.05 for the decarboxylation step k5, these numbers suggest that decarboxylation is not totally rate-determining, rather Schiff-base interchange (transimination) and C—C scission are both participating in rate-limitation ( 4 k5) see Scheme 13. 

Thiamine pyrophosphate (TPP) is the coenzyme required by enzymes that catalyze the transfer of a two-carbon fragment. Biotin is the coenzyme required by enzymes that catalyze carboxylation of a carbon adjacent to a carbonyl group. Pyridoxal phosphate (PLP) is the coenzyme required by enzymes that catalyze certain transformations of amino acids decarboxylation, transamination, racemiza-tion, C —Cp bond cleavage, and a,j8-elimination. In a transimination reaction, one imine is converted into another imine in a transamination reaction, the amino group is removed from a substrate and transferred to another molecule. 

Preexistence of a Schiff base between pyridoxal phosphate and the enzyme may account for the greatly enhanced rates of enzymatic reactions is compared to the rates of the corresponding non-enzymatic reactions. Subsequent reactions of the enzyme with amino acids must involve Schiff base formation via a fast transimination step (see section IV.D). Once the new Schiff base is formed, the e-amino group of the lysine residue that was originally bound to pyridoxal phosphate is free and is in a favorable position to act as a catalyst in subsequent steps of the enzymatic reaction. 

The binding of a symmetric chromophore to a protein or nucleic acid often induces CD in that chromophore. For example, the bands of enzyme-bound pyridoxal and pyridoxamine phosphates shown in Fig. 14-9 are positively dichroic in CD, but the band of the quinonoid intermediate at 20,400 cm-1 (490 nm) displays negative CD. When "transimination" occurs to form a substrate Schiff base (Eq. 14-26), the CD is greatly diminished. While the coenzyme ring is known to change its orientation (Eq. 14-39 Fig. 14-10), it is not obvious how the change in environment is related to the change in CD. 

The thermodynamic and kinetic characteristics of an intramolecular transimination reaction observed in solutions containing pyridoxal-5-phosphate and ethylenediamine have been investigated (75JA6530). The open-chain structure Schiff bases and the cyclic tautomers such as 54 are in equilibrium in aqueous solution over the pH range 7.5-14, but these equilibria are rather complex owing to the different states of the ionization (protonation) in both tautomers. The ring-chain equilibrium constant (the sum of all cyclic tautomers versus all open-chain tautomers) varies by less than a factor of 4 over the pH range 7-14. At pH 14, KT = 1.2 at pH 10,... 

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