Quinonoid-carbanionic intermediate

Loss of the a-hydrogen (Group a). Dissociation of the a-hydrogen from the Schiff base leads to a quinonoid-carbanionic intermediate whose structure in depicted in Fig. 14-5. The name reflects the characteristics of the two resonance forms drawn. 

Figure 14-5 Some reactions of Schiff bases of pyridoxal phosphate, (a) Formation of the quinonoid intermediate, (b) elimination of a (3 substituent, and (c) transamination. The quinonoid-carbanionic intermediate can react in four ways (1—4) if enzyme specificity and substrate structure allow.

These also presumably lead to a transient quinonoid-carbanionic intermediate. Addition of a proton at the original site of decarboxylation followed by breakup of the Schiff base completes the sequence. Decarboxylation of amino acids is nearly irreversible and frequently appears as a final step in synthesis of amino compounds. For example, in the brain glutamic acid is decarboxy-lated to y-aminobutyric acid (Gaba),193 196b while 3,4-dihydroxyphenylalanine (dopa) and 5-hydroxy-... 

The donor amino acid forms a Schiff base with pyridoxal phosphate within the enzyme s active site. After a proton is lost, a carbanion forms and is resonance-stabilized by interconversion to a quinonoid intermediate. After an enzyme-catalyzed proton transfer and a hydrolysis, the a-keto product is released. A second a-keto acid then enters the active site. This acceptor a-keto acid is converted to an a-amino acid product as the mechanism just described is reversed. 

Although not a subject of this chapter, Toney and coworkers have quantitated the reaction coordinate of a PLP-dependent L-alanrne racemase [15]. Despite the expectation that the cofactor provides resonance stabilization of the carbanion/enolate anion (quinonoid) intermediate derived by abstraction of the a-proton, the spectroscopic and kinetic analyses for the wild type racemase at steady-state provided no evidence for the intermediate in the reaction catalyzed by the wild type enzyme. Indeed, Toney had previously demonstrated that a kinetically competent quinonoid intermediate accumulates in the impaired R219E mutant [16] Arg 219 is hydrogen-bonded to the pyridine nitrogen of the cofactor. For the wild type racemase, the derived transition state energies for conversion of the bound enantiomers of alanine,... 

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