Properties of 4-ethenyl and 4-ethynyl analogs of pyridoxal phosphate and their reactions with the apo form of asparatate aminotransferase.

The binding to apoaspartate aminotransferase of analogs of pyridoxal phosphate bearing vinyl, cis- and trans-methylvinyl, and ethynyl groups in place of the fomyl group of the coenzyme has been studied. Details of synthesis of the ethynyl compound are given. The absorption spectra of all of the compounds have been analyzed and pKa values have been determined. The positions of the absorption bands can be related to those of pyridoxine but with bathochromic shifts induced by the ethenyl and ethynyl groups. However, this shift is almost completely lacking for the cis-methylvinyl compound suggesting nonplanarity of the molecule. Binding of the analogs to the apoenzyme is accompanied by a strong bathochromic shift which, from a study of solvent effects on the free analogs, appears to indicate a hydrophobic environment on the enzyme. Nevertheless, the analogs are bound as dipolar ions exclusively. Binding is accompanied by a distinct perturbation of the protein spectrum in the aromatic region. An effect on the spectrum of 1 or more tryptophan residues is indicated. Bands of the bound analogs exhibit positive circular dichroism except for that of the 4-vinyl analog. The 4-ethynyl analog reacts in a more complex way, giving at least two successive products in addition to the initial complex. The final product is reducible by sodium borohydride, is released from the enzyme by boiling, and appears to have the properties of a Schiff base. We postulate that the addition of an amino group of the enzyme to the ethynyl group is followed by tautomeric rearrangement to a Schiff base in which the ring is in a p-quinonoid structure.