Properties of the charge-transfer transition observed in glyceraldehyde-3-phosphate dehydrogenase from sturgeon muscle alkylated by 3-chloroacetylpyridine--adenine dinucleotide. Characterisation of the modified amino acid.

An absorption band at 340 nm is shown to be formed concomitantly with the covalent bond between the affinity label 3-chloroacetylpyridine--adenine dinucleotide (clac3PdAD+) and glyceraldehyde-3-phosphate from sturgeon. This band corresponds to a charge-transfer transition. Its intensity depends upon the pH and the ionic strength but is almost independent of the nature of the anions present in the medium. The pH dependence shows an inflection point at pH 7.1. This result suggests the participation of a residue with a pKa of 7.1 within the active site of the enzyme in the formation of this transition. Using various techniques, the amino acid alkylated by clac3PdAD+ is shown to be the essential Cys-149, thus excluding the participation of this residue in the formation of the charge-transfer transition. On the other hand, the modification of Cys-153 seems not to affect this charge-transfer band. Other possible donors are proposed, such as the invariant His-176 or Tyr-317 residues. These amino acids might be implicated in the formation of the Racker band.