The general scheme of PLP-mediated bioconjugation. (a) In the first step, a
protein is incubated with PLP (1) under mild, aqueous conditions. This
oxidizes the N-terminus of the protein to a ketone or an aldehyde, providing
a unique functional group for further modification. (b) In the second step
the ketone is conjugated to an alkoxyamine-bearing reagent (2) through oxime
formation. (c) The proposed mechanism begins with Schiff base formation
between the N-terminal amine and the PLP aldehyde. Tautomerization, followed
by hydrolysis, affords the keto-protein product. Based on this mechanism,
the reaction rate is expected to depend on both the concentration of protein
and PLP.