Question

Explain why the ability of PLP to catalyze an amino acid transformation is greatly reduced if a PLP- requiring enzymatic reaction is carried out at a pH at which the pyridine nitrogen is not protonated.

Short answer

If the pyridine nitrogen is not protonated on to the positively charged protonated nitrogen of pyridine ring then it would be less attractive to electrons and thereby decreases enzymatic reactions.

Step-by-step solution

Reason behind less ability of PLP

Amino acid reacts with imine formed by PLP and the lysine side chain converts to another imine by transimination reaction. The next step after this reaction is breaking substituents from $\mathit{\alpha }$- carbon of amino acid. The electrons left behind when bond breaks are delocalized on to the positively charged protonated nitrogen of pyridine ring. If the pyridine nitrogen is not protonated, then it would be less attractive to electrons. Hence, pyridine nitrogen less crowded with electrons.

Imine between PLP and amino acid

Here the structure represents an imine linkage in between PLP coenzyme and an amino acid containing hydrocarbon side chain.

Thus, the reason behind the reduction in PLP to catalyze an amino acid transformation is discussed above.