Question

Explain why the ability of PLP to catalyze an amino acid transformation is greatly reduced if the OH substituent of pyridoxal phosphate is replaced by OCH₃.

Short answer

The OH group in the pyridine ring forms a hydrogen bond with nitrogen of the imine linkage. This helps in gaining a partial positive charge on nitrogen atom, which makes it to attach amino acid to imine carbon.

Step-by-step solution

Role of OH group in pyridine ring

Transamination reaction attaches an amino acid with coenzyme forming a product with imine between PLP and amino acid. The OH group in the pyridine ring forms a hydrogen bond with nitrogen of the imine linkage. This helps in gaining a partial positive charge on nitrogen atom, which makes it to attach amino acid to imine carbon. It also helps to remove substituents from$\mathit{\alpha }$-carbon of amino acid easily. Thus, if OH group is replaced with OCH₃ coenzyme loses these activities.

Hydrogen bond linkage in pyridine ring

This is the diagram showing PLP the coenzyme, attached to an amino acid via imine linkage. Also, the OH group in PLP forms a hydrogen bond with nitrogen.

Thus, the draw backs of inserting OCH₃ group in pyridoxal phosphate is discussed above.