Question

Aspartate is the next-most common amine donor (after glutamate) in transamination reactions. What prosthetic group is involved in these reactions what is the a-keto acid product of one of these reactions when the a-amino acid substrate is aspartate?

Answer 1

Transamination reactions require the enzymes called transaminases or aminotransferases. The enzymes differ only for the amino acid but has same mechanism of action and have the same prosthetic group- Pyridoxal phosphate (PLP), which is the coenzyme form of Vitamin B₆ also called pyridoxine. The role of PLP is to act as an intermediate that carries the specific amino group to the active site of the transaminase enzyme. It is then the enzyme transfers the amino group to the α-keto acid making the corresponding amino acid.

The PLP prosthetic group exists in two forms: pyridoxal phosphate, that accepts the amino group from an amino acid (α-amino acid) and pyridoxamine phosphate, that donates the amino group to the α-keto acid (the type of mechanism exhibited by transaminase in conjugation with PLP is called ping-pong mechanism or reaction where one amino acid attaches to the enzyme, donates its amino group and leaves the active site, the amino group is then transferred to the α-keto acid, forming a new amino acid). Pyridoxal phosphate is attached to the enzyme's active site covalently through a Schiff base linkage with the ε-amino group of lysine amino acid residue in the active site.

For example:

α-keto acid of glutamate is α-ketoglutarate.

Similarly, when the α-amino acid is aspartate, the α-keto acid formed is oxaloacetate, while the α-keto acid that accepts the amino group of aspartate is α-ketoglutarate, forming the amino acid glutamate.

The reaction is as follows:

ОИ O= C O=C o=0 Aspartate cu ИС-ри, I CAN autuff transaminase eu Ma o= c J-8-5-u- CH2 PLP ordu o=c on 0= Aspartate X-ketoglutarate Oxaloacetate Glutamate o - P =0 o-p=0 o- O ch 0- 0 =C HN 1- -Ch3 би и Pyridoxial phosphate haté xamine phosphate