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Aspartate Transaminase (AST): Definition, Function & Structure

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Proteins are one source of energy, as each amino acid undergoes transamination in order to be used for energy. Aspartic acid uses the enzyme Aspartate Transanimase (AST); in this lesson we will learn how AST is used.

Transanimase

Our bodies are very good at using food for energy. One source of energy from food is protein. But how does the body use this protein for energy? In general, proteins are broken down into individual amino acids which then undergo transdeanimation. Transdeanimation is the process by which nitrogen from amino acids is transferred to glutamate for removal and the resulting carbon chain can be used for energy in the citric acid cycle.

Each amino acid has its own transanimase enzyme, or an enzyme to transfer the nitrogen group from it to alpha-ketoglutamate. For aspartate, it is the Aspartate Transanimase or AST enzyme. AST takes aspartate and alpha-ketoglutarate and forms L-Glutamate and alpha-keto-aspartate. Alpha-keto-aspartate can be used to make oxaloacetate, which is one of the main carbon chains in the citric acid cycle.

General Transanimase Structure

All transanimase enzymes include a pyridoxal phosphate (PLP) group. This group is formed from vitamin B6, which is one reason this vitamin is so important in our bodies. PLP works by moving the amino acid into the active site of the enzyme. It also acts as the intermediate in transferring the nitrogen from the amino acid to the alpha-ketoglutarate.

PLP is found in the active site of the enzyme, circled in this image
PLP active site

The active site typically has a lysine which binds to the PLP. Once the PLP has bound to lysine it can remove the nitrogen from the amino acid. When water is added this allows PLP to be released from the lysine. Once the PLP has been released, the nitrogen is also released onto the alph-ketoglutarate.

So we end up with the alpha-ketoacid and L-Glutamate.

AST Structure

Aspartate Transanimase (AST) is a transanimase enzyme specifically for aspartate. It has the same general structure as most other transanimase enzymes. For example, it has a lysine in the active site to bind PLP, and this is where the nitrogen is transferred.

The difference lies in how it specifically selects for aspartate instead of the other amino acids. AST contains two arginine residues. These arginine residues are able to bind to the two carboxylic acids on aspartate. Since aspartate becomes bound in the enzyme it specifically allows the nitrogen to be transferred off of nitrogen and onto glutamate.

Aspartic acid has two carboxylic acids which is what allows AST to identify and bind it
Aspartic acid

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