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Catabolism of Some Amino Acids

In this article we will discuss about the catabolism of amino acid. Read on to learn more about catabolism of amino acid!

Amino acids are compounds containing carbon, hydrogen, oxygen and nitrogen. These are monomeric constituents of proteins. There are 20 different amino acids. They act as precursors of other nitrogen containing biologically important compounds, like hormones, neurotransmitters etc. and can be used as energy source.

Catabolism of Amino Acids

1. Transamination

It is the process by which an amino group, from amino acid, is transferred to an a-keto acid (mostly alfa-keto glutarate) with formation of the corresponding keto and glutamate. Thus, transamination provides a route for redistribution of amino acids. Transamination reactions are catalyzed by transaminases (aminotransferases). Each amino transferase is specific for one or a few amino group donors. 

Catabolism of Amino Acids Steps

In an α-keto acid, the carbonyl or keto group is located on the carbon atom adjacent to the carboxyl group of the acid.

All amino acids except histidine, threonine and proline participate in transamination reactions. Transamination reactions have equilibrium constants close to one. Therefore, the direction of a transamination reaction proceeds in large part as a function of the intracellular concentrations of the reactants. This means that transamination can be used not only for amino acid synthesis, but also for degradation of amino acids that accumulate in excess of need. 

Example

Transamination is an exchange of functional groups between any amino acid (except lysine, proline, and threonine) and an α-keto acid. The amino group is usually transferred to the keto carbon atom of  α-ketoglutarate, converting the α-keto acid to glutamate. Transamination reactions are catalyzed by specific transaminases (also called aminotransferases), which require pyridoxal phosphate as a coenzyme.

Catabolism of Amino Acids Steps

Two Transamination Reactions. In both reactions, the final acceptor of the amino group is α-ketoglutarate, and the final product is glutamate.

2. Oxidative Deamination

Deamination is also an oxidative reaction that occurs under aerobic conditions in all tissues but especially the liver. During oxidative deamination, an amino acid is converted into the corresponding keto acid by the removal of the amine functional group as ammonia and the ketone group replaces the amine functional group. The ammonia eventually goes into the urea cycle.

Catabolism of Amino Acids Steps

Small number of amino acids undergoes oxidative or non-oxidative deamination. For example, glutamate is oxidised to a-ketoglutarate by glutamate dehydrogenase, which is an anaerobic dehydrogenase and needs NAD or NADP as a coenzyme. This enzyme is present in most tissues and occurs both in cytoplasm and mitochondria. It has high activity. It deaminates glutamic acid resulting in a-ketoglotaric acid and ammonia.

3. The Fate of the Carbon Skeleton

Any amino acid can be converted into a citric acid cycle intermediate. After the amino group is removed, usually by transamination, the remaining α-keto acid is catabolized by a pathway specific to that acid, which consists of one or more reactions. 

Before it splits into fumarate and acetoacetate, phenylalanine, for example, goes through six reactions. Fumarate is an intermediate in the citric acid cycle, whereas acetoacetate must first be converted to acetoacetyl-CoA and then to acetyl-CoA before entering the citric acid cycle.

Fates of the Carbon Skeletons of Amino Acids

Those amino acids that can form any of the intermediates of carbohydrate metabolism can subsequently be converted to glucose via a metabolic pathway known as gluconeogenesis. These amino acids are called glucogenic amino acids. Amino acids that are converted to acetoacetyl-CoA or acetyl-CoA, which can be used for the synthesis of ketone bodies but not glucose, are called ketogenic amino acids. Some amino acids fall into both categories. Leucine and lysine are the only amino acids that are exclusively ketogenic. Figure summarizes the ultimate fates of the carbon skeletons of the 20 amino acids. 

The table above summarizes the Glucogenic and Ketogenic Amino Acids. Image Source: https://chem.libretexts.org/

Summary

The removal of the amino group is usually the first step in the breakdown of amino acids, which is done through a reaction known as transamination. The carbon skeletons of amino acids undergo additional reactions to form compounds that can be used in the synthesis of glucose or ketone bodies.

Further Readings