Useful Notes on the Transformation of Amino Acids

During this process the amino group is removed from the a-amino acid to form ?-keto acid The disposition of ?-keto acid is almost an individual process, differing in a manner dependent on to amino acid from which it was derived.

For the most part the amino group is converted to urea in man and excreted. It may also be excreted as ammonia or as other nitrogenous waste products. The entire process is caused by several enzymes which are either of oxi­dizing or of reducing type.

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2. Oxidative deamination:

In some cases, the deamination process may be accompanied by an oxidation reaction such a combi­nation is referred to as “oxidative deamination”.

This process, al­though, prominent in tissues, is by no means the only way in which amino groups are removed from the donor amino acids.

A series of non-oxidative deamination enzymes are associated with certain speci­fic amino acids such as the hydroxy amino acids (dehydrase) the sulphur containing amino acids (desulphydrases), histidine (histidase), tryptophan (tryptophanase) and others.

In general these reactions lead to one or the other of two familiar residues, pyruvate or acetate.

Dietary experiments with individually labelled amino-acids have shown that in mammals the non-essential amino-acids usually produce a pyruvate residue (glycogenic amino- acids), while most of the essential amino-acids form an acetate resi­due (ketogenic amino-acids). The significance of the terms glycogenic and ketogenic is obvious. The first group of amino-acids in large quantities leads to the production and storage of carbohydrate, while fat metabolism is emphasized in the case of the ketogenic group.

Through deamination at least half of the common amino-acids are reduced to these two familiar carbon residues, and from this point, as acetyl CoA, they can enter the energy-yielding reactions of the citric acid cycle.

3. Transamination:

Transamination is a process which brings about the transfer of the amino group from the donor amino-acid to a recipient keto acid, under the influence of a transaminase or amino transferase enzyme. This is represented by the following chemical reaction.

The donor amino-acid thus becomes a keto acid and the reci­pient keto acid becomes an amino-acid. The co-enzyme required for this reaction is pyridoxal phosphate.

In both transamination and deamination the original ac-amino- acid is converted into an cc-keto acid.

Most of the a-keto acids formed in these conversions are compounds involved in carbohydrate meta­bolism.

This is precisely the way in which carbohydrates and proteins are interrelated. Some specific examples of transamination are as follows:

Glutamic acid Pyruvic acid a-keto glutamic Alanine (from proteins) (from carbo- acid (a-amino-acid) ?-keto acids other than pyruvic acid may react with glutamic acid. Two such compounds are a-keto butyric acid and oxaloacetic

There are, however, certain limitations to the reaction while most amino-acids may act as donor or recipient and must be either ?-oxoglutaric acid or oxaloacetic acid or pyruvic acid.

It is important to note that all of these keto acids are components of the tricarboxylic acid and are, therefore, common metabolites in the cell.

The amino- acids formed from them are glutamic acid, aspartic acid and alanine respectively.

There are, thus, three main types of transaminase, the most important reaction is that involving glutamic acid its correspon­ding keto acid, a-oxoglutaric acid.

For example, the reaction between aspartic acid and a-oxoglutaric acid is catalyzed by the aspartic amino transferase, sometimes referred to as glutamic oxaloacetate transaminase

These reactions reveal that transamination represents the major mechanism leading toward the eventual disposition of nitrogen as waste products and also results in the production of carbon com­pounds which may be metabolized for energy purpose.

The process of transamination takes place chiefly in the liver but also occurs in the kidneys, brain and heart, etc.

The process of amino-acid catabolism by combined action of an amino transferase (transaminase) and glutamate dehydrogenase may be summarized as follows:

4. Decarboxylation:

It is a process by which —COOH group leaves a primary amine as CO2 and is probably not an important pathway for amino-acid metabolism in man.

This process is carried out with the help of decarboxylases which are found in a variety of animal tissues. Pyridoxal phosphate is required as a co-factor.

5. Transmethylation:

Transmethylation is the process where­by methyl groups are transferred from one compound to another.

In this way, the body can synthesize some essential compounds. Methio­nine is an efficient supplier of methyl groups.