Protein Metabolism

Glycolysis &
Citric Acid Cycle
Lipid Metabolism &
Review Metabolism
 Elmhurst College
Transamination Urea Cycle Energy Summary  Chemistry Department
Oxidative Deamination Bilirubin - Heme Catabolism  PKU  Virtual ChemBook


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Transamination Reaction

Introduction:

Transamination as the name implies, refers to the transfer of an amine group from one molecule to another. This reaction is catalyzed by a family of enzymes called transaminases. Actually, the transamination reaction results in the exchange of an amine group on one acid with a ketone group on another acid. It is analogous to a double replacement reaction.

The most usual and major keto acid involved with transamination reactions is alpha-ketoglutaric acid, an intermediate in the citric acid cycle. A specific example is the transamination of alanine to make pyruvic acid and glutamic acid.

Other amino acids which can be converted after several steps through transamination into pyruvic acid include serine, cysteine, and glycine.

Link to: Transamination and Deamination (move cursor over arrows)
Jim Hardy, Professor of Chemistry, The University of Akron.

 
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Other Transamination Reactions:

Aspartic acid can be converted into oxaloacetic acid, another intermediate of the citric acid cycle. Other amino acids such as glutamine, histidine, arginine, and proline are first converted into glutamic acid.

Glutamine and asparagine are converted into glutamic acid and aspartic acid by a simple hydrolysis of the amide group.

All of the amino acids can be converted through a variety of reactions and transamination into a keto acid which is a part of or feeds into the citric acid cycle. The interrelationships of amino acids with the citric acid cycle are illustrated in the graphic on the left.

Quiz: List the amino acids that can be converted into glucose.  
List the amino acids that can be converted into lipids.  

 
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Amino Acids in Overall Metabolism:

Once the keto acids have been formed from the appropriate amino acids by transamination, they may be used for several purposes. The most obvious is the complete metabolism into carbon dioxide and water by the citric acid cycle.

However, if there are excess proteins in the diet those amino acids converted into pyruvic acid and acetyl CoA can be converted into lipids by the lipogenesis process. If carbohydrates are lacking in the diet or if glucose cannot get into the cells (as in diabetes), then those amino acids converted into pyruvic acid and oxaloacetic acids can be converted into glucose or glycogen.

The hormones cortisone and cortisol from the adrenal cortex stimulate the synthesis of glucose from amino acids in the liver and also function as antagonists to insulin.

 

Synthesis of New Amino Acids:

In addition to the catabolic function of transamination reactions, these reactions can also be used to synthesize amino acids needed or not present in the diet. An amino acid may be synthesized if there is an available "root" ketoacid with a synthetic connection to the final amino acid. Since an appropriate "root" keto acid does not exist for eight amino acids, (lys, leu, ile, met, thr, try, val, phe), they are essential and must be included in the diet because they cannot be synthesized.

Glutamic acid usually serves as the source of the amine group in the transamination synthesis of new amino acids. The reverse of the reactions mentioned earlier are the most obvious methods for producing the amino acids alanine and aspartic acid.

Several nonessential amino acids are made by processes other than transamination. Cysteine is made from methionine, and serine and glycine are synthesized from phosphoglyceric acid - an intermediate of glycolysis.