INTRODUCTORY METABOLISM MODULE

PROTEIN METABOLISM
Use these links to navigate to selected parts of this section of the Introductory Metabolism module.

Introduction
A major component of food is protein. The proteins ingested as part of our diet are not the same proteins required by the body, nor can large molecules be absorbed from the gut. Therefore these proteins are digested and their component amino acids absorbed into the blood stream.

Uses of amino acids
Amino acids are used in three ways in the body :
The synthesis of new proteins is very important during growth. In adults new protein synthesis is directed towards replacement of proteins as they are constantly turned over.
  • synthesis of a variety of other compounds

Examples of compounds synthesised from amino acids include

purines and pyrimidines (components of nucleotides)

catecholamines (adrenaline and noradrenalin)

neurotransmitters (serotonin)

histamine

porphyrins (the central oxygen binding component of haemoglobin)

  • as a biological fuel
About 10% of energy production in humans is from amino acids. The percentage is much higher in carnivores, whose diet is almost entirely protein.

Amino acid catabolism
The other biological fuels discussed (carbohydrates & fats) contain only the elements carbon, hydrogen and oxygen. Amino acids contain nitrogen as well. The first step in amino acid catabolism is the removal of the nitrogen (the amino group).

 

Deamination
The removal of the amino groups of all twenty amino acids begins with the transfer of amino groups to just one amino acid - glutamic acid (or glutamate ion). This is catalysed by transaminase enzymes which transfer the amino group from amino acids to a compound called alpha-ketoglutarate. The product is an alpha-keto acid formed from the amino acid and glutamate (formed from the addition of the amino group to alpha-ketoglutarate.

 

 
Once the amino groups have all been "collected" in the form of the one amino acid, glutamate, this amino acid has its amino group removed (termed "oxidative deamination"). This reaction reforms alpha-ketoglutarate with the other product being ammonia (NH4 +).
 

 

 

Ammonia and urea
Ammonia is toxic to the nervous system and its accumulation rapidly causes death. Therefore it must be detoxified to a form which can be readily removed from the body. Ammonia is converted to urea, which is water soluble and is readily excreted via the kidneys in urine.

 

Amino acid carbon skeletons
The remainder of the amino acid is referred to as the "carbon skeleton". Depending on the particular amino acid being catabolised, its carbon skeleton will be converted to :
  • acetyl CoA
Those carbon skeletons which end up as acetyl CoA are committed to energy production. They will either be immediately oxidised via the citric acid cycle or they may be converted to ketone bodies. Because the amino acids whose carbon skeletons yield acetyl CoA are potentially a source of ketone bodies they are referred to as ketogenic amino acids.
  • or pyruvate
The carbon skeletons which end up as either pyruvate or a citric acid cycle intermediate may be used for energy production or they may be used to synthesis glucose by the pathway known as gluconeogenesis. Because the amino acids whose carbon skeletons yield pyruvate or a citric acid cycle intermediate are potentially a source of glucose they are referred to as glucogenic amino acids.

 

Catabolism summary
A summary of amino acid catabolism is shown below.

 

Amino acid synthesis
A detailed description of the processes by which amino acids are synthesised is outside the aim of this introductory module. Only a few brief relevant points are included.
Amino acids are divided into two classes depending on whether they can be synthesised in the human body or whether they must be supplied in the diet. The former group are referred to as non-essential and the latter group as essential. The table below shows which of the twenty are in each group. Note that there are ten in each of the two groups.
 

 
Non-essential amino acids are synthesised from the products of their catabolism - i.e. acetyl CoA, pyruvate or the relevant citric acid cycle intermediate. The amino group is donated by glutamate and added by the reverse of the transamination reactions discussed above.
The essential amino acids are synthesised in micro-organisms (bacteria and yeasts) and passed through the food chain until they reach us in our diet.
One of the pathways essential to life which is carried out by bacteria is the "fixation" of atmospheric nitrogen initially as inorganic nitrate and ultimately as amino groups in amino acids. Higher organisms cannot perform this function.

This completes this section on metabolism of the subunits of proteins, amino acids.

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