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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).
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.
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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 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.
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 :
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 a citric acid cycle intermediate
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.
A
summary of amino acid catabolism is shown below.
| 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.
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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.
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