Protein metabolism

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Protein metabolism

• By
• Dr OJ Tsotetsi

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protein

• A major component of foods. It is digested
  firstly in the stomach, and then in the duodenum
  to dipeptides and amino acid.
• Absorbed using symport active transport with
  sodium.
• Stored in liver and muscles

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Uses

• Protein synthesis -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 synthesized from amino
  acids include purines and pyrimidines (components
  of nucleotides), catecholamines (adrenaline and
  noradrenalin) neurotransmitters (serotonin)

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as a biological fuel - About 10 of energy
production in humans is from amino acids
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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).

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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 catalyzed 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
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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 or a citric
acid cycle intermediate
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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

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Non-essential amino acids are synthesised from
the products of their catabolism - i.e. acetyl
CoA, pyruvate or the relevant Krebs 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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Figure 25.3 Nutrient Use in Cellular Metabolism
Figure 25.3
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Hormone Principal metabolic
actions Insulin Increases glucose
uptake in peripheral tissues. Stimulates
protein synthesis.Inhibits lipolysis and
glycolysis Glucagon Increases cyclic AMP
levels in the liver and adipose tissue,
with stimulation of fatty acid
mobilization, glycogenolysis, glycolysis and
gluconeogenesis Catecholamines Increase
cyclic AMP levels in the liver, skeletal
muscle and adipose tissue, with release of
glucose, free fatty acids and
lactate Corticosteroids Increase
gluconeogenesis. Increase amino acid
mobilization from the periphery (chiefly
skeletal muscle), Increase fatty acid release
from extremities. Decrease glucose
utilization by peripheral tissues GH
AND TH
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Fed State

• Insulin
• stimulates LPL
• increased uptake of FA from chylomicrons and VLDL
• stimulates glycolysis
• increased glycerol phosphate synthesis
• increases esterification
• induces HSL-phosphatase
• inactivates HSL
• net effect TG storage

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Figure 25.15 The Absorptive State
Figure 25.15
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Figure 25.17 The Postabsorptive State
Figure 25.17
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Acknowlegdement

• http//www.elmhurst.edu/chm/vchembook/630proteinm
  et.html