hepatic protein metabolism & amino acids in nitrogen balance Flashcards
why is the liver important in protein metabolism (3)
- stores more proteins than other tissues
- can rapidly synthesise or degrade proteins.
- can quickly synthesise and degrade amino acids, unlike most other tissues.
how many different amino acids are needed in the human body for protein synthesis
20
10 can be synthesized but 10 cannot
what are essential amino acids
the 10 that cannot be synthesised and are needed in the diet
what is the process of amino acid synthesis called
transamination
describe transamination process
- A precursor alpha-keto acid such as pyruvate is needed
- A donor of the amino group is needed, and this is commonly glutamic acid
- An aminotransferase catalyses the reaction such as alanine transaminase (ALT) – this is one of the main liver enzymes alongside aspartate transaminase (AST)
what happens once the cells of the body have reached their limit of protein storage
the left-over amino acids in the blood are metabolised in the liver, prior to their conversion into lipids or glucose.
via transamination and deamination
where does amino acid degradation and catabolism take place
hepatocytes of liver
which amino acids undergo degradation
any that that are not required as building blocks for protein synthesis
what does amino acid catabolism require
for the the alpha amino group (nitrogen containing) to be removed
what does amino acid catabolism produce (2)
Nitrogen - which is incorporated in other compounds or excreted
Carbon skeleton - which can then be metabolised and used in the Kreb’s
cycle
what are the 2 main catabolism processes
oxidative deamination
transamination
what does oxidative deamination result in
removes an amine group as free ammonia
amine is replaced by an
oxygen atom from water to form a alpha-keto acid
results in the formation of an alpha-keto acid (e.g alpha-ketoglutarate) &
ammonia
what is the only amino acid that undergoes rapid oxidative deamination
glutamate
how is the alpha keto acid used
in the Kreb’s cycle for use in glucose
production - GLUCONEOGENESIS
which enzyme is used in oxidative deamination
glutamate dehydrogenase
what is the co enzyme used in oxidative deamination
forward reaction - NAD+
backward reaction - NADPH
what is oxidative deamination dependent on
on concentrations of;
glutamate
alpha-ketoglutarate
ammonia
what happens when there is excess amonnia
it can easily cross the blood-brain barrier and then react with alpha-ketoglutarate because it is a reversible reaction
what happens after a protein rich meal
glutamate concentration will be high, oxidative
deamination will degrade the amino acid glutamate
resulting in ammonia formation
what happens in transamination in amino acid catabolism
an alpha amino group is trasnferred from amino acid to a keto-acid to form an alpha-keto-acid
what enzyme involved in transamination in amino acid catabolism
aminotransferases such as ALT/AST
each aminotransferase is specific to one or a few amino group donors
where are aminotransferases found
in the cytosol of the mitochondria throughout the body and particularly in the kidneys & liver
why do degradation and oxidative deamination work in unison
transamination is readily reversible
degradation will occur after a protein-rich meal and amino acid synthesis will occur depending on dietary supply and cellular demand
what is nitrogen balance
the difference between nitrogen excreted from the body and nitrogen ingested in the diet
this reflects gain or loss of total body protein
what is anabolic nitrogen balance
positive balance
i.e net gain in amino acids
nitrogen intake > nitrogen loss
what is catabolic nitrogen balance
negative balance i.e. net loss in amino acids
nitrogen intake < nitrogen loss
what does it mean if nitrogen balance is in equilibrium
the person is healthy
what if someone doesnt have the essential amino acids in their diet
they have a negative nitrogen balance (loss greater than gain)
most common cause of a positive nitrogen balance
pregnancy
what is the recommended daily intake of amino acids to remain in nitrogen balance
0.8g/kg bodyweight
most common cause of a negative nitrogen balance
malnutrition
other causes of negative nitrogen balance
multiple trauma or extensive trauma where there is a lot of tissue damage
what happens in the glucose alanine cycle
move proteins from muscles to the liver when glycogen stores are low to generate additional ATP
which is the main amino acid transported to liver in glucose alanine cycle
alanine
what is alanine used for
gluconeogenesis
describe glucose alanine cycle process
- excess alanine released into
bloodstream and transported into liver - in liver, alanine is converted
back to pyruvate by transamination - the pyruvate can be used as a source of carbons for glucose production via
gluconeogenesis - The glucose then enters the blood and can be used in the muscles which in turn can produce pyruvate via glycolysis which can then be used again to remove excess ammonia (NH3) - this is the glucose-alanine cycle
- The glutamate produced can then be
converted to ammonium (NH4+) via
oxidative deamination producing
ammonium (NH4+) which then rapidly
disassociates into ammonia (NH3) which in turn can be converted to urea via the urea cycle
