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Samenvatting Learning goals case 5
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Notes made in turtorial included as well, they are written in red. I succeeded this course with an 8, so all the cases are complete.
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Learning goals case 51. How do you get ATP(energy) from protein?
When amino acids enter the liver via the portal vein, the liver has three choices (fed state)..
- Protein synthesis
- Gluconeogenesis or novo lipogenesis
- Send it off to other tissues, such as muscle, where they can use it for their own synthesis.
Amino acids enter the liver (fasted state)…
- Fatty acids are converted into glucose in the liver
- Catabolism amino acid
The liver plays a special role in amino acid oxidation, not least because it is the organ that
first receives the dietary amino acids, which enter the circulation via the portal vein. It is also
the only organ capable of eliminating the nitrogen from amino acids, by synthesizing urea
(exception are the branched chain amino acids (BCAA), catabolism is largely in the muscle).
- The three branched-chain amino acids are valine, leucine, and isoleucine. These three
essential amino acids constitute about 20% of dietary protein, but represent about 70%
of the amino acids leaving the liver after a meal. The implication is that other amino acids
have been preferentially retained in the liver. The branched-chain amino acids are
instead preferentially removed by muscle after a meal. Since muscle removes these
amino acids preferentially, it cannot require them simply for protein synthesis, or they
would not be matched in proportion by other amino acids. In fact, skeletal muscle has
the ability to oxidize the branched-chain amino acids.
Amino acid oxidation provides about half the liver’s energy requirements.
, The most amino acids we ingest are ultimately oxidized. Amino acid oxidation contributes
around 10–20% of the total oxidative metabolism of the body under normal conditions
The difference between amino acids and carbs and fats is that amino acids cannot be stored
without having a function apart from storage.
Urea is the form in which we, as humans, excrete most of the amino nitrogen that is “left
over” after amino acid oxidation, although we also excrete some nitrogen in the form of
ammonia, especially during starvation. The enzymes of the urea cycle are found at low levels
also in the brain and adipose tissue, but the liver is the only organ contributing significant
amounts of urea to the circulation.
When an amino acid (AA) is oxidised the products are NH2, a carbon skeleton or alfa-keto
acids.
AA NH2 + carbon skeleton/alfa-keto acid
The carbon skeleton can be used for..
- Energy generation
- Glucose
- Ketone body production
- Cholesterol production
- Fatty acid production
Amino acids fall into three categories:
- Glucogenic glucogenic amino acids are those that give rise to a net production of
pyruvate or TCA cycle intermediates, such as 2-oxoglutarate (α-ketoglutarate) or
oxaloacetate, all of which are precursors to glucose via gluconeogenesis. All amino acids
except lysine and leucine are at least partly glucogenic.
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