Amino acid catabolism

15 important questions on Amino acid catabolism

Amino acid catabolism - C and N part of AA follow different pathways:

- C --> gluconeogenesis or TCA cycle
- N --> urea cycle
- C-backbone: de-aminated by either oxidative de-amination or transamination

Which amino acid has the highest nitrogen percentage:

arganine

Semi-essential amino acids:

- cysteine from methionine
- tyrosine from phenylalanine
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Non-essential amino acids:

- alanine
- aspartic acid
- glutamic acid

Which vitamin can be involved in transamination reactions?

Vitamin B6 --> in a 2 step process, depending on the amino acid needed

Transaminations of AA to common intermediates:

- Alanine to pyruvate
- Aspartic acid to oxaloacetate
- Glutamic acid to alpha-ketoglutarate

Hepatic urea synthesis:

- Only liver cells are able to synthesize urea

Example ATP yield alanine de-aminated to pyruvate:

- pyruvate provides +12.5 ATP
- Excretion of N: +2.5 ATP (de-amination) 
- Urea cycle: -2 ATP
--> total: 13 ATP

ME (kJ/gram) that can be obtained from muscle protein in body:

18.4 kJ/g

GE (kJ/gram) of protein:

23.6 kJ/g

Protein loss via urea via urea cycle:

5.2 kj/g

Rate of protein turnover can be influenced by

external factors (nutrients)

Why so much protein needed in the body?

- 60% of theorine requirement is for mucus synthesis
- Cortisol induces gluconeogenesis and breakdown of muscle protein (acts on liver only)

Costs protein synthesis:

4 kJ/g

Ratio of depostion and maintenance

5:1

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