Amino Acid Metabolism

Question 1

BCAAs

Answer 1

• lysine, leucine, isoleucine

- 1/3 of skeletal muscle

Question 2

Conditionally essential amino acids

Answer 2

• can make these, but not in the quantities needed
• methionine (needed to produce cysteine in the diet)
• arginine (needed for urea)
• phenylalanine (needed to form tyrosine)

Question 3

Digestion of protein

Answer 3

1) acid stomach environment cleaves proteins into peptides via pepsin
2) alkaline environment from small intestine cleaves further with trypsin, chymotrypsin and elastase
3) further cleaved into amino acids via aminopeptidase, carboxypeptidase and dipeptidases secreted from the gut wall

Question 4

Uptake of amino acids

Answer 4

• driven my enterocytes in the gut
• create a strong Na+ gradient by using Na+ pump in the basolateral membrane
• Na-linked amino acid carriers operate within intestinal brush border, which allows for movement of amino acids in

Question 5

4 important amino acids and their function (Ala, Glu, Asp, Gln)

Answer 5

• glutamate and aspartate: excitatory neurotransmitters in CNS
• glutamine can be converted to glutamine which is central N carrier in the blood
• alanine is released from tissues during starvation, and is an important substrate for hepatic gluconeogenesis

Question 6

Transamination (AST and ALT examples)

Answer 6

• important to have transamination between aspartate and glutamate as this forms key components of the urea cycle
• uses pyridoxal phosphate (vitamin B6) as a cofactor, ring structure provides stabilisation as amino group transiently binds
• is water comes in and keto acid produced, B6 holds onto amine group until another substrate comes along to take it
• AST and ALT are good examples of this
• AST: aspartate aminotransferase forms important part of malate- aspartate shuttle to get NADH into mitochondria from cytosol
• ALT: alanine amino transferase, important for gluconeogenesis or TCA cycle. Alanine released from muscles during starvation, transamination produces pyruvate and glutamate (which can be detox using urea cycle)

Question 7

Deamination: glutamate dehydrogenase

Answer 7

• 75% AA follow this route
• glutamate + NAD(P)H cofactor produces NH4+ and oxoglutarate
• beginning of urea cycle

Question 8

Amino acid metabolism: trans-deamination

Answer 8

• most amino acids share common substrates
  1) transaminase uses oxoglutarate, releasing keto acid (C skeleton can be used for metabolism i.e acetyle coA, pyruvate) and glutamate
  2) glutamate dehydrogenase with NAD(P)H produces NH4+ and regenerates oxoglutarate

Question 9

The urea cycle

Answer 9

1) arginine from the diet is cleaved by arginase in the cytosol to form urea and ornithine
2) a new urea residue is built on ornithine, which perpetuates the cycle. HCO3- and NH4+ in the mitochondria are combined using 2ATP and CPS-I enzyme to make carbomyl phosphate
3) ornithine transcarbamoylase then removes a phopshate, making citrulline in the cytosol
4) cirtulline reacts with aspartate in the cytosol using ATP and arginosuccinate synthetase, generating arginosuccinate
5) arginosuccinate is then cleaved by arginosuccinate lyase to release fumarate and arginine
6) arginine can then be cleaved by arginase to produce urea and ornithine

Question 10

Nitric oxide synthase (NOS)

Answer 10

• rather than being cleaved by arginase, arginine can be cleaved by nitric oxide synthase to produce NO (an ROS) and citrulline (using NADPH)
• cirtulline can be reconverted to arginine using 2 urea cycle enzymes
• Endothelial NO involved in relaxing of smooth muscle
• neuronal NO is involved with neuronal signalling
• inducible NO used for WBC (macrophages)