protein breakdown and urea formation

Question 1

what is nitrogen balance?

Answer 1

→ Nitrogen balance is a measure of nitrogen input minus nitrogen output.

Question 2

what are the two parts of an amino acid and how is the carbon skeleton broken down?

Answer 2

→ the carbon skeleton
→ nitrogen
→The carbon skeleton is broken down by energy metabolism and biosynthetic pathways.

Question 3

why do we need to remove nitrogen, and what is it converted into to be removed?

Answer 3

→Nitrogen is toxic, so it has to be removed safely.

→ In mammals, the nitrogen is converted to the non-toxic, neutral compound urea and excreted in urine.

Question 4

what are the three steps in which amino acid nitrogen is transferred to urea?

Answer 4

→transamination
→ formation of ammonia
→formation of urea

Question 5

describe transamination

Answer 5

→chemical reaction that transfers an amino group to a keto acid to form new amino acids.

→Transaminase (aka aminotransferases) is the enzyme involved in this reaction.

→It catalyses a transamination reaction between an amino acid and a α-keto acid.

→The nitrogen group of one amino acid is transferred to a particular keto acid to give us a second amino acid.

→The synthesised molecules can be metabolised more readily.

Question 6

what are some alpha keto acids and what can they be oxidized to?

Answer 6

→ α-ketoglutarate
→pyruvate
→oxaloacetate
→α-keto acids are important metabolic intermediates. They can be oxidised or converted to glucose.

Question 7

what are two important aminotransferases and what are the chemical reaction that they catalyze?

Answer 7

→ALANINE (ALT) Alanine will react with α-ketoglutarate to give pyruvate and glutamate.

→In the context of urea formation, this reaction predominates.

→ASPARTATE (AST) Aspartate will react with α-ketoglutarate to give oxaloacetate and glutamate.

→In the context of urea formation, the opposite of this reaction predominates.

→Both generate glutamate, and both reactions are fully reversible.

→These reactions require pyridoxal phosphate derived from Vit B6.

Question 8

how can the levels of transaminases be used diagnostically?

Answer 8

→Transaminases are primarily liver enzymes, so high levels of ALT and AST in the blood can be indicative of liver damage

→they’re not meant to be released into circulation

Question 9

what happens to the glutamate after transamination?

Answer 9

→Glutamate can release ammonia directly by the action of glutamate dehydrogenase.

→The reaction is fully reversible and can use either NAD or NADP; however, it is usual for NAD to be used for degradation and NADPH for synthesis.

Question 10

why is having the transamination to glutamate and then the oxidative deamination back to α-ketoglutarate of amino acids important ?

Answer 10

→ conversion of many amino acids from their original state into glutamate

→ can be transported and then re-converted back into something the body can use for energy (or transamination again)

→ re-synthesizing the ammonia that is fed into the urea cycle

Question 11

how do we eliminate free ammonia?

Answer 11

→Free ammonia generated in tissue combines with Glutamate to give Glutamine.

→Glutamate + NH4+ + ATP –> Glutamine + ADP

→This reaction is catalysed by Glutamine Synthase.

Question 12

what is the importance of glutamine in the transport of nitrogen?

Answer 12

→Glutamine is the main transporter of nitrogen.

→ formed from glutamate which, in addition to having already accepted amino-groups to α-ketoglutarate, accepted more nitrogen to form glutamine.

→Glutamine can donate nitrogen for the biosynthesis of amino acids, nucleotides, amino sugars and NAD+.

Question 13

describe the structure of urea

Answer 13

→Urea is made up of two amine groups joined to a C=O.

→One amine group is donated from aspartate, while the other comes from glutamine/glutamate.

→The carbon C=O comes from the carbon skeleton, through using CO2 that has been produced from its breakdown.

→detrimental products of amino acid degradation can be used to combine to form urea, a non-toxic, soluble compound that can be readily excreted.

Question 14

describe the urea cycle

Answer 14

→CO2 comes from the bicarbonate and reacts with the ammonium ion that has come from glutamine/glutamate (formed by transamination of α-ketoglutarate and α-amino acid).

→They form carbamoyl phosphate (in the mitochondria). Carbamoyl phosphate then reacts with Ornithine to produce Citrulline.

→Citrulline reacts with Aspartate to form Argininosuccinate.

→ Argininosuccinate then is metabolised to Arginine (urea cycle) and Fumarate (TCA).

→The Arginine is acted upon by the enzyme arginase which is how urea is formed.

→The urea cycle continues.

→ Fumarate is converted to Malate which is transported back into the mitochondria and converted into oxaloacetate.

→The TCA then continues.

Question 15

how are muscles involved in the breakdown of amino acids?

Answer 15

→Muscles don’t have the enzymes needed to form urea, so the urea cycle doesn’t take place in muscles.

→muscles do break down amino acids for energy during prolonged exercise or starvation.

Question 16

what are the two ways in which remaining amino acids are dealt with in the muscle?

Answer 16

→1) Nitrogen is transferred to alanine (via glutamate and pyruvate)

→2) Circulating/intracellular glutamate can be made into glutamine (and return to the liver)

Question 17

how do muscles come into the removal of nitrogen?

Answer 17

→muscle can export alanine, as it is one of the major exports of muscle that is actively being broken down (due to exercise or starvation).

Question 18

describe the glucose-alanine cycle between the muscle and the liver

Answer 18

→In the muscle, branched amino acids are taken and broken down.

→The carbon skeleton is used for energy production.

→ the NH4 can be used to convert to pyruvate to Alanine. Alanine is then exported into the blood and travels to the liver.

→ alanine is then converted to glutamate via transamination (reacting with α-ketoglutarate) also producing a pyruvate.

→The pyruvate can enter the gluconeogenic pathway to form glucose, and the glucose can be transported in the blood back to the muscle where it can be used for energy.

→The glutamate will then be used along with the CO2 generated to produce urea in the liver.

Question 19

why is glutamate a useful molecule?

Answer 19

→Glutamate is a very useful molecule
→ freely interchangeable with the α-keto acids
→ability to donate and accept ammonium ions.