Protein breakdown and Urea Formation

Question 1

LOOK AT DIAGRAM IN NOTES TO SEE THE INTERACTION OF THE UREA CYCLE WITH THE CITRIC ACID CYCLE

Answer 1

LOOK AT DIAGRAM IN NOTES TO SEE THE INTERACTION OF THE UREA CYCLE WITH THE CITRIC ACID CYCLE

Question 2

What happens to excess protein?

Answer 2

Excess protein is broken down and excreted.

Question 3

What happens if there is too much protein?

Answer 3

There is positive nitrogen balance. Look at diagram in notes. Majority is taken into the amino acid pool and used to increase body protein with urea and other products also being made in smaller amounts.
This occurs if there is a lot of exercise and tissue hypertrophy (Hypertrophy is the increase in the volume of an organ or tissue due to the enlargement of its component cells). This is driven by anabolic hormones

Question 4

What happens if there too little protein?

Answer 4

Negative nitrogen balance. Look at diagram in notes. The amino acid pool is not given much so it cannot therefore create as much body protein so there is a decrease.
Occurs when there is deficiency in diet – protein is lost as other processes do continue. This gives a net loss of protein.

Question 5

What are the parts of an amino acid?

Answer 5

• the carbon skeleton and is used for:
• Energy metabolism
• Biosynthetic pathways
• Nitrogen
• Has to be removed as it is toxic
• In mammals the nitrogen is converted to the non-toxic neutral compound urea and excreted in the urine
• Amino acid nitrogen is transferred to urea in three steps: transamination, formation of ammonia and formation of urea

Question 6

What are the steps involved in amino acid breakdown?

Answer 6

1) transamination
2) formation of ammonia
3) urea formation

Question 7

What occurs in the first step of amino acid break down, transamination?

Answer 7

-The enzymes involved in this step are called transaminase/aminotransaminase and examples o these enzymes are alanine and aspartate transaminase. Their role is to transfer a a-amino group from an amino acid to a a-keto acid.
* These enzymes are predominately liver enzymes so high levels of AST and ALT in the blood are indicative of liver damage e.g. cirrhosis
-First the nitrogen group of one amino acid is transferred to keto acid to give to another amino acid. Look at diagram in notes. Examples of a a-keto acid include α-ketoglutarate, pyruvate and oxaloacetate which are all important metabolic intermediates. These can be metabolised easily.
* α-ketoglutarate, pyruvate and oxaloacetate can be oxidised or converted to make glucose –
-This whole reaction requires pyridoxal phosphate derived from vitamin B.
NOTE:
In the context of urea formation although the reactions of these two enzymes are almost fully reversible the equilibrium of the ALT is towards the formation of pyruvate and glutamate. The equilibrium of the AST however is over towards the left and the generation of aspartate and α-ketoglutarate.

Question 8

What occurs in the second step of amino acid break down, formation of ammonia?

Answer 8

-One of the things that can happen to the glutamate can be converted to α-ketoglutarate but through a different enzymatic route.
-The enzyme important in this is 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
-This reaction generates free ammonia. This ammonia is a substrate for the urea cycle.
-This process is called oxidative deamination
Look at diagram in notes.

Question 9

What happen to the free ammonia generates in tissue?

Answer 9

It combines with glutamate to give glutamine.
The glutamate gains Nitrogen. This requires energy and the enzyme Glutamine synthetase and produces glutamine. Glutamine is readily soluble that can be transported around the body and so Glutamine is the main transporter of nitrogen.
Glutamine can donate nitrogen for the biosynthesis of amino acids, nucleotides, amino sugars and NAD+
Look at diagram in notes.
NOTE:
In some cases the ion used is NH3+ because the ammonium ions and ammonia are used synonymously. Under normal pH of about 7-7.2 the ammonia will be present as ammonium ions.

Question 10

What occurs in the third step of amino acid break down, urea formation?

Answer 10

• The urea cycle occurs. The urea cycle is the means of excreting nitrogen, the enzymes for this cycle are localised so they are present only in the liver and not muscle. It takes place in the mitochondria and the cytoplasm. The substrates for urea formation are bicarbonate, aspartate and ammonium ions (released from either glutamine or glutamate).
• The urea cycle is sort of combined with TCA cycle. There are two parts to it: one part leads to generation of aspartate (transaminase reaction we talked about) and the other is the ammonium ion that comes from glutamate.
  1) The ammonia reacts to form carbamoyl phosphate.
  2) This combines with ornithine to give citrulline.
  3) Citrulline and aspartate react to give arginino-succinate.
  4) This will break down to give arginine which under the influence of arginase gives urea.
  5) Fumarate is converted to malate and then oxaloacetate. This can be used to react with further amino acid to give a-keto acid and aspartate.

Question 11

Why is it that in muscles, it is unable to do the citric acid cycle?

Answer 11

• It does not have the arginase or components of the cycle. This process is however is important for muscle in prolonged exercise and starvation. In prolonged exercise or starvation the muscle proteins will break down to produce primarily alanine. The alanine is used to generate glucose with the help of the liver (branched amino acids are used for energy)
• Enzymes of the urea cycle not present in muscle

Question 12

What are the two routes that nitrogen can be transported to the liver?

Answer 12

-Alanine
*Nitrogen transferred to alanine via glutamate and
pyruvate
-Glutamine
*Glutamate is made into glutamine

Question 13

What happens in the glucose alanine cycle?

Answer 13

Look at diagram

Is the series of reactions in which amino groups and carbons from muscle are transported to the liver

Question 14

What happens to the carbon skeleton that is created in the glucose alanine cycle?

Answer 14

Some of the carbon skeleton from the breakdown of amino acids will form a-keto acids i.e. pyruvate, oxaloacetate and a-ketoglutarate. Some of the backbone of other amino acids can also feed into different components of the TCA cycle.

Question 15

All amino acids can be interconverted to a-keto acids expect for two, what are they?

Answer 15

Lysine and Threonine

Question 16

After a meal, how are the protein and amino acid metabolised by a person with a normal metabolism?

Answer 16

(high insulin, low glucagon)
In a normal individual most amino acids from a protein meal will be used for protein synthesis in peripheral tissues such as skeletal muscle.
Excess amino acids can also be used as sources of energy, and the nitrogen derived from their oxidation will be incorporated into urea in the liver and excreted.

Question 17

During starvation, how are the protein and amino acid metabolised by a person with a normal metabolism?

Answer 17

In a normal individual: insulin is low, glucagon is high
During short-term starvation there will be a net flow of amino acids from muscle to the liver, with increased production of glucose and urea.
During long-term starvation, tissue protein is “spared” because ketone bodies replace glucose as a major energy fuel for the brain.

Question 18

How are the protein and amino acid metabolised by a person with untreated diabetes?

Answer 18

Here the negative nitrogen balance associated with short term starvation persists even though the subject is fed, leading to muscle wasting.
Negative nitrogen balance due to decreased protein synthesis and/or increased protein breakdown is also seen in conditions of chronic infections, late stage cancer or trauma, including that following surgery or burns injury.
Some of these effects are mediated by cytokines

Question 19

What else can breakdown proteins to form glutamate?

Answer 19

Non -muscle tissues also breakdown proteins forming glutamate which can be converted to glutamine. This is transported back to the liver. In muscle there are physiological circumstances where turnover of protein is much higher.
Look at diagram.