PoH: Nitrogen

Question 1

What two molecules in the body contain nitrogen?

Answer 1

Amino acids and nucleotides

Question 2

How many AAs are there?

Answer 2

20 (around 20)

Question 3

What’s an essential AA and how many are essential?

Answer 3

An AA we can’t synthesise in the body (in sufficient quantity) so we must get it from our diet

There’s 9 essential AAs

Question 4

What’s a conditionally essential AA?

Answer 4

AA required to some degree in young, growing and/or sometimes during illness

Question 5

What mnemonic is used to remember essential AAa?

Answer 5

PVT TIM HALL

Phenylalanine
Valine
Trytophan

Threonine
Isolucine
Methoinine

Histadine
(Arginine)
Lysine
Leucine

Question 6

During N metabolism, AAs can become carbon skeletons (deaminated AAs). What happens to them next?

Answer 6

We use them to form glucose or fats, or if we’re hungry, convert them to a metabolic intermediate that can be oxidised by the citric acid cycle

Question 7

N metabolism features 3 potential cycles. What are they?

Answer 7

Urea cycle

Aspartate-arginino-succinate shunt of citric acid cycle

Citric acid cycle

Question 8

Describe the fate of dietary protein

Answer 8

They’re enzymatically hydroysed by
Pepsin in the stomach
Trypsin and chymotrypsin in the SI
Aminopeptidase and carboxypeptidases A and B in the SI

Question 9

What’s the aim of the urea cycle

Answer 9

To take toxic ammonia and turn it into harmless urea - ridding the body of excess nitrogen

Question 10

Where does the urea cycle take place

Answer 10

Mitochondria and cytoplasm

Question 11

The body catalyses proteins that are… (4 things)

Answer 11

From the diet
Misfolded
Foreign
Unwanted

Question 12

How is protein catalysed?

Answer 12

It’s turned into proteasomes
Then peptide fragments
Proteolysis breaks them into amino acids

Question 13

What causes PKU?

Answer 13

Normally, phenylalanine is converted to tyrosine by enzyme phenylalanine hydrogase

In PKU there’s an absence of the enzyme. Substrates are instead converted to toxic products which can cross the blood-brain barrier and cause irreversible damage

Question 14

Explain screening for PKU

Answer 14

Infants are screened via a heel-prick “Guthrie Card” test on Day 5

Question 15

How is PKU treated?

Answer 15

It’s treated by reducing protein in the diet and supplementing tyrosine. They’re then monitored through blood tests.

Question 16

Define metabolic disorders relating to urea cycle and give 2 examples

Answer 16

Caused by any step in the urea cycle not functioning properly. The faulty step can be identified by systematically testing concentrations of molecules in the pathway and finding which levels are too high and which levels are too low

PKU
Ornithine transcarbonmoylase (OTC) deficiency

Question 17

Symptoms of OTC deficiency and # who have it

Answer 17

Hyperammonaemia which is toxic. It’s most often noticed in babies who suffer from seizures where blood tests show high ammonia levels. Around 1 in 40,000 have it.

Question 18

Define transamination

Answer 18

moving amino groups (NH2) from one AA to another AA, to make a new AA and alpha-keto acid. Glutamate is both a donor and an acceptor of amino groups.

Question 19

Describe the role of glutamate in transfer of nitrogen to, from and between amino acids.

Answer 19

Glutamate is central to nitrogen entering the body.

Once it is broken down, the nitrogen from those amino acids is transferred (transaminated) to alpha-ketoglutarate to form glutamate.

Glutamate is then used as an amino group donor for the synthesis of non-essential amino acids.

Question 20

What’s special about glutamate

Answer 20

Glutamate helps N get into body. It’s a precursor in the biosynthesis of other AAs like proline, arginine and histidine

Question 21

How is N transported through plasma to the liver?

Answer 21

1. We eat N
2. The GI tract catabolises it to AAs
3. AAs are absorbed into bloodstream
4. Cells take AAs
5. Cells leave toxic ammonia
6. Ammonia goes into bloodstream
7. Liver converts ammonia to urea via urea cycle