T7. AMINO ACID METABOLISM

Question 1

Why is nitrogen important in amino acid metabolism?

Answer 1

Because most organisms lack nitrogen stores needed for synthesizing amino acids and nitrogenous bases.

Question 2

What forms of nitrogen exist in nature?

Answer 2

Nitrate and dinitrogen

Question 3

What is the main source of nitrogen in humans?

Answer 3

Dietary protein.

Question 4

What is nitrogen balance in healthy individuals?

Answer 4

Protein synthesis equals protein degradation (consumption = excretion).

Question 5

What is positive nitrogen balance?

Answer 5

When nitrogen consumption exceeds excretion (e.g.

Question 6

What is negative nitrogen balance?

Answer 6

When nitrogen excretion exceeds consumption (e.g.

Question 7

What are the origins of amino acids?

Answer 7

Digestion of dietary proteins

Question 8

What are the fates of amino acids?

Answer 8

Used to synthesize proteins and nitrogenous compounds or degraded for energy (10–15% of body energy).

Question 9

What are the four steps of amino acid catabolism?

Answer 9

1. Transamination
2. Oxidative deamination
3. Urea cycle
4. Carbon skeleton utilization

Question 10

What is transamination?

Answer 10

Formation of glutamate from α-ketoglutarate in a reversible reaction catalyzed by specific aminotransferases.

Question 11

What coenzyme is involved in transamination?

Answer 11

Pyridoxal phosphate (PLP)

Question 12

What is the clinical relevance of elevated aspartate aminotransferase (AST)?

Answer 12

It can indicate liver disease or infarction.

Question 13

What is the clinical relevance of elevated alanine aminotransferase (ALT)?

Answer 13

It is used as a diagnostic marker of liver disease.

Question 14

What happens to ammonia during transamination?

Answer 14

It is transferred to another amino acid but not released.

Question 15

Where does oxidative deamination of glutamate occur?

Answer 15

In the hepatic mitochondria.

Question 16

What enzyme catalyzes oxidative deamination?

Answer 16

Glutamate dehydrogenase.

Question 17

What does oxidative deamination of glutamate produce?

Answer 17

Free ammonium ion (NH4+) and α-ketoglutarate.

Question 18

Which coenzyme is used in oxidative deamination?

Answer 18

NADP+ is used as an electron acceptor.

Question 19

What activates glutamate dehydrogenase?

Answer 19

ADP.

Question 20

What inhibits glutamate dehydrogenase?

Answer 20

GTP

Question 21

What is the purpose of detoxifying the ammonium group?

Answer 21

To prevent toxic accumulation of ammonia in the body.

Question 22

Which organ is responsible for detoxifying ammonia?

Answer 22

The liver.

Question 23

In what form is nitrogen transported from extrahepatic tissues to the liver?

Answer 23

As glutamine and alanine.

Question 24

What enzyme synthesizes glutamine in peripheral tissues?

Answer 24

Glutamine synthase adds ammonia to glutamate to form glutamine.

Question 25

What happens to glutamine in the liver?

Answer 25

It enters mitochondria

Question 26

What role does alanine play in nitrogen transport?

Answer 26

Alanine carries ammonia and pyruvate carbon skeleton from muscle to liver.

Question 27

What happens to alanine in the liver?

Answer 27

Ammonia is excreted and pyruvate is used to make glucose (returned to muscle).

Question 28

Why must NH4+ be excreted?

Answer 28

Because excess ammonium is toxic to the body.

Question 29

How do ammoniotelic animals excrete nitrogen?

Answer 29

They excrete ammonia (NH4+) directly (e.g.

Question 30

How do ureotelic animals excrete nitrogen?

Answer 30

They excrete nitrogen as urea (e.g.

Question 31

How do uricotelic animals excrete nitrogen?

Answer 31

They excrete nitrogen as uric acid (e.g.

Question 32

What is urea?

Answer 32

A small molecule that contains N an it is use as final product to secrete N

Question 33

Where does the urea cycle occur?

Answer 33

Partially in the mitochondria (first two steps) and partially in the cytosol of hepatic cells.

Question 34

What does the urea cycle do?

Answer 34

It synthesizes urea from ammonium and carbon dioxide in a cyclic process.

Question 35

How many enzymes are involved in the urea cycle?

Answer 35

Five enzymes.

Question 36

What is the global reaction of the urea cycle?

Answer 36

HCO₃⁻ + NH₄⁺ + 3 ATP + Asp + H₂O → Urea + 2 ADP + 4 Pi + AMP + Fumarate.

Question 37

What is the function of carbamoyl-phosphate synthetase I (CPS-I)?

Answer 37

Condenses CO₂ with ammonium to form carbamoyl-phosphate using 2 ATP.

Question 38

What does ornithine transcarbamoylase (OTC) do?

Answer 38

Combines carbamoyl-phosphate with ornithine to form citrulline.

Question 39

What is the role of argininosuccinate synthetase (ASS)?

Answer 39

Combines citrulline (from mitochondria) with aspartate in cytosol to form argininosuccinate (with 2 amino groups).

Question 40

What is the function of argininosuccinate lyase (ASL)?

Answer 40

Breaks down argininosuccinate into fumarate and arginine.

Question 41

What happens to fumarate produced by ASL?

Answer 41

It is derived from the carbon skeleton of aspartate and enters the Krebs cycle.

Question 42

What does arginase do?

Answer 42

Breaks arginine with water to produce urea and regenerate ornithine.

Question 43

What is the link between the urea cycle and the Krebs cycle?

Answer 43

Fumarate from urea cycle enters Krebs cycle

Question 44

What enzyme transfers an amino group from glutamate to oxaloacetate?

Answer 44

Aspartate aminotransferase (ASAT)

Question 45

What is the role of aspartate in the urea cycle?

Answer 45

It provides the second nitrogen group for urea synthesis.

Question 46

What regulates the urea cycle long term?

Answer 46

Transcriptional regulation—enzyme levels increase with a protein-rich diet or fasting.

Question 47

What regulates the urea cycle short term?

Answer 47

Activation of CPS-I by N-acetylglutamate

Question 48

What signals high amino acid availability?

Answer 48

High glutamate concentration → synthesis of N-acetylglutamate → activates CPS-I → stimulates urea cycle.

Question 49

What role does arginine play in regulating the urea cycle?

Answer 49

It activates N-acetylglutamate synthase

Question 50

What happens to the carbon skeletons of amino acids?

Answer 50

They are degraded through pathways that converge into 7 metabolic intermediates.

Question 51

What are the 5 gluconeogenic intermediates from amino acids?

Answer 51

1. Pyruvate 2. Oxaloacetate 3. Succinyl-Coa 4. Fumarate 5. a-Ketoglutarate

Question 52

What are ketogenic amino acids?

Answer 52

Amino acids that generate acetyl-CoA and acetoacetyl-CoA

Question 53

Which amino acids are exclusively ketogenic?

Answer 53

Leucine and lysine.

Question 54

Which amino acids can mammals synthesize?

Answer 54

Only the non-essential amino acids; essential ones must come from diet.

Question 55

What are the carbon skeleton precursors for nonessential amino acid biosynthesis?

Answer 55

From glycolysis: 3-phosphoglycerate

Question 56

What causes amino acid metabolism disorders?

Answer 56

Defects in urea cycle enzymes or amino acid catabolic enzymes leading to toxic metabolite accumulation.

Question 57

What happens when urea synthesis is absent or defective?

Answer 57

Ammonium accumulates

Question 58

What is phenylketonuria (PKU)?

Answer 58

A defect causing phenylalanine buildup in the blood

Question 59

What is maple syrup urine disease?

Answer 59

Accumulation of branched-chain amino acids (leucine

Question 60

How does maple syrup urine disease present clinically?

Answer 60

With food refusal

Question 61

What is tyrosinemia?

Answer 61

Tyrosine and derivatives accumulate