Biochemistry Endocrine

Question 1

What is the ultimate fate of most amino acid nitrogen in the body?

Answer 1

Converted to urea and excreted

This is a key metabolic process in the urea cycle.

Question 2

What cofactor is required by all transaminases?

Answer 2

PLP

PLP stands for pyridoxal phosphate, derived from vitamin B6.

Question 3

Which two products are formed in the ALT reaction?

Answer 3

Pyruvate and glutamate

ALT stands for alanine aminotransferase.

Question 4

What product results when AST acts on aspartate and α-ketoglutarate?

Answer 4

Glutamate and oxaloacetate

AST stands for aspartate aminotransferase.

Question 5

PLP is formed from which B vitamin?

Answer 5

B6

Vitamin B6 is essential for amino acid metabolism.

Question 6

Which two amino acids are the major nitrogen carriers in blood?

Answer 6

Alanine and glutamine

These amino acids play a crucial role in nitrogen transport.

Question 7

What enzyme converts glutamate to α-ketoglutarate and releases NH4+?

Answer 7

Glutamate dehydrogenase

This enzyme catalyzes reversible reactions in amino acid metabolism.

Question 8

Which enzyme hydrolyzes glutamine to release ammonium ion?

Answer 8

Glutaminase

Glutaminase is important for nitrogen entry into the urea cycle.

Question 9

What role does α-ketoglutarate play in nitrogen metabolism?

Answer 9

Accepts nitrogen to form glutamate

α-ketoglutarate is a key intermediate in the Krebs cycle.

Question 10

Which enzyme catalyzes the first step in the urea cycle?

Answer 10

Carbamoyl phosphate synthetase I

This enzyme initiates the urea cycle by converting ammonia and bicarbonate.

Question 11

What is the fate of fumarate in the urea cycle?

Answer 11

Converted to malate

Fumarate is an intermediate that links the urea cycle to the Krebs cycle.

Question 12

What happens to ammonia levels if the urea cycle is impaired?

Answer 12

They increase

Elevated ammonia levels can lead to toxicity and other metabolic issues.

Question 13

How does ketone body use by the brain during fasting spare muscle protein?

Answer 13

Reduces need for gluconeogenesis

This process helps preserve muscle mass during prolonged fasting.

Question 14

What does an elevated BUN indicate?

Answer 14

Impaired kidney function

BUN stands for blood urea nitrogen, a common diagnostic marker.

Question 15

The urea breath test is primarily used to detect infection by which organism?

Answer 15

H. pylori

Helicobacter pylori is associated with gastric ulcers and other gastrointestinal disorders.

Question 17

What is the ultimate fate of most amino acid nitrogen?

Answer 17

Converted to urea in the liver and excreted in urine by the kidneys

This process detoxifies ammonia, a byproduct of amino acid catabolism.

Question 18

What is the structure of urea?

Answer 18

O=C(NH2)2

Question 19

What do transamination reactions involve?

Answer 19

Transfer of an amino group from an amino acid to an α-keto acid

Catalyzed by transaminases (aminotransferases) and require pyridoxal phosphate (PLP).

Question 20

What is the cofactor required for transamination reactions?

Answer 20

Pyridoxal phosphate (PLP)

Question 21

What reaction does alanine transaminase (ALT) catalyze?

Answer 21

Alanine + α-ketoglutarate ⇌ Pyruvate + Glutamate

Question 22

What role does aspartate transaminase (AST) play?

Answer 22

Catalyzes Aspartate + α-ketoglutarate ⇌ Oxaloacetate + Glutamate

Question 23

What is the function of pyridoxal phosphate (PLP)?

Answer 23

Active coenzyme for transaminases that forms a Schiff base with amino acids

Question 24

What are the two major nitrogen carriers in blood?

Answer 24

• Alanine
• Glutamine

Question 25

What happens to branched-chain amino acids in the fasting state?

Answer 25

Transaminated to form glutamate, which releases NH4+

Question 26

How does glutamine contribute to nitrogen entry into the urea cycle?

Answer 26

Hydrolyzed by glutaminase to release NH4+

Question 27

What are the key enzymes involved in the urea cycle?

Answer 27

* ALT * AST * Glutaminase * Glutamate dehydrogenase * CPS-1

Question 28

What is the role of α-ketoglutarate in nitrogen metabolism?

Answer 28

Acts as an amino group acceptor to form glutamate

Question 29

Where does the urea cycle occur?

Answer 29

Partly in mitochondria and partly in cytosol

Question 30

What is the ATP cost per cycle of the urea cycle?

Answer 30

3 ATP per cycle

Question 31

What is the connection between aspartate and fumarate?

Answer 31

Fumarate is hydrated to malate, oxidized to oxaloacetate, and transaminated to form aspartate

Question 32

What are the effects of impaired urea cycle?

Answer 32

Leads to hyperammonemia, which is neurotoxic

Question 33

What happens to the urea cycle during fasting?

Answer 33

Protein catabolism provides amino acids whose nitrogen is excreted via the urea cycle

Question 34

What does an elevated Blood Urea Nitrogen (BUN) test indicate?

Answer 34

Impaired kidney function or increased protein catabolism

Question 35

What is the purpose of the urea breath test?

Answer 35

Detects Helicobacter pylori infection

Question 36

Fill in the blank: The urea cycle involves _______ as a key intermediate.

Answer 36

Citrulline

Question 37

True or False: Glutamate can release ammonia, form glutamine, or participate in transamination reactions.

Answer 37

True

Question 38

What hormonal shift promotes hepatic ketogenesis during fasting?

Answer 38

Insulin levels fall and glucagon levels rise ## Footnote This shift activates hormone-sensitive lipase in adipose tissue, promoting lipolysis.

Question 39

What is the primary starting material for ketogenesis?

Answer 39

Acetyl-CoA ## Footnote Acetyl-CoA is derived primarily from fatty acid β-oxidation during fasting or carbohydrate restriction.

Question 40

Where does ketogenesis occur?

Answer 40

Mitochondria of liver hepatocytes

Question 41

Which enzyme catalyzes the formation of HMG-CoA from acetoacetyl-CoA?

Answer 41

Mitochondrial HMG-CoA synthase

Question 42

What is the fate of HMG-CoA in ketogenesis?

Answer 42

Cleaved to form acetoacetate and acetyl-CoA by HMG-CoA lyase

Question 43

What are the two primary ketone bodies used as fuel?

Answer 43

* Acetoacetate * β-hydroxybutyrate

Question 44

What determines the interconversion between acetoacetate and β-hydroxybutyrate?

Answer 44

NAD+/NADH ratio

Question 45

How is acetone formed during ketogenesis?

Answer 45

Non-enzymatically by the spontaneous decarboxylation of acetoacetate

Question 46

What is the primary fate of acetone produced during ketogenesis?

Answer 46

Excreted in urine or exhaled

Question 47

Which hormone promotes ketogenesis during fasting?

Answer 47

Glucagon

Question 48

What is the role of β-hydroxybutyrate dehydrogenase in ketogenesis?

Answer 48

Interconverts acetoacetate and β-hydroxybutyrate

Question 49

What gives breath a fruity odor during fasting?

Answer 49

Acetone exhalation

Question 50

What happens to acetyl-CoA when it exceeds the capacity of the TCA cycle?

Answer 50

It is diverted to ketogenesis

Question 51

True or False: Acetone is a significant energy source.

Answer 51

False

Question 52

What is the ultimate fate of most amino acid nitrogen?

Answer 52

Most amino acid nitrogen is ultimately converted to urea in the liver and excreted in the urine by the kidneys.

Question 53

What is the structure of urea?

Answer 53

O=C(NH2)2

Question 54

What are transamination reactions?

Answer 54

Transamination reactions transfer an amino group from an amino acid to an α-keto acid.

Question 55

What enzyme catalyzes transamination reactions?

Answer 55

Transaminases (aminotransferases)

Question 56

What cofactor is required for transamination reactions?

Answer 56

Pyridoxal phosphate (PLP)

Question 57

What reaction does alanine transaminase (ALT) catalyze?

Answer 57

Alanine + α-ketoglutarate ⇌ Pyruvate + Glutamate

Question 58

What reaction does aspartate transaminase (AST) catalyze?

Answer 58

Aspartate + α-ketoglutarate ⇌ Oxaloacetate + Glutamate

Question 59

What is the role of α-ketoglutarate in nitrogen metabolism?

Answer 59

Acts as an amino group acceptor to form glutamate.

Question 60

What are the major nitrogen carriers in the blood?

Answer 60

* Alanine * Glutamine

Question 61

How is glutamine formed in the muscle during fasting?

Answer 61

Branched-chain amino acids are transaminated to form glutamate, which is then converted to glutamine.

Question 62

What is the starting point for nitrogen entry into the urea cycle?

Answer 62

Ammonium enters via carbamoyl phosphate.

Question 63

What are the key reactions of the urea cycle?

Answer 63

* CPS-1: NH4+ + HCO3- → Carbamoyl phosphate * Ornithine transcarbamoylase: Carbamoyl phosphate + Ornithine → Citrulline * Argininosuccinate synthetase: Citrulline + Aspartate + ATP → Argininosuccinate * Argininosuccinate lyase: Argininosuccinate → Arginine + Fumarate * Arginase: Arginine → Urea + Ornithine

Question 64

What is the effect of impaired urea cycle?

Answer 64

Leads to hyperammonemia, which is neurotoxic.

Question 65

What does an elevated Blood Urea Nitrogen (BUN) test suggest?

Answer 65

Impaired kidney function or increased protein catabolism.

Question 66

What is the urea breath test used for?

Answer 66

Detects Helicobacter pylori infection.

Question 67

What are essential amino acids?

Answer 67

Amino acids that must be obtained from the diet.

Question 68

What are nonessential amino acids?

Answer 68

Amino acids synthesized by the body.

Question 69

How is serine synthesized from glucose?

Answer 69

From 3-phosphoglycerate via oxidation, transamination, and dephosphorylation.

Question 70

What enzyme synthesizes glycine from serine?

Answer 70

Serine hydroxymethyltransferase

Question 71

What is the pathway for methionine catabolism?

Answer 71

Methionine is converted to homocysteine through S-adenosylmethionine (SAM) intermediates.

Question 72

What are the entry points for amino acids into metabolism?

Answer 72

* Pyruvate * Acetyl-CoA * Acetoacetyl-CoA * α-ketoglutarate * Succinyl-CoA * Fumarate * Oxaloacetate

Question 73

What are the branched-chain amino acids (BCAAs)?

Answer 73

* Leucine * Isoleucine * Valine

Question 74

What is the significance of elevated phenylalanine in PKU?

Answer 74

Classical PKU is due to deficiency of phenylalanine hydroxylase.

Question 75

What is the precursor for glutathione?

Answer 75

Glutamate, cysteine, and glycine.

Question 76

What is the role of PLP in amino acid metabolism?

Answer 76

Required as a cofactor for various transaminases.

Question 77

What enzyme catalyzes the conversion of aspartate to asparagine?

Answer 77

Asparagine synthetase

Question 78

What are aromatic amino acids?

Answer 78

* Phenylalanine * Tyrosine * Tryptophan

Question 79

What is the source of carnitine?

Answer 79

From lysine.

Question 80

What is the enzyme that catalyzes the reaction of creatine synthesis?

Answer 80

Creatine kinase

Question 81

What is the role of glutamate in neurotransmitter synthesis?

Answer 81

Precursor for GABA and other neurotransmitters.

Question 82

Fill in the blank: The urea cycle costs ______ ATP per cycle.

Answer 82

3