Chapter 30

Question 1

True or False: Amino acids are obtained from the diet when proteins are digested

Answer 1

True

Question 2

Cellular proteins are degraded to amino acids because of ______ or for _______________

a) damage; regulatory purposes
b) modification; glycolysis
c) the synthesis of peptides; regulatory purposes

Answer 2

a) damage; regulatory purposes

Question 3

True or False: First priority for use of amino acids is as precursors for proteins and other biomolecules

Answer 3

True

Question 4

True or False: Amino acids are stored

Answer 4

False

Amino acids are not stored, so any excess amino acids are degraded

Question 5

Amino Acids Degradation

Answer 5

 Alpha-amino groups are converted into ammonium ions by the oxidative deamination of glutamate

 Aminotransferases (transaminases) transfer amino groups from an amino acid to a-ketoglutarate to generate glutamate

 Examples:
- Aspartate + a-ketoglutarate  oxaloacetate + glutamate
- Alanine + a-ketoglutarate  pyruvate + glutamate

–> aspartate aminotransferase catalyzes transfer of the a-amino group of aspartate to a-ketoglutarate

–> alanine aminotransferase catalyzes transfer of amino group of alanine to a-ketoglutarate

 Glutamate can be oxidatively deaminated by glutamate dehydrogenase (a mitochondrial enzyme)
- Amino group forms NH4+

 Some amino acids can be directly deaminated using a dehydratase
 Serine and threonine can be directly deaminated

• Serine  pyruvate + NH4+
• Threonine  a-ketobutyrate + NH4+

 In most terrestrial vertebrates, the ultimate fate of NH4+ is the formation of urea

 Muscle uses branched-chain amino acids as fuels

• Nitrogen atoms transported to liver by glucose-alanine cycle
• Nitrogen atoms transported as glutamine by glutamine synthetase

Question 6

Clinical Insight: Blood Levels of Aminotransferases serve as a Diagnostic Function

Answer 6

 The presence of alanine and aspartate aminotransferase in blood = indication of liver damage

 Liver damage can occur for several reasons, including

• viral hepatitis
• long-term excessive alcohol consumption
• reaction to drugs such as acetaminophen

 Under these conditions, liver cell membranes are damaged and some cellular proteins, including the aminotransferases, leak into the blood

 Normal blood values for alanine and aspartate aminotransferase activity are 5–30 units/l and 40–125 units/l, respectively. Depending on the extent of liver damage, the values will reach 200–300 units/l

Question 7

The glucose-alanine cycle

Answer 7

1. glycogen breakdown (muscle)
2. glycolysis (muscle)
3. CAC (muscle)
4. oxidative phosphorylation (muscle)
5. gluconeogenesis (liver)
6. urea cycle (liver)

During prolonged exercise or fasting, muscle cells will use branched chain amino acids as fuel. The nitrogen will be removed and transferred through glutamate to alanine which is released into the bloodstream. In the liver, alanine is taken up and converted to pyruvate for subsequent synthesis of glucose

Question 8

In terrestrial vertebrates, excess NH4+ is converted into urea by __________

a) gluconeogenesis
b) the urea cycle
c) glycolysis
d) fatty acid degradation

Answer 8

b) the urea cycle

Question 9

Organisms that excrete excess NH4+ as urea are called ______________

a) urea-ammonia organisms
b) ureotelic insects
c) proteolytic organisms
d) ureotelic organisms

Answer 9

d) ureotelic organisms

Question 10

In humans, the urea cycle occurs in the _______

a) liver
b) kidney
c) small intestine

Answer 10

a) liver

Question 11

Describe features of ammonia

Answer 11

• Small and mobile
• Volatile
• Basic properties:
  –> accepts proton to become NH4+ (ammonium)
  –> can increase pH

Question 12

Describe features of urea

Answer 12

• Soluble in water
• Neutral
• Non-toxic
• Can be stored

Question 13

Urea Production ~ The Urea Cycle Overview

Answer 13

Urea has 2 nitrogen atoms
- 1 N-atom is transferred from amino acid ~ aspartate
- 1 N-atom is derived directly from free ammonia via glutamate dehydrogenase

1. The carbamoyl group is transferred to ornithine by ornithine transcarbomoylase to form citrulline
2. Citrulline is transported out of the mitochondria into the cytoplasm in exchange for ornithine
3. In the cytoplasm, citrulline condense w/ aspartate to form argininosuccinate catalyzed by argininosuccinate synthetase
4. Argininosuccinate is cleaved into arginine and fumarate by argininosuccinase
5. Arginine is cleaved by arginase into urea and ornithine

Question 14

Urea Cycle: Step 1

Answer 14

 The first step in urea cycle is the coupling of ammonia (NH3) with bicarbonate (HCO3)

 This reaction, which occurs in the mitochondria, is catalyzed by carbamoyl phosphate synthetase I (CPS I)

• Carbamoyl phosphate synthetase I (CPS I) is key regulatory enzyme

 In this step, 2 ATP are required (irreversible)

 Synthetase required N-acetylglutamate (NAG) for activity
- N-acetylglutamate is synthesized when proteins are abundant
- N-acetylglutamate is an allosteric activator
- N-acetylglutamate is formed from acetyl-CoA and glutamate ~ this formation occurs when there are excess amino acids

Question 15

Urea Cycle: Step 2

Answer 15

Carbamoyl group transferred to ornithine via ornithine transcarbamoylase –> forms citrulline –> which is transported out of mitochondria and into cytoplasm in exchange for ornithine

Question 16

Urea Cycle: Step 3

Answer 16

Citrulline condenses w/ aspartate via argininosuccinate synthetase –> forming argininosuccinate

Question 17

Urea Cycle: Step 4

Answer 17

Argininosuccinate is cleaved into arginine and fumarate via argininosuccinase

Question 18

Urea Cycle: Step 5

Answer 18

• Arginine is cleaved via arginase into urea and ornithine
• Urea is excreted and ornithine is transported to the mitochondria

Question 19

Urea Cycle ~ Linked to TCA & Gluconeogenesis

Answer 19

• Fumarate can be converted into oxaloacetate by the CAC
• Oxaloacetate cane have different fates

1. Can be transaminated into aspartate
   –> aspartate can supply nitrogen in urea cycle
2. Can be converted to phosphoenolpyruvate in gluconeogenesis
3. Can be converted into citrate

Question 20

Fate of Carbon Atoms

Carbon skeletons of amino acids metabolized into 7 major metabolic intermediates. Name them

Answer 20

• Pyruvate
• Acetyl-CoA
• Acetoacetyl-CoA
• Succinyl-CoA
• A-ketoglutarate
• Fumarate
• Oxaloacetate

Question 21

Fate of Carbon Atoms

Amino acids metabolized to acetyl-CoA and acetoacetyl-CoA are called __________

a) ketogenic amino acids
b) gluconeogenic amino acids

Answer 21

a) ketogenic amino acids

• b/c they can form fats but not glucose
• only leucine and lysine are solely ketogenic

Question 22

Fate of Carbon Atoms

Amino acids degraded to the remaining major intermediates are called _________________

a) gluconeogenic amino acids
b) ketogenic amino acids
c) proteolytic amino acids

Answer 22

a) gluconeogenic amino acids

• b/c they can be used to synthesize glucose

Question 23

Fate of Carbon Atoms

Additional steps are required to process other amino acids into carbon skeleton entry points. Name the entry points and number of carbon skeletons for each major metabolic intermediates

Answer 23

• Pyruvate ~ entry point of 3-C amino acids (alanine, serine & cysteine)
• Oxaloacetate ~ entry point of 4-C amino acids (aspartate & asparagine)
• Fumarate ~ entry point for components of phenylalanine/tyrosine
• a-ketoglutarate ~ entry point for 5-C amino acids (glutamine, proline, arginine, and histidine)
• Succinyl-CoA ~ entry point of several non-polar amino acids (methionine, isoleucine, threonine, and valine)
• Acetyl-CoA and acetoacetate ~ entry point of branched amino acids

Question 24

Clinical Insight: Inherited Effects of the Urea Cycle cause Hyperammonia

Answer 24

All defects in the urea cycle can lead to an elevated level of NH4+ in the blood (hyperammonia)

• Some of the genetic defects become evident a day or two after birth, when infant becomes lethargic and vomits periodically ~ coma and irreversible brain damage may soon follow, called hepatic encephalopathy

Excessive alcohol consumption can result in hyperammonia

• Concerning the effects of ethanol consumption on the liver, much of the damage is due to excessive production of NADH

Question 25

Which amino acids have the following fate: Oxaloacetate

Answer 25

asparagine and aspartate

Question 26

Which amino acids have the following fate: fumarate

Answer 26

aspartate, phenylalanine, tyrosine

Question 27

Which amino acids have the following fate: Succinyl-CoA

Answer 27

isoleucine, methionine, threoinine, valine

Question 28

Which amino acids have the following fate: a-ketoglutarate

Answer 28

arginine, glutamate, glutamine, histidine, proline

Question 29

Which amino acids have the following fate: acetoacetyl-CoA

Answer 29

leucine, lysine, phenylalanine, tryptophan, tyrosine

Question 30

Which amino acids have the following fate: acetyl-CoA

Answer 30

isoleucine, leucine, threoinine, tryptophan