Chapter 18 Part 1

Question 1

When does amino acid oxidation happen in animals?

Answer 1

1. Normal synthesis and degradation of cellular proteins.
2. A diet rich in protein produces more amino acids than are required. The amino acids are oxidized as there is no mechanism for storage.
3. During starvation or uncontrolled diabetes, carbohydrates are scarce or not properly used and cellular proteins are degraded and their amino acids are oxidized for fuel.

Question 2

What is the first step in amino acid catabolism?

Answer 2

Deamination

Question 3

What are the ammonium and carbon skeletal remains called?

Answer 3

alpha-keto acids

Question 4

Amino Group Catabolism

Answer 4

Amino groups from amino acids are transferred to alpha-ketoglutarate in the liver to form glutamate.

Glutamate and glutamine enter the mitochondria and gives up the amino group as NH4+.

These groups are either used for resynthesis or converted to an excretory end product.

Question 5

How does the body degrade dietary protein?

Answer 5

1. Acidic gastric juice denatures proteins allowing internal peptide bonds to be more accessible.
2. Pepsinogen (proenzyme/zymogen) is converted to pepsin hydrolyzing the N-terminal side of F,W, & Y.
3. Bicarbonate is released into the small intestine bringing the pH to ~7.
4. Zymogens trypsinogen & chymotrypsinogen are cleaved to trypsin & chymotrypsin which hydrolyze peptides at distinct sequences.
5. Carboxypeptidases and aminopeptidases hydrolyze the shortened peptides to amino acids.

Question 6

What is an Aminotransferase

Answer 6

Catalyze the transfer of the amine group of L-amino acids to α- ketoglutarate forming a glutamate and α- keto acids.

They are specific for a certain amino acid.

They use Pyridoxal Phosphate (PLP)

Question 7

What is PLP?

Answer 7

PLP is a prosthetic group used by aminotransferases. It comes from vitamin B6.

It carries the amine intermediately.

It is covalently attached to the N of lysine via an aldimine linkage.

Question 8

What happens during the PLP reaction?

Answer 8

The NH3 group of the amino acid replaces the N on lysine.

Question 9

What kind of reactions does PLP participate in?

Answer 9

1. Transaminations
2. Racemizations
3. Decarboxylations

Question 10

Where does the deamination of glutamate happen?

Answer 10

Mitochondria

Question 11

What enzyme is responsible for the deamination of glutamate?

Answer 11

L-glutamate dehydrogenase

Question 12

What does L-glutamate dehydrogenase do?

Answer 12

Catalyzes the oxidative deamination of glutamate to produce α-ketoglutarate and NH4+.

Question 13

What is special about L-glutamate dehydrogenase?

Answer 13

It is one of the only enzymes that can use NAD+ or NADP+ as the electron acceptor.

Question 14

Glutamine Transport

Answer 14

Ammonia in extrahepatic tissues is transported in the blood to the liver by converting it to glutamine.

Question 15

What does Glutamine synthetase do?

Answer 15

Catalyzes the ATP-dependent attachment of ammonia to glutamate.

Question 16

What does glutaminase do?

Answer 16

Converts the glutamine back to glutamate and an ammonium ion once it reaches the liver mitochondria.

Question 17

Glucose-Alanine Cycle

Answer 17

In the muscle, amino groups can also be transferred to pyruvate to form alanine.

Alanine can then travel in the blood to the liver where it undergoes transdeamination regenerating pyruvate which is used in gluconeogenesis.

Question 18

What does alanine aminotransferase do?

Answer 18

Transfers an amino group to pyruvate to form alanine.

Question 19

What is transdeamination?

Answer 19

Combination of the transamination and deamination reactions.

Question 20

What is Ammonotelic?

Answer 20

Species which excrete nitrogen as ammonia; aquatic species e.g. bony fish.

Question 21

What is Ureotelic?

Answer 21

Species which excrete nitrogen as urea; most terrestrial animals, humans; occurs in the liver.

Question 22

What is Uricotelic?

Answer 22

Species which excrete nitrogen as uric acid; birds and reptiles.

Question 23

What happened to Ammonium in the mitochondria?

Answer 23

It is immediately used with HCO3- and ATP to form carbamoyl phosphate.

Question 24

What does Carbamoyl phosphate synthetase I do?

Answer 24

Catalyzes the formation of carbamoyl phosphate in the mitochondria.

Question 25

What is the function of carbamoyl phosphate?

Answer 25

As an activated carbamoyl group donor.

Question 26

What does Ornithine transcarbamoylase do?

Answer 26

Catalyzes the transfer of the carbamoyl group to ornithine releasing inorganic phosphate to form Citrulline.

Question 27

What is special about Ornithine?

Answer 27

Ornithine plays an analogous role in the urea cycle as oxaloacetate plays in the citric acid cycle (the beginning and end of the cycle).

Question 28

What does Argininosuccinate synthetase do?

Answer 28

Catalyzes the ATP- dependent reaction via a citrullyl-AMP intermediate to produce argininosuccinate. This happens in the cytosol.

Question 29

What does argininosuccinase do?

Answer 29

Catalyzes the cleavage of agininosuccinate into arginine and fumarate.

Question 30

What happens to fumarate after it is produced by argininosuccinase?

Answer 30

It enters the mitochondria and feeds the citric acid cycle.

Question 31

What does arginase do?

Answer 31

Cleaves the arginine to yield urea and ornithine.

Question 32

What happens to Ornithine at the end of the cycle?

Answer 32

Once regenerated, the ornithine is transported back into the mitochondria to begin the cycle again.

Question 33

Where does each step in the urea cycle happen?

Answer 33

In liver.

Step 1 in the mitochondria.

Steps 2, 3 and 4 in the cytosol.