Amino Acid Metabolism

Question 1

Sources and Sinks of amino acids

Answer 1

Question 2

Arginine

Answer 2

Required only during periods of growth and is generally not required by the adult

Technically “essential”

Question 3

Essential amino acids

Answer 3

Pvt. Tim Hall:

Phenylalanine, valine, thryptophan, threonine, isoleucine, methionine, histidine, arginine, leucine, lysine

Question 4

Waste product of amino acid catabolism

Answer 4

Question 5

hepatic encephalopathy

Answer 5

Encephalopathy caused by a loss of funtion of the liver and buildup of ammonia, resulting in ammonia poisoning.

Question 6

Amino acid metabolism outline

Answer 6

Question 7

Pathway of dietary amino acid absorption

Answer 7

Question 8

Amino acid transit via enterocytes

Answer 8

Question 9

_____ pass through the liver and are used primarily as an alternative energy source by peripheral muscle (especially in fasting) or adipose tissue (in the fed state).

Answer 9

leucine, isoleucine, and valine (the branched nonpolar amino acids) pass through the liver and are used primarily as an alternative energy source by peripheral muscle (especially in fasting) or adipose tissue (in the fed state).

This is because the liver lacks an enzyme required for metabolism of branched chain amino acids.

Question 10

Pancreatic response to dietary amino acids

Answer 10

Question 11

The ability of glucagon to stimulate ____ is important to maintain blood glucose levels after ____.

Answer 11

The ability of glucagon to stimulate gluconeogenesis is important to maintain blood glucose levels after a high protein meal.

Question 12

Regulation of muscle protein breakdown

Answer 12

Question 13

In skeletal muscle, cortisol exerts its effects at the level of ____.

Answer 13

In skeletal muscle, cortisol exerts its effects at the level of transcription.

Question 14

Akt/PKB in amino acid metabolism

Answer 14

Question 15

FOXO

Answer 15

Regulatory transcription factor that promotes the expression of E3’s that recognize and ubiquitinate skeletal muscle protein, leading to their degradation. (in addition to many other functions)

Question 16

Muscle is thought to be broken down by ___ in the fasted state.

Answer 16

Muscle is thought to be broken down by the ubiquitin proteasome pathway AND autophagy in the fasted state.

Question 17

Fate of absorbed amino acids

Answer 17

Question 18

The glucose-alanine cycle

Answer 18

Question 19

_____ are often the first step in amino acid metabolism.

Answer 19

Transamination reactions are often the first step in amino acid metabolism.

Question 20

Reaction catalyzed by aminotransferase

Answer 20

Question 21

All aminotransfer reactions share a common _____.

Answer 21

All aminotransfer reactions share a common mechanism AND cofactor (B₆, pyridoxal phosphate).

Question 22

High levels of pyruvate in muscle promotes. . .

Answer 22

. . . transfer of amino groups to pyruvate to form alanine.

Question 23

The common amino group donor is ____.

Answer 23

The common amino group donor is glutamate.

Question 24

Maple Syrup Urine Disease

Answer 24

The urine in these patients has the odor of maple syrup due to the presence of α-keto acids.

Caused by a partial or complete deficiency in the branched-chain α-keto acid dehydrogenase complex, which causes these amino acids and their ketoacids to accumulate in the blood. Their accumulation interferes with brain function and infants with this disease show symptoms within the first several days after birth. This disease is lethal within the first few weeks of life if left untreated.

Question 25

Treatment of maple syrup urine disease

Answer 25

The disease is treated using a formula that contains just enough branched chain amino acids for normal growth, but not enough for toxic accumulation of the amino acids or their metabolites.

Question 26

Branched-chain amino acid metabolism

Answer 26

Question 27

Aspartate, glutamate, and glutamine metabolism in the liver

Answer 27

Question 28

Only the ____ from aspartic acid is consumed by the urea cycle

Answer 28

Only the **amino group** from aspartic acid is consumed by the urea cycle **its carbon skeleton is recycled,** via the TCA cycle by forming **fumarate**

Question 29

Oxidative deamination of glutamate

Answer 29

Question 30

Catabolism of amino acid carbon skeletons

Answer 30

Question 31

Roles of the urea cycle

Answer 31

1. To get rid of nitrogenous waste 2. To supply arginine

Question 32

The Urea Cycle

Answer 32

Question 33

Link between urea and TCA cycles

Answer 33

Question 34

The main form of urea cycle regulation is. . .

Answer 34

. . . **substrate availability**

Question 35

Major activators of urea cycle

Answer 35

1. High protein diet 2. Fasting

Question 36

Arginine plays a special role in the urea cycle because it. . .

Answer 36

. . . accelerates flux through the urea cycle

Question 37

The urea cycle produces ____ which enters the TCA cycle and generates ____ that can be transaminated to ____ for the urea cycle

Answer 37

The urea cycle produces **fumarate** which enters the TCA cycle and generates **oxaloacetate** that can be transaminated to **aspartate** for the urea cycle

Question 38

Glutamine and glutamate transport between peripheral tissues and the liver

Answer 38

**Glutamate synthetase** in **peripheral tissues** uses energy from **ATP hydrolysis** to fix a free **ammonia** onto glutamate, forming **glutamine**. That glutamine is then shuttled to the liver, where **hepatic glutaminase** **hydrolyzes off the ammonia**, targeting it for the urea cycle, **regenerating glutamate.**

Question 39

Why does the glutamate/glutamine system work so well for transferring ammonia?

Answer 39

Because the amino group transferred in this cycle is **not an α-amino group**. Remember that **transaminases act only on α-amino groups, not on side chains.** This **separates the reaction** from the rest of transaminase nitrogen metabolism.

Question 40

The only oxidative deaminase

Answer 40

**Glutamate dehydrogenase**. This is the only one the body really needs, since all the other amino acids can feed into the glutamate pathway via transaminases.

Question 41

\_\_\_ and ___ aminotransferases are the most highly expressed in the liver.

Answer 41

**Aspartate** and **alanine** aminotransferases are the most highly expressed in the liver. This is because of the special **role of aspartate in the urea cycle** and the **role of alanine in the alanine cycle.**

Question 42

In the urea cycle, the ammonia that goes into carbamoyl phosphate comes from \_\_\_\_. The aspartate amino group comes from \_\_\_\_.

Answer 42

In the urea cycle, the ammonia that goes into carbamoyl phosphate comes from **glutamate dehydrogenase**. The aspartate amino group comes from **glutamate via aspartate aminotransferase**. ## Footnote **Both are ultimately utilizing glutamate as their source for nitrogen.**

Question 43

High AST, low ALT

Answer 43

Not likely to be liver damage

Question 44

High AST, High ALT

Answer 44

Highly suggestive of liver damage

Question 45

The ____ carbon skeleton **must be obtained from the diet**.

Answer 45

The **arginine/ornithine/citrulline** carbon skeleton **must be obtained from the diet**.

Question 46

Anaplerotic

Answer 46

Something which fills up a cycle and thus speeds it up.

Question 47

Anaplerotic describes the role of ____ in the urea cycle.

Answer 47

Anaplerotic describes the role of **arginine** in the urea cycle.

Question 48

\_\_\_ is to the urea cycle as oxaloacetate is to the krebs cycle.

Answer 48

**ornithine** is to the urea cycle as oxaloacetate is to the krebs cycle.

Question 49

\_\_\_\_ is highly characteristic of hepatic encephalopathy.

Answer 49

**asterixis** is highly characteristic of hepatic encephalopathy. Asterixis is when a patient has an involuntary tremor "flapping" in their hand.

Question 50

What is the importance of insulin produced in response to gluconeogensis in a high protein meal?

Answer 50

It **keeps skeletal muscle from being degraded** in the presence of high levels of dietary amino acids, **even when the meal contains no glucose whatsoever.**

Question 51

No ___ is produced by high protein meals.

Answer 51

No **glycogen** is produced by high protein meals.