Metabolic pathways: amino acids

Question 1

Are all amino acids be converted to acetyl-CoA?

Answer 1

No, but all amino acids can be oxidised in the TCA cycle

Question 2

Can amino acids be stored in the body?

Answer 2

No, any amino acid that is not involved in protein synthesis has to be degraded

Question 3

Where are amino acids that are not used in protein synthesis degraded?

Answer 3

Liver

Question 4

Where do the amino acids for degradation come from?

Answer 4

Mostly from diet

Protein turnover

Question 5

What happens to proteins/amino acids in the stomach and small intestine?

Answer 5

Proteolytic enzymes break proteins into single amino acids, and di- and tri- peptides
These are absorbed into intestinal cells and released into blood for absorption by other tissues

Question 6

What is the main problem for amino acid breakdown?

Answer 6

Amino acids contain nitrogen - in amino groups and sometimes in side chains
The breakdown of amino acids produces ammonia and ammonium ions which can be toxic

Question 7

What is the main molecule for excreting nitrogen from amino acid metabolism?

Answer 7

Urea

Question 8

Where is urea formed?

Answer 8

Liver

Question 9

How much of the nitrogen from amino acid breakdown is excreted by urea?

Answer 9

80%

Question 10

What is the first step of urea synthesis and what does this involve?

Answer 10

Transamination

An amino group from an amino acid is transferred onto a keto acid, forming a new amino acid and a keto acid

Question 11

What is the second step of urea synthesis and what does this involve?

Answer 11

De-amination

The amino group is removed from the newly formed amino acid, releasing a free ammonium group and an NADH

Question 12

What happens to the free amino group produced in the second step of urea synthesis?

Answer 12

It enters the urea cycle, where it is converted to urea

Question 13

How many nitrogens does urea contain and where do they come from?

Answer 13

2
One from the de-amination of an amino acid forming a free ammonium ion
Another from aspartic acid

Question 14

Where does transamination occur?

Answer 14

In all tissues

Question 15

Where does de-amination occur?

Answer 15

Liver

Question 16

What must happen to the glutamate (newly formed amino acid from transamination) for it to be transported to the liver?

Answer 16

Amino group of glutamate is transferred to pyruvate
OR
Glutamine synthase adds NH4 to glutamate, giving glutamine

Question 17

What is the most common amino acid involved in transamination of urea synthesis?

Answer 17

Usually alpha-ketoglutarate receives an amino group from the alpha-amino acid to form glutamate

Question 18

What are the major carries of nitrogen in blood to the liver?

Answer 18

Glutamine

Alanine

Question 19

Where is the carbon in urea derived from?

Answer 19

CO2

Question 20

How many ATP are cleaved to allow urea synthesis?

Answer 20

3

Question 21

What happens to the carbon skeletons left after the removal of alpha amino groups?

Answer 21

Converted to major metabolic intermediates

e.g. can be converted to glucose or oxidised in the TCA cycle

Question 22

What are ketogenic amino acids converted into?

Answer 22

Degraded to acetyl-CoA or acetoacetyl-CoA

Can give rise to ketone bodies or fatty acids

Question 23

What are glucogenic amino acids converted to?

Answer 23

Degraded to pyruvate or TCA cycle intermediates

Can be converted into phosphoenolpyruvate and then into glucose

Question 24

What is alcaptonuria?

Answer 24

A disorder in which the degradation of phenylalanine and tyrosine is blocked

Question 25

What is maple syrup urine disease?

Answer 25

A disorder in which the degradation of valine, isoleucine and leucine is blocked
If untreated, can cause mental retardation

Question 26

What is phenylketonuria?

Answer 26

Phenylalanine accumulates in all body tissues as it cannot be broken down