Metabolic Pathways: Amino acid catabolism

Question 1

What happens to amino acid that is not used for protein synthesis?

Answer 1

Degraded in the liver

Question 2

Describe absorption of amino acids?

Answer 2

Proteolytic enzymes in stomach and intestine produce single amino acids, di and try peptides where they are released into blood stream for absorption

Question 3

Describe protein turnover

Answer 3

It is tightly regulates
Important for rapid changes
Damages proteins have to be removed

Question 4

What is the main problem for amino acid breakdown?

Answer 4

They contain nitrogen in the amino group and some in the side chain

Question 5

What does breakdown of amino acids produce?

Answer 5

Ammonia and ammonium ions which at high concentration are toxic

Question 6

What is urea?

Answer 6

The main excretion molecule for nitrogen

Formed in liver NOT kidneys

Question 7

What other molecules are responsible to excrete molecules?

Answer 7

Uric acid
Creatinine
Ammonium ions

Question 8

What is the initial step of urea synthesis?

Answer 8

Amino group of alpha amino acid is transferred onto a alpha keto acid usually a-ketpglutarate (transamination)

Catalysed by amino transferases

The amino group most commonly ends up in glutamic acid

OR

glutamine synthase adds free ammonium ion to glutamate giving glutamine

Question 9

What is the second step of urea synthesis?

Answer 9

Removal of amino group from amino acid (de-amination)

Free ammonium ions are used for the synthesis of urea

Question 10

What is the third step of urea synthesis?

Answer 10

Urea (ornithine cycle)

Free ammonium ion is used to synthesise urea, one nitrogen from free ammonium, the other from aspartic acid, carbon from CO2

Question 11

What are the main carriers of nitrogen in the blood to the liver?

Answer 11

Alanine and glutamine

Question 12

What is the remainder of amino acids when the amino groups are removed?

Answer 12

Carbon skeletons

Question 13

What are carbon skeletons used for?

Answer 13

Converted into metabolic intermediates

Converted to glucose or oxidised in the TCA cycle

Question 14

What are the two classes of amino acids?

Answer 14

Ketogenic amino acids

Glucogenic Amino acids

Question 15

Describe ketogenic amino acids?

Answer 15

Degraded to acetyl-CoA or Acetoacetyl-CoA

Can give rise to ketone bodies or fatty acids

Question 16

Describe glycogenic amino acids?

Answer 16

Degraded to pyruvate or TCA cycle intermediates

Can be converted into phosphoenolpyruvate and then into glucose

Question 17

What is alcaptonuria?

Answer 17

Blockage of the breakdown of phenoalanine and tyrosine

Question 18

What is maple syrup urine disease?

Answer 18

Degradation of valine, isoleucine and leucine is blocked

Urine smells like maple syrup

Mental and physical retardation

Prevented by appropriate diet

Question 19

What is phenylketonuria?

Answer 19

Blockage of breakdown of phenylalanine

Accumulates in all body fluids

Question 20

What results from a defect in a urea cycle enzyme?

Answer 20

Accumulation of urea cycle intermediates

Glutamine levels increase in the circulation

Question 21

What happens when glutamine levels increase?

Answer 21

a-ketoglutarate is no longer regenerated

- they become low and cause build up of ammonium ions

Question 22

What is done to treat urea cycle disorders?

Answer 22

Low-protein diet
Drugs which remove nitrogen
-form complexes with amino acids which are excreted
-gene therapy in hepatocytes