Amino Acid Catabolism

Question 1

Nitrogen + Bacteria/fungi/plants

Answer 1

N deficient so don’t excrete it, store it

Question 2

Nitrogen + animals

Answer 2

Have excess N (protein) so don’t store but excretes

Question 3

Pepsin

Answer 3

cleaves between hydrophobic and aromatic residues optimum activity at pH 2 (stomach)

Question 4

Chymotrypsin

Answer 4

cleaves aromatic carboxyl side residues (duodenum)

Question 5

Trypsin

Answer 5

cleaves carboxyl side of lysine or arginine (duodenum)

Question 6

Amino peptidases

Answer 6

breaks down oligopeptides to amino acids and tri/dipeptides

Question 7

Proteolytic enzymes

Answer 7

breaks down proteins to a.a. and oligopeptides

Question 8

What are the main sources for protein degradation?

Answer 8

~ mis-folding of proteins
~ errors in translation
~ regulation of pathways
~ damaged proteins

Question 9

Ubiquitin

Answer 9

the signal for protein death

Question 10

Ubiquitination

Answer 10

ATP + ubiquitin + target (lysine residue) –> AMP + ubiquitin-target
Enzyme = Ubiquitin ligase
~ more and more molecules get added in a chain

Question 11

How does ubiquitin ligase choose its target?

Answer 11

N terminal rule:
the N-terminal amino acid of a protein determines its half-life
~ diff a.a. have different half lives

Question 12

If amino acids are not needed then…

Answer 12

~ their amino group is removed by aminotransferase

~ carbon skeleton degraded

Question 13

Aminotransferase

Answer 13

removes amino group from amino acid and produces a keto acid + glutamate
~ all specific for their substrate

Question 14

Glucogenic amino acids

Answer 14

when an a.a.’s carbon skeleton can be feed into the citric acid cycle via gluconeogenesis

Question 15

Which 3 main keto acids do glycogenic a.a. donate their carbon skeletons to?

Answer 15

~ pyruvate
~ oxaloacetate
~ alpha-ketoglutarate

Question 16

Ketogenic amino acids

Answer 16

~ help synthesis of fatty acids or ketone bodies (not glucose)
~ metabolised to acetyl CoA or acetoacetyl CoA
~ a.a. that cannot be metabolised to citric acid intermediates

Question 17

Which amino acid is both glycogenic and ketogenic?

Answer 17

Tryptophan

Question 18

What is the fate of the amino group?

Answer 18

~ amino group loaded onto glutamate via transamination reaction
~ glutamate dehydrogenase breaks down glutamate to ammonia + alpha-ketoglutarate

Question 19

What happens to the toxic ammonia produced?

Answer 19

aquatic animals = release it directly into he water
birds/reptiles = secrete as uric acid in faeces
land vertebrates = synthesise urea and excrete as urine (urea cycle)

Question 20

How is aspartate regenerated?

Answer 20

in the krebs ‘bi’cycle

Question 21

How do the nitrogens enter the urea cycle?

Answer 21

one as carbamoyl phosphate

one as aspartate

Question 22

Name two amino acid metabolism disease.

Answer 22

Phenyketonuria

Argininosuccinase deficiency

Question 23

Phenyketonuria

Answer 23

~ no phenylalanine hydroxylase
~ accumulation of phenylalanine in tissues
~ risk of severe retardation

Question 24

Argininosuccinase deficiency

Answer 24

~ lack of enzyme argininosuccinase
~ ammonia buildup –> lead to death
~ arginine surplus required