Amino Acids

Question 1

Role of proteins/AAs

Answer 1

Breakdown for fuel
Nitrogen excretion in urea cycle
Carbon skeleton cycling

Question 2

Pathways of protein degradation

Answer 2

Lysosomal
Ubiquitin-protease
Intestinal

Question 3

Intestinal degradation

Answer 3

Dietary proteins = AAs for oxidative metabolism, gluconeogenesis
Proteins hydrolysed to AA via proteolytic enzymes, enter intestinal cells, and exit to bloodstream

Question 4

AA handling

Answer 4

No storage form - must be used
AA amino group nitrogen removed – urea, excreted
Carbon skeleton – AcCoA, acetoacetylCoA, pyruvate, CAC intermediates
Can also form glucose, FAs, ketone bodies

Question 5

Protein turnover

Answer 5

70-100g intake
10-20% of total oxidative metabolism
High rates in structally rearranging tissues e.g. uterine tissue in pregnancy, skeletal in starvation
Low rates in long lasting structural proteins

Question 6

Oxidative degradation of AAs

Answer 6

Occurs in protein-rich states, starvation (cellular AAs) and in normal processes
Carbon skeleton (left from amine removal) is oxidised in CAC

Question 7

Transamination (Step 1. ox degradation)

Answer 7

Alpha amino group transferred to a-ketoglutarate = glutamate and carbon skeleton
with transaminases

Question 8

Deamination (step 2.)

Answer 8

Amino group removed from glutamate by glutamate dehydrogenase (GDH) in liver
Regenerates a-ketoglutarate, releases ammonia

Question 9

Urea cycle

Answer 9

Liver, to detoxify ammonia
GDH (in matrix) mediates deamination = ammonia, sequestered in matrix to prevent cell damage
Urea cycle uses 3ATP equivalents to produce urea (4 part II in notes)

Question 10

Urea cycle and CAC

Answer 10

Linked via aspartate-argininosuccinate shuttle
Fumarate is also an intermediate in both, converted to malate for citrilline shuttle in CAC
Aspartate, produced by CAC, used for shuttle for urea cycle too

Question 11

AA carbon skeleton fates

Answer 11

Keto acid produced by transamination (i.e. amino acid with amino group removed), converted to metabolic intermediates by oxidative metabolism
Pyruvate
Oxaloacetate
Fumarate
Succinyl CoA
A-ketoglutarate
Citrate
Acetyl CoA
Acetoacetyl CoA

Question 12

Amino acid entry to CAC

Answer 12

Oxidation of AAs allows them to enter the cycle, used for glucose (glucogenic: producing pyruvate/CAC precursor when catabolised) or FAs/ketones (ketogenic: if acetoacetate/precursors are formed)

Question 13

Gluconeogenesis from AAs

Answer 13

AA and lactate = major precursors, converted to pyruvate

Question 14

FA synthesis from AAs

Answer 14

Only leucine and lysine are solely ketogenic

Excess dietary AAs converted to fat

Question 15

Branched chain AA degradation?

Answer 15

Valine, isoleucine and leucine
NOT degraded in liver - only in muscle, kidneys and brain
This is due to an aminotransferase (not present in the liver) = transamination
Branched chain a-keto acid dehydrogenase (BCD) complex then catalyses oxidative decarboxylatoin of a-keto acids, giving CO2 and acCoA

Question 16

Regulation of branched chain AA degradation?

Answer 16

BCD complex inactivated by phosphorylation when dietary AA are low

Question 17

AA diseases

Answer 17

Maple syrup urine disease - missing/defective BCD complex = accumulation of a-keto acids and AAs

Phenylketonuria
Absence of phenylalanine hydroxylase or cofactor = no phenylalanine degradation

Question 18

Nitrogen transport

Answer 18

To liver for urea synthesis

Branched chain AAs contribute a lot of nitrogen = toxic ammonia

Transported as glutamine (glutamine synthetase combines glutamate + ammonia = glutamine)

Or as alanine
(alanine transaminase converts pyruvate to alanine in alanine cycle)

Question 19

Alanine cycle

Answer 19

Two transamination reactions, in muscle and liver
a-amino group from glutamate – pyruvate, leaving alanine
Alanine – liver, reverse occurs by transamination = a-ketoglutarate and pyruvate

Catalysed by alanine transaminase

Question 20

Overall route of AAs?

Answer 20

AAs – liver for disposal and carbon harvest – liver for glucose synthesis

Question 21

Regulation of AA degradation

Answer 21

Substrate concentration (enzymes have high Km = MM kinetics)
Glutamate dehydrogenase catalysing deamination = key regulatory point
Allosterically - nucleotides (ATP, GTP or ADP, GDP) indicating energy status

Question 22

Amino acid components (carbon skeleton, amino) sources

Answer 22

Nitrogen - mostly from glutamine/glutamate

Carbon skeletons - intermediates of glycolysis, pentose phosphate pathway, CAC

Question 23

Different AA synthesis?

Answer 23

Depends on complexity of molecule and whether it is essential (usually more complex)

Question 24

6 pathways of amino acid biosynthesis?

Answer 24

Grouped based on intermediate they are formed from

a-ketoglutarate
3-phosphoglycerate
Oxaloacetate
Pyruvate
Phosphoenolpyruvate and erythrose 4-phosphate
Ribose 5-phosphate

Question 25

Regulation of AA biosynthesis

Answer 25

Allosteric regulation

Feedback inhibition where end product blocks first step

Question 26

Regulation; threonine – isoleucine

Answer 26

Isoleucine is an allosteric inhibitor of threonine dehydratase, blocking this pathway i.e. stopping its own synthesis from threonine

Question 27

Regulation; glutamate to glutamine

Answer 27

Very popular pathway, used in nitrogen transport

7 allosteric inhibitors of glutamine synthetase

Question 28

Regulation; aspartate

Answer 28

Branched pathway - nested feedbacl inhibition

Aspartokinase governs with 3 isoenzymes for aspartate – lysine, methionine or threonine
Each of which is allosterically inhibited by their end product

Question 29

AAs as metabolic precursors

Answer 29

Hormones, alkaloids, antibiotics, coenzymes, NTs etc, usually through providing nitrogen

e. g. glycine contributes carbon for haem groups
e. g. purine ring formation