hepatic protein metabolism & amino acids in nitrogen balance

Question 1

why is the liver important in protein metabolism (3)

Answer 1

• stores more proteins than other tissues
• can rapidly synthesise or degrade proteins.
• can quickly synthesise and degrade amino acids, unlike most other tissues.

Question 2

how many different amino acids are needed in the human body for protein synthesis

Answer 2

20

10 can be synthesized but 10 cannot

Question 3

what are essential amino acids

Answer 3

the 10 that cannot be synthesised and are needed in the diet

Question 4

what is the process of amino acid synthesis called

Answer 4

transamination

Question 5

describe transamination process

Answer 5

1. A precursor alpha-keto acid such as pyruvate is needed
2. A donor of the amino group is needed, and this is commonly glutamic acid
3. An aminotransferase catalyses the reaction such as alanine transaminase (ALT) – this is one of the main liver enzymes alongside aspartate transaminase (AST)

Question 6

what happens once the cells of the body have reached their limit of protein storage

Answer 6

the left-over amino acids in the blood are metabolised in the liver, prior to their conversion into lipids or glucose.

via transamination and deamination

Question 7

where does amino acid degradation and catabolism take place

Answer 7

hepatocytes of liver

Question 8

which amino acids undergo degradation

Answer 8

any that that are not required as building blocks for protein synthesis

Question 9

what does amino acid catabolism require

Answer 9

for the the alpha amino group (nitrogen containing) to be removed

Question 10

what does amino acid catabolism produce (2)

Answer 10

Nitrogen - which is incorporated in other compounds or excreted

Carbon skeleton - which can then be metabolised and used in the Kreb’s
cycle

Question 11

what are the 2 main catabolism processes

Answer 11

oxidative deamination

transamination

Question 12

what does oxidative deamination result in

Answer 12

removes an amine group as free ammonia

amine is replaced by an
oxygen atom from water to form a alpha-keto acid

results in the formation of an alpha-keto acid (e.g alpha-ketoglutarate) &
ammonia

Question 13

what is the only amino acid that undergoes rapid oxidative deamination

Answer 13

glutamate

Question 14

how is the alpha keto acid used

Answer 14

in the Kreb’s cycle for use in glucose
production - GLUCONEOGENESIS

Question 15

which enzyme is used in oxidative deamination

Answer 15

glutamate dehydrogenase

Question 16

what is the co enzyme used in oxidative deamination

Answer 16

forward reaction - NAD+

backward reaction - NADPH

Question 17

what is oxidative deamination dependent on

Answer 17

on concentrations of;
glutamate
alpha-ketoglutarate
ammonia

Question 18

what happens when there is excess amonnia

Answer 18

it can easily cross the blood-brain barrier and then react with alpha-ketoglutarate because it is a reversible reaction

Question 19

what happens after a protein rich meal

Answer 19

glutamate concentration will be high, oxidative

deamination will degrade the amino acid glutamate

resulting in ammonia formation

Question 20

what happens in transamination in amino acid catabolism

Answer 20

an alpha amino group is trasnferred from amino acid to a keto-acid to form an alpha-keto-acid

Question 21

what enzyme involved in transamination in amino acid catabolism

Answer 21

aminotransferases such as ALT/AST

each aminotransferase is specific to one or a few amino group donors

Question 22

where are aminotransferases found

Answer 22

in the cytosol of the mitochondria throughout the body and particularly in the kidneys & liver

Question 23

why do degradation and oxidative deamination work in unison

Answer 23

transamination is readily reversible

degradation will occur after a protein-rich meal and amino acid synthesis will occur depending on dietary supply and cellular demand

Question 24

what is nitrogen balance

Answer 24

the difference between nitrogen excreted from the body and nitrogen ingested in the diet

this reflects gain or loss of total body protein

Question 25

what is anabolic nitrogen balance

Answer 25

positive balance

i.e net gain in amino acids

nitrogen intake > nitrogen loss

Question 26

what is catabolic nitrogen balance

Answer 26

negative balance i.e. net loss in amino acids

nitrogen intake < nitrogen loss

Question 27

what does it mean if nitrogen balance is in equilibrium

Answer 27

the person is healthy

Question 28

what if someone doesnt have the essential amino acids in their diet

Answer 28

they have a negative nitrogen balance (loss greater than gain)

Question 29

most common cause of a positive nitrogen balance

Answer 29

pregnancy

Question 30

what is the recommended daily intake of amino acids to remain in nitrogen balance

Answer 30

0.8g/kg bodyweight

Question 31

most common cause of a negative nitrogen balance

Answer 31

malnutrition

Question 32

other causes of negative nitrogen balance

Answer 32

multiple trauma or extensive trauma where there is a lot of tissue damage

Question 33

what happens in the glucose alanine cycle

Answer 33

move proteins from muscles to the liver when glycogen stores are low to generate additional ATP

Question 34

which is the main amino acid transported to liver in glucose alanine cycle

Answer 34

alanine

Question 35

what is alanine used for

Answer 35

gluconeogenesis

Question 36

describe glucose alanine cycle process

Answer 36

1. excess alanine released into
   bloodstream and transported into liver
2. in liver, alanine is converted
   back to pyruvate by transamination
3. the pyruvate can be used as a source of carbons for glucose production via
   gluconeogenesis
4. The glucose then enters the blood and can be used in the muscles which in turn can produce pyruvate via glycolysis which can then be used again to remove excess ammonia (NH3) - this is the glucose-alanine cycle
5. The glutamate produced can then be
   converted to ammonium (NH4+) via
   oxidative deamination producing
   ammonium (NH4+) which then rapidly
   disassociates into ammonia (NH3) which in turn can be converted to urea via the urea cycle