case 5 - protein metabolism in the liver

Question 1

why is the liver especially important in protein metabolism

Answer 1

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

Question 2

out of the 20 amino acids in the body, how many can be synthesised

Answer 2

only ten

Question 3

what is the process via the synthesis of non-essential amino acids happens

Answer 3

transamination

Question 4

what is needed for transmamination

Answer 4

A precursor alpha-keto acid such as pyruvate is needed

A donor of the amino group is needed, and this is commonly glutamic acid

Question 5

what catalyses transamination

Answer 5

an aminotransferase catalyses the reaction such as ALT or AST

Question 6

what is protein synthesis stimulated by

Answer 6

insulin and growth hormone

Question 7

what are the important plasma proteins synthesised by the liver

Answer 7

Albumin
Globulin
Fibrinogen
CRP (an infection marker)
Clotting factors – Factors II, VII, IX and X are Vitamin K dependent
Thrombopoietin
Angiotensinogen

Question 8

what plays an important role in maintaining the amino acid equilibrium in the blood

Answer 8

the plasma proteins synthesised by the liver

Question 9

what happens in times of tissue amino acid depletion

Answer 9

these plasma proteins can be degraded and released back into the blood as amino acids for tissues to use in protein synthesis

Question 10

what do the plasma protein provide that is vital

Answer 10

oncotic pressure in the blood, meaning they hold water in the plasma

Question 11

where are all clotting factors synthesised

Answer 11

in the liver

Question 12

what is essential to the carboxylase enzyme that is used to make clotting factors II,VII,IX,X and protein C

Answer 12

vitamin K

Question 13

what is needed for vitamin K digestion and absorption and why

Answer 13

as vitamin K is fat soluble, bile is needed for its digestion and absorption.

Question 14

what happens if there is biliary obstruction

Answer 14

vitamin K is not appropriately absorbed and thus the vitamin K dependant clotting factors are not adequately synthesised, leading to a further tendency towards bleeding

Question 15

what are the amino acids converted to in transamination

Answer 15

keto acids

Question 16

what does this use

Answer 16

an acceptor to accept the amine group

Question 17

what is this reaction catalysed by

Answer 17

aminotransferase sich as AST/ALT

Question 18

please give a list of some examples of transamination

Answer 18

Amino acid + a-ketoglutarate –> glutamate and keto acid

Amino acids + oxaloacetate –> aspartate and ketoacid

Aspartate can then be metabolised further:

Aspartate + a-ketoglutarate –> oxaloacetate and glutamate

Question 19

what is the end result of this step

Answer 19

the generation of glutamate

Question 20

describe deamination

Answer 20

Glutamate is metabolised further by glutamate dehydrogenase. The amine group is removed and rapidly forms ammonia (NH3) and subsequently ammonium (NH4+), alongside a-ketoglutarate. The ammonium is highly toxic and must therefore be removed through the urea cycle. Meanwhile, the a-ketoglutarate can enter the TCA cycle.

Question 21

where does the urea cycle occur

Answer 21

both within the mitochondria and the cytoplasm of the hepatocyte

Question 22

what happens within the mitochondria

Answer 22

ammonia reacts with ATP and CO2 to generate citruline from ornithine

Question 23

what happens to the citrulline

Answer 23

the citrulline then enters the cytoplasm and passes through a series of reactions, consuming asparate along the way, to regenerate ornithine

Question 24

what happens to the ornithine

Answer 24

it re enters the mitochondrion and the cycle repeats

Question 25

what happens through this process

Answer 25

the toxic ammonia is converted into harmless urea which is easily excreted

Question 26

the liver is important in the metabolic acitvation of what vitamin

Answer 26

vitamin D

Question 27

what happens to the vitamin D

Answer 27

it is carried to the liver in the blood where it is first converted to the prohormone calcifediol via hydroxylation

Question 28

what happens to this calcifediol

Answer 28

it is then transported to the kidneys where it is converted into calcitriol

Question 29

what is the biologically active form of vitamin D

Answer 29

calcitriol

Question 30

what is the conversion of calcifediol to calcitriol catalysed by

Answer 30

25-hydroxyvitamin D3 1-alpha-hydrocylase

Question 31

what is this conversion stimulated by

Answer 31

parathyroid hormone and low calcium