nitrogen and urea cycle

Question 1

what waste can’t the kidney deal with on its own?

Answer 1

ammonia

the liver must step in

Question 2

ammonia

Answer 2

major toxin from metabolism of amino acids

Question 3

where does free ammonia go

Answer 3

to the liver for the urea cycle

ammonia –> urea

Question 4

where does urea cycle occur?

Answer 4

in the mitochondria

so it doesn’t affect cytoplasm by being toxic

Question 5

how does ammonia get to liver? (way 1)

Answer 5

1) glutamine synthetase adds ammonia to glutamate to make glutamine
2) glutamine enters blood and transports ammonia to a liver cell

Question 6

what happens when glutamine gets into liver mitochondria?

Answer 6

1) glutaminase cleaves glutamine back into glutamate and ammonia
2) ammonia enters urea cycle

Question 7

2nd method of getting ammonia into liver cells

Answer 7

skeletal muscle cells

glutamate dehydrogenase adds ammonia to alpha-ketoglutarate to make glutamate

Question 8

what is the problem with glutamate?

Answer 8

unlike glutamine, glutamate can’t leave the cell on its own

Question 9

how does glutamate fix this problem

Answer 9

alanine transaminase (ALT) converts glutamate and pyruvate into alpha-ketoglutarate and alanine
alanine brings ammonia to liver mitochondria

Question 10

what happens when alanine gets into liver mitochondria?

Answer 10

ALT converts Alanine and alpha-ketoglutarate back into pyruvate and glutamate
ammonia is now part of glutamate again

Question 11

glutamate: first outcome in liver mitochondria

Answer 11

glutamate dehydrogenase converts glutamate into alpha-ketoglutarate and free ammonia which enters urea cycle
this is reversible!

Question 12

glutamate: second outcome in liver mitochondria

Answer 12

aspartate transaminase (AST)
combines glutamate with oxaloacetate to form aspartate and alpha-ketoglutarate
aspartate is carrying ammonia and can enter urea cycle

Question 13

what is the only amino acid to enter the urea cycle?

Answer 13

aspartate

Question 14

urea cycle overview

Answer 14

modify chemical structure of substrates to create urea

Question 15

urea

Answer 15

2 nitrogen groups and 1 carbonyl group

Question 16

urea cycle: where does it occur

Answer 16

first 2 steps in mitochondria, rest of the steps in the cytosol

Question 17

urea cycle rxn 1

Answer 17

2 ATP, bicarbonate, and ammonia become carbamoyl phosphate

enzyme: carbamoyl phosphate synthetase 1 (CPS1)

Question 18

carbamoyl phosphate

Answer 18

carb = carboxyl group from CO2
amoyl = nitrogen group from ammonia
phosphate = phosphate group from 1 of ATP

Question 19

CPS1 regulation

Answer 19

N-acetylglutamate allosterically activates CPS1
modifies physical shape of CPS1 so it processes ammonia more efficiently
increases urea synthesis

Question 20

how is N-acetylglutamate made?

Answer 20

made by mitochondrial enzyme: N-acetylglutamate synthetase combines glutamate and acetyl-CoA to make N-acetylglutamate

Question 21

low N-acetylglutamate levels

Answer 21

CPS1 has very low affinity for ammonia, ammonia can build up to toxic levels

Question 22

urea cycle step 2

Answer 22

ornithine trans-carbamoylase combines ornithine with carbamoyl phosphate to make citrulline
releases free phosphate

Question 23

what does ornithine trans-carbamoylase cause?

Answer 23

increase in ammonia levels

Question 24

urea cycle step 3

Answer 24

citrulline goes from mitochondria where it is joined to aspartate by argininosuccinate synthetase to form arginino succinate

uses ATP

Question 25

urea cycle step 4

Answer 25

arginino succinate lyase cuts arginino succinate into fumarate and arginine

Question 26

what happens to fumarate

Answer 26

1) fumarate is converted to malate
2) malate dehydrogenase converts malate to oxaloacetate
3) oxaloacetate and glutamate are converted to aspartate and alpha-ketoglutarate by AST (aspartate transaminase)

Question 27

why is AST important

Answer 27

it regenerates aspartate to enter the next urea cycle

Question 28

what happens to arginine

Answer 28

1) arginine broken down into urea and ornithine by arginase
2) urea enters bloodstream to be broken down by kidneys
3) ornithine enters mitochondria for the next urea cycle

Question 29

what organic linkage buffers ammonia within cells?

Answer 29

glutamic acid

Question 30

what organic linkages allow amino group transport in blood?

Answer 30

alanine and glutamine

Question 31

2 functions of urea cycle

Answer 31

1) waste disposal

2) synthesis of Arginine

Question 32

why is glutamine special?

Answer 32

major Nitrogen donor in biosynthesis of organic N compounds (purines, pyrimidines, other amino acids)
SO, GS is highly regulated

Question 33

what does glutamate dehydrogenase (GDH) do?

Answer 33

alpha-KG to glutamate
organically links ammonia
NADP to NADP+
releases H2O

Question 34

glutamine synthetase

Answer 34

link amino groups as nitrogen
glutamate to glutamine
phosphate released and displaced by ammonia

Question 35

glutaminase

Answer 35

hydrolysis to convert glutamine back to glutamate

uses H2O

Question 36

which amino acids use transamination?

Answer 36

all but threonine (T) and lysine (K)

Question 37

CPS1 location and function

Answer 37

in mitochondria
3 enzyme activities connected
2 ATP, bicarbonate, ammonia –> first intermediate of urea cycle

Question 38

high glutamate concentration =

Answer 38

more N-acetyl-glutamate = more CPS1 = more urea cycle flux

Question 39

urea cycle regulation is based on…

Answer 39

NEED (amount of protein in diet), not hormones