liver in urea cycle and elimination

Question 1

what happens when cells reach their limit of protein storage

Answer 1

• left over AA in the blood are metabolized in the liver prior to their conversion into lipids or glucose
• results in toxic ammonia (NH3) accumulation

Question 2

what does the urea cycle do in mammals

Answer 2

converts ammonia to urea, which is eliminated in urine

Question 3

sources of ammonia - AA

Answer 3

liver via trans-deamination reactions

Question 4

sources of ammonia - glutamine

Answer 4

kidney by renal glutaminase and glutamine dehydrogenase

Question 5

sources of ammonia - amino acids and urea

Answer 5

intestinal lumen via bacterial enzymes

Question 6

what is hyperammonemia

Answer 6

high blood ammonia levels

Question 7

what is transamination

Answer 7

amino acids are converted into keto acids, using an acceptor to accept the amine group

Question 8

transamination reactions are catalyzed by:

Answer 8

aminotransferases

Question 9

what is an example of a transamination reaction

Answer 9

amino acid + a-ketoglutarate -> glutamate + keto acid

or

AA + oxaloacetate -> asparate + keto acid —» asparate + a-ketoglutarate -> glutamate + oxaloacetate

Question 10

what is the desired product of a transamination reaction

Answer 10

glutamate

Question 11

what are the two specific aminotransferases

Answer 11

• alanine aminotransferase (ALT)
• aspartate aminotransferase (AST)

Question 12

almost all AA undergo transamination except:

Answer 12

lysine and threonine

Question 13

what is deamination

Answer 13

removal of NH3

Question 14

____ is deaminated in mitochondria by ______

Answer 14

glutamate; glutamate dehydrogenase (GDH)

Question 15

what does GDH do

Answer 15

liberates NH3 from glutamate for elimination via the urea cycle

Question 16

the amino group of glutamate is removed to yield:

Answer 16

ammonia (NH3) and a-ketoglutarate

Question 17

ionized form of NH3 is highly reactive ____, which is toxic and must be removed through the ____

Answer 17

ammonium (NH4); urea cycle

Question 18

what does the urea cycle do

Answer 18

converts toxic ammonia into harmless urea, which is excreted in urine

Question 19

where does the urea cycle take place

Answer 19

mitochondria and cytoplasm of hepatocytes

Question 20

in mitochondria, ammonia reacts with ATP and CO2 to generate ____ from ____

Answer 20

citrulline; ornithine

Question 21

the citrulline exported to the cytoplasm and passes through a series of reactions consuming aspartate along the way to regenerate _ + _

Answer 21

ornithine; urea

Question 22

what are the 4 steps of the urea cycle

Answer 22

1. detox step; NH4 +CO2 and ATP makes the rate limiting step - NAG = essential CPS cofactor; makes citrulline from ornithine
2. citrulline gets exported to cytoplasm
3. asparate provides an additional N; makes arginiosuccinate
4. arginase breaks arginine into ornithine and urea

Question 23

what do deficiencies in the urea cycle cause

Answer 23

hyperammonemia

Question 24

in nearly all tissues, ammonia is stored and transported to liver as:

Answer 24

glutamine

Question 25

in various tissues, ammonia is stored via ____ (detoxification)

Answer 25

glutamine synthetase

NH3 + glutamate -> glutamine

Question 26

in liver, ammonia is released from glutamine via:

Answer 26

glutaminase

glutamine -> NH3 + glutamate

Question 27

glutamate is further deaminated by GDH to yield NH3 and

Answer 27

a-ketoglutarate

Question 28

what is the glucose-alanine cycle

Answer 28

nitrogen elimination in skeletal muscle, muscle transports alanine to liver for NH3 elimination and glucose production

Question 29

ammonia excretion via kidneys

Answer 29

• some ammonia (NH4) is generated in kidneys via glutamine deamination (glutaminase) and excreted in urine
• important for maintaining body’s acid-base balance