Lecture 40:Nitrogen cycle/urea cycle

Question 1

What are the three types of nitrogen secretion?

Answer 1

1. Ammonotelic
2. Ureotelic
3. Uricotelic

Question 2

Ammonotelic

Answer 2

• Ammonia

- Aquatic Animals

Question 3

Ureotelic

Answer 3

• Urea

- Terrestrial

Question 4

Uricotelic

Answer 4

• Uric Acid

- Birds and reptiles

Question 5

How do plants obtain NH3?

Answer 5

from soil or through symbiosis

Question 6

NH3 source for bacteria and microorganisms

Answer 6

must ingest amino acids (or nucleic acids)

-performed by extracellular digestion and diffusion of amino acids through the cell wall

Question 7

NH3 source in vertebrates

Answer 7

• digestive system partially breaks down protein in the stomach further (pH1 + pepsin)
• broken down further in the small intestine by proteases into amino acids
• the amino acids are absorbed through epithelial layer and are transported to the liver and other tissues

Question 8

Oxidative Degradation of amino acids

Answer 8

removes the NH3 group through transamination

Steps

1) the removal of NH3 through transamination is usually an early step in degradation
2) the transfer of NH3 to glutamate and subsequent oxidant of glutamate

-a-ketogluterate is used as an acceptor and L-glutamate is the result. L-glutamate is subsequently oxidized back to a-ketogluterate

Question 9

Issues with NH3 transport

Answer 9

-humans derive most of their needed energy from carbohydrate so oxidation of proteins for energy is a small part of our metabolism

Question 10

location of glutamine synthetase

Answer 10

cytosol of peripheral tissues

Question 11

glutaminase location

Answer 11

liver mitochondria

Question 12

Glutamate dehydrogenase location

Answer 12

liver mitochondria

Question 13

Transport NH3 via glutamine

Answer 13

DELIVER EXCESS NH3 to liver by glutamine

• In starvation, or in diabetes mellitus proteins will be oxidized and NH3 will be recycle or excreted. NH3 is stored and transported intracellularly as glutamate
• Glutamine IMPORTANT IN INTRACELLULAR TRANSPORT from peripheral tissues to liver

Question 14

Glucose Alanine cycle

Answer 14

Transport of NH3 from muscle

• involves amination of pyruvate to make alanine
• pyruvate is from glycolysis
• muscle cannot perform gluconeogenesis especially if there is low O2 concentration this cycle provides a way to reenergize the muscle with glucose

Question 15

Glucose Alanine cycle steps

Answer 15

• Alanine is transported to the liver where transamination occurs to make glutamate
• glutamate feeds NH4+ to the urea cycle
• in the liver the resulting pyruvate can be used in gluceoneogenesis, and then glucose is recirculated to the muscle

Question 16

NH3 excretion via the urea cycle

Answer 16

In the liver cytosol:

• condensation of NH4+ with HCO3- and PO42- to generate carbamoyl phosphate
• Carbamoyl phosphate is reacted with ornithine
• aspartate is used in a second condensation resulting in argininosuccinate

Fumarate is released

Terminal (NH2)2CO is released as urea

Question 17

why is the Urea cycle called a cycle?

Answer 17

because a ornithine is regenerated

Question 18

Issues with urea cycle?

Answer 18

reactions that are part of the cycle occur in different areas:

• extrahepatic tissues deliver glutamine
• hepatocyte cytosol- glutamate from alanine
• Mitochondrial matrix- glutamate dehdyrogenase

Question 19

Krebs cycle

Answer 19

what the combination of the urea cycle and the TCA cycle

Question 20

What are the fates of the carbon backbones from amino acid oxidation in urea cycle?

Answer 20

the of the carbon backbones can be used by:

• feeding into the citric acid cycle (glycogenic)
• as energy sources by other pathways (ketogenic)

Question 21

Fates of carbon backbones from urea cycle: glucogenic

Answer 21

Depending on the state of the cell- the glucogenic ones can be oxidized further in the TCA cycle or can be exported to make oxaloacetate for gluconeogenesis

Question 22

Fates of carbon backbones from urea cycle: ketogenic

Answer 22

acetoacetate is the product

-this is degraded to acetone and B-hydroxybutyrate which circulate in the blood and are converted to acetyl-CoA

Question 23

What happens if there is excessive production of Ketone bodies?

Answer 23

when diabetes or starvation

• the accumulation of acetoacetate and B-hydroxybutyrate results in acidosis of the blood
• Extreme acidosis can result in coma/death