Lecture 40:Nitrogen cycle/urea cycle Flashcards
What are the three types of nitrogen secretion?
- Ammonotelic
- Ureotelic
- Uricotelic
Ammonotelic
- Ammonia
- Aquatic Animals
Ureotelic
- Urea
- Terrestrial
Uricotelic
- Uric Acid
- Birds and reptiles
How do plants obtain NH3?
from soil or through symbiosis
NH3 source for bacteria and microorganisms
must ingest amino acids (or nucleic acids)
-performed by extracellular digestion and diffusion of amino acids through the cell wall
NH3 source in vertebrates
- 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
Oxidative Degradation of amino acids
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
Issues with NH3 transport
-humans derive most of their needed energy from carbohydrate so oxidation of proteins for energy is a small part of our metabolism
location of glutamine synthetase
cytosol of peripheral tissues
glutaminase location
liver mitochondria
Glutamate dehydrogenase location
liver mitochondria
Transport NH3 via glutamine
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
Glucose Alanine cycle
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
Glucose Alanine cycle steps
- 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
NH3 excretion via the urea cycle
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
why is the Urea cycle called a cycle?
because a ornithine is regenerated
Issues with urea cycle?
reactions that are part of the cycle occur in different areas:
- extrahepatic tissues deliver glutamine
- hepatocyte cytosol- glutamate from alanine
- Mitochondrial matrix- glutamate dehdyrogenase
Krebs cycle
what the combination of the urea cycle and the TCA cycle
What are the fates of the carbon backbones from amino acid oxidation in urea cycle?
the of the carbon backbones can be used by:
- feeding into the citric acid cycle (glycogenic)
- as energy sources by other pathways (ketogenic)
Fates of carbon backbones from urea cycle: glucogenic
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
Fates of carbon backbones from urea cycle: ketogenic
acetoacetate is the product
-this is degraded to acetone and B-hydroxybutyrate which circulate in the blood and are converted to acetyl-CoA
What happens if there is excessive production of Ketone bodies?
when diabetes or starvation
- the accumulation of acetoacetate and B-hydroxybutyrate results in acidosis of the blood
- Extreme acidosis can result in coma/death
