Nitrogen Flashcards
State the main nitrogen-containing molecules of the body.
Amino acids are the main nitrogen containing molecules of the body.
Describe the fate of dietary protein.
Dietary proteins are degraded amino acids in the small intestine. (cellular proteins targeted for destruction (misfolded, foreign, unwanted proteins) are also broken into amino acids.
Describe the concept of catabolism of body protein.
Body protein undergoes a constant turnover. Some proteins last for months on end, and some have a half-life that can be measured in minutes.
Regardless of this, all proteins are eventually turned over, and hence there is the need to replace these proteins.
New proteins need a source of amino acids from our diet.
So dietary protein is digested to produce amino acids for use in anabolic pathways.
However, unlike sugars and fats, there is no storage facility for excess protein. So ingested protein that is surplus to the body needs to be catabolised.
Describe the formation of urea.
The urea cycle is split between the mitochondrial matrix and the cytosol.
Describe the process of transamination.
Proteins are ingested, and are broken down by different proteases into amino acids.
The amino acids are absorbed in intestinal epithelial cells and are then moved into the blood.
When amino acids are broken down (in the hepatocytes of the liver) the amine part is removed and accepted by Alpha ketoglutarate acid, which converts it to glutamate.
(Releasing alpha ketoglutarate as a side product)
Describe the process of deamination.
The amine group is now in glutamate form.
It is released from glutamate and converted (by glutamate dehydrogenase) to alpha immunoglutarate intermediate, from which ammonia is released and then excreted as urea - this leaves a final product of alpha ketoglutarate acid.
Describe the urea cycle.
The urea cycle takes place in liver hepatocytes, partially in the mitochondrial matrix and partially in the cytosol.
Starting in the mitochondrial matrix, the ammonia from the deamination process is converted (by carbamoyl phosphate synthetase) to carbamoyl phosphate.
Carbamoyl phosphate combines with ornithine in the cytosol and this produces citrulline.
Citrulline then begins the urea cycle by combining with aspartate to produce:
Arginosuccinate that then advances to produce: Arginine which can release urea, excreting the toxic ammonia out of the cell via urine.
By releasing urea from Arginine, this produces ornithine, which allows the cycle to continue to excrete urea.
Describe the fate of the carbon skeleton of amino acids in the body (following removal of the amine group).
The carbon skeleton of the amino acid is used to form intermediates of the TCA cycle, glycolysis, gluconeogenesis or lipid metabolism pathways.
Describe the genetic disorder, Phenylketonuria (PKU).
Phenylketonuria is caused by the absence or deficiency of the enzyme, phenylalanine hydroxylase.
This enzyme is responsible for the conversion of phenylalanine to tyrosine.
Tyrosine is important for production of thyroxine, L DOPA (and therefore melanin).
When there is no conversion of phenylalanine, this causes a build up of the amino acid eventually causing the generation of phenylpyruvate and sequentially phenylacetate.
These esters are able to cross the blood brain barrier and can cause irreversible (but preventable) impaired brain development.
It is treatable by a reduced protein diet, supplemented with tyrosine.
Describe the proteases found in the body.
Pepsin - stomach
Trypsin and chymotrypsin - small intestine
Aminopeptidase and carboxypeptidase - small intestine.
