Protein Breakdown Flashcards
What is nitrogen balance?
Synthesis of new proteins is balanced by the breakdown of old proteins. If this is altered then you can get a positive or a negative balance
Describe a positive nitrogen balance
Describe a negative nitrogen balance
What happens following the metabolism of an amino acid?
Aa metabolism forms an alpha ketoacid + ammonia. Enzymes involved are transaminase or aminotransferase. Ammonia is metabolised by the liver to form urea in the urea cycle which is then excreted by the kidney.
The nitrogen part of the aa is used in the urea formation and nitrogenous compounds
The carbon skeleton part of the aa is used for production of glucose, ketone bodies and/or energy
What is transamination?
Also give the main alpha keto acids
a.acid A + α-keto acid B—> α-keto acid A a.acid B
What are the most important transaminases?
Two most important are alanine (ALT) and aspartate (AST) transaminase
How does glutamate form ammonia?
Glutamate can form ammonia w glutamate dehydrogenase. The reaction is fully reversible and can use either NAD or NADP
Usually NAD is used for degradation and NADP for synthesis
The purpose of the 2 reactions= to release NH3 which is picked up by glutamate to form glutamine
Describe the elimination of free ammonia
Free ammonia generated in non-hepatic tissue combines with glutamate to give glutamine
What is the urea cycle?
The means of excreting nitrogen
Enzymes are present in the liver but not muscle
Takes place in the mitochondria and the cytoplasm
Substrates required are bicarbonate, aspartate and ammonium ions (released from glutamine or glutamate)
Draw and describe the urea cycle
Ammonia and bicarbonate form Carbamoyl phosphate, which forms citrulline. Citrulline is then transported to the cytosol
Citrulline and aspartate are condensed to form argininosuccinate.
Argininosuccinate is broken down into arginine and fumarate
Arginine is broken down into urea and ornithine by arginase.
Where does the urea cycle occur?
All reactions in the urea cycle take place in the cytosol except the formation of citrulline from carbamoyl phosphate and ornithine that takes place in the mitochondria.
The citrulline formed is then exported across the mitochondrial membranes in exchange for ornithine.
Glutamate dehydrogenase and aspartate aminotransferase are found in the mitochondria of liver
Does the urea cycle occur in muscles?
Enzymes of the urea cycle not present
In prolonged exercise or starvation branched amino acids are used for energy (leucine, isoleucine and valine)
Two routes are used to transport nitrogen to the liver: Alanine and glutamine
