24. Nitrogen Balance Flashcards
What is the ‘nitrogen balance’ a balance between?
Explain the different categories of AAs.
How can ingested proteins be metabolised by the body?
+ve: N intake exceeds output (periods of growth, pregnancy) and -ve: N output exceeds ingestion (body proteolysis, fasting, trauma, protein malnutrition). Balance imp b/c no significant N store in humans.
N from AA in diet. 20 AAs with differing R groups. Essential (9): must get in diet. Conditional essential (6): can make but body has limited capacity to do so - becomes essential when exceeds capacity to make it.
AA taken off and used to make proteins directly (for growth/repair), OR can use C skeleton to make glucose/KB -> energy, OR can be important substrates to make other things e.g. nucleotides, NTs, coenzymes
List some of the things the following can be metabolised to:
a) Tyrosine (phenylalanine can be broken down into it too)
b) Tryptophan
c) Arginine
d) Histadine
e) Glycine, glutamate, aspartate
a) melanin, DA, adrenaline, NA, thyroxine
b) serotonin, melatonin
c) NO - vasodilation!
d) histamine
e) used directly as NTs
Describe how an amino acid’s C skeleton can be used in transamination.
If you remove the amino group from the following AAs, what keto acids do you get: alanine, glutamate, aspartate
What are glucogenic AAs? Give examples.
Are all amino acids glucogenic?
Make one AA from another. AA + keto acid (C skeleton with no amino group), and move the amino group from the AA onto the keto skeleton to make a new amino acid and keto acid.
Alanine = Pyruvate, glutamate = alpha-ketoglutarate, aspartate = oxaloacetate
AA that can be converted to glucose via GNG e.g. aspartate (feeds into oxaloacetate), alanine (feeds into pyruvate), glutamine/glutamate (feeds into alpha-ketoglutarate)
No - not leucine and lysine
What are ketogenic AAs? Give examples.
The AA C skeleton is used to make e.g. NT or melanin. The amino group is turned into NH3
a) Why is it hard to excrete?
b) How is excess ammonia transported/excreted in: fish, us, birds
An AA that can be degraded directly into acetyl CoA, which is the precursor of ketone bodies. Leucine and lysine -> acetoacetyl CoA, isoleucine -> acetyl CoA. Some are ketogenic AND glucogenic e.g. tryptophan, some only one or the other e.g. glycine is only glucogenic.
a) Toxic, v reactive, not v water soluble - need alot of water to excrete it.
b) Fish: excrete excess NH3. Us: Convert to urea (contains 2 amino groups) which is more soluble and less reactive, so need less H2O to excrete. Birds: uric acid - concentrate it more so use less water = less water weight when flying
Describe the glucose-alanine cycle.
Overall: Transports peripheral amino groups into the liver, and delivers amino groups to urea cycle in liver.
Muscle: Periphery takes glucose -> pyruvate -> alanine transamination (take amino group from glutamate). Glutamate can have it’s amino group turned into NH4+ in muscle when conveted to alpha-ketoglutarate by glutamate dehydrogenase (reversible). NH3 group added to more glutamate to make glutamine via glutamine synthase (oneway). To reverse = glutaminase, releases NH3
Alanine transported in blood -> liver, converted back to pyruvate by traansferring amino group back onto alpha-ketoglutarate to make glutamate, which enters urea cycle. Only prod little NH3, can also fo to urea cycle from blood.
Kidney: uses glutamine to help buffer blood and for GNG and immune system
Give the roles of glutamate in the following tissues:
a) skeletal muscle
b) kidney
c) CNS
d) immune cells
a) synthesised, stored and released in fasting. Can be used as fuel: convert to alphaKG an feed to TCA
b) substrate for gluconeogenesis; formation of NH3 for buffering protons - balanace pH
c) shuttle for maintenance of glutamate as a NT
d) fuel and enhances T-lymph response to infection; also supports phagocytosis by neutrophils and macrophages
List the sources of surpluss NH3 that accumulate in the liver for excretion.
What happens to this NH3?
1) Gluamate (made from alanine), glutamate dehydrogenase used -> NH3.
2) Liver breaks down other AA releasing amino groups and excess nucleotides -> NH3
3) When immune system uses glutamine as fuel -> releases NH3 into blood -> liver
4) Gut microbiota break down proteins and produce urea -> NH3 -> liver.
All converted to urea and sent to kidney -> excrete
Describe the urea cycle.
What is the point of control/energy utilising step? How is it controlled?
What intermediate can be taken from the cycle?
Omithine cycle.
Glutamate -> NH3 and CO2 by glutamate dehydrogenase. NH3 (plus more NH3 from blood etc.) reacts with carbomyl phosphate synthase to make carbomyl phosphate, which reacts with omithine to make citrulline. Citrulline + aspartate -> argino-succinate -> arginine -> omithine which releases urea.
Carbomyl phosphate synthase. Allosteric control from N-acetyl glutamate (when NAG is high, CPS is active, when low its inactive). NAG is only high when levels of glutamate are high (glutamate + acetyl CoA -> NAG + CoA), so this links urea cycle to amount of excess N in liver; the more glutamate, the more this is turned on.
Creatine (used as energy store in muscle)
