amino acids Flashcards
Why is there continuous synthesis and degradation?
AAs can’t be stores
Obtained from- diet, synthesis in body, normal protein turnover (recycled)
If not needed- rapidly degraded
Proteins- source of energy
What are the types of AAs?
Essential from diet
From body-
Non-essential
Conditionally essential
How are proteins digested?
Hydrolysed by pepsin in stomach, trypsin from pancreas etc
AAs actively transported into epithelial cells and enter blood
Cystinuria- defective carrier system—>AAs in urine, crystals of cysteine form kidney stones
How are AAs synthesised?
AAs can be be made form glucose and nitrogen
Can be made from intermediates of glycolysis and TCA cycle
Eg. Oxaloacetate forms aspartate, pyruvate forms alanine, valine, leucine
Non essential AAs can be made from essential AAs
Eg. Methionine donates sulphur for cysteine, phenylalanine forms tyrosine
How are AAs degraded?
Some no longer needed/Diet exceeds need for protein
Glucogenic- carbons converted to glucose
Ketogenic- converted to acetyl coA or acetoacetate (ketone bodies)
Some AAs return to their precursors, some dont
How are AAs degraded via nitrogen? Part 1
Transamination-
~amino group transferred from one AA to another (alpha-keto acid 1 + AA2–> AA1 + alpha-keto acid 2)
~alpha-ketoglutarate and glutamate usually one pair
~co-factor- pyridoxyl phosphate (derived from Vit B6)
~reversible reaction- also for synthesis
How are AAs degraded via nitrogen? Part 2
Remove nitrogen as ammonia-
~glutamate can collect nitrogen form other AAs
~nitrogen in glutamate converted to ammonia by glutamate dehydrogenase
~ammonia enters urea cycle
~allosteric regulation by ATP and GTP (inhibits), ADP and GDP (activates)
Eg. If energy is low, enzyme activated so energy from AA carbon skeletons
What is a proteasome?
Protease complex- unfolds and degrades proteins (needs energy from ATP)
Bit like intracellular
What is ubiquitin?
Small proteins that target proteins for degradation
How are proteins degraded?
Continuously synthesised and degraded
-varying half lives
Proteins recycled within cells
Proteolytic enzymes in lysosomes can break down proteins
Ubiquitin and proteasome play role
How is nitrogen relevant to the body?
Nitrogen not usable in body
NH3 (ammonia) usable and crosses membranes but NH4+ (ammonium ion) is toxic
What is the nitrogen balance?
Nitrogen ingested=nitrogen excreted
Children/pregnant- +ve balance
Disease/starvation- -ve balance
What is the urea cycle?
5 main steps
- and 2. In mitochondria
- , 4. and 5. In cytosol
Nitrogen enters as ammonia between 1. and 2.
Nitrogen enters as aspartic acid between 2. and 3.
Urea leaves cycle at 5.
Orthinine initiates and is regenerated
How is the urea cycle controlled?
Feed forward regulation- higher rate of ammonia production, higher rate of urea
Allosteric activation of enzymes
Eg. Arginine stimulates carbamoyl phosphate
High protein diet/fasting induces urea cycle enzymes
Protein is energy source in fasting so need to get rid of nitrogen
What is the glucose-alanine cycle? (Cahill)
AAs used as emergency source of energy
Cycling of nutrients between muscles and liver
Precise reactions and location depends on physiological state
Fasting- muscle protein—> AAs
~transaminated to form alanine which is transported to liver
Nitrogen enters urea cycle and pyruvate enters TCA cycle for energy
