Lecture 14 Protein-AA Metabolism Flashcards
How big is the protein pool?
very limited pool, high conservation, not stored so constant turnover
* ~150g (dietary protein ~100g and endogenous protein ~75g)
* flux 400-500g/day
Major metabolic fates of amino acids in the body
- use for protein synthesis
- use as precursors for the synthesis of numerous non-protein nitrogenous molecules
- catabolism with excretion of nitrogen and use of carbon chains as energy substrates
What are the major nitrogen movements in AAs?
- protein synthesis from AAs
- protein degradation to AAs
- NH3 from AAs
- urea from free AA
- urea from NH3
- urinary from urea
Where is the start of AA metabolism?
The liver
* site of synthesis of many different proteins (both structural & plasma proteins) from AAs
What are some non-protein nitrogenous molecules that come from AAs?
just for interest
What are the two most important reactions of protein metabolism?
- transamination
- oxidative deamination
Describe the transamination reaction
Involves the transfer of an amino group to an 𝝰-keto acid to form a new amino acid.
* different enzymes and differnt substrates but the reaction is the same
* essentially amino acid with an ammonia group and keto acid with no ammonia group and the reaction causes a switch
What are the enzymes called that catalyze transamination?
aminotransferases
* pyridoxal 5’-phosphate [PLP] dependant
What AAs are actively transaminated in human tissue?
- Ala
- Asp
- Glu
- Ser
- Val
- Ile
- Leu
What AAs do not participate in transamination reactions?
- Lys
- Thr
- Pro
What 𝝰-keto acid is used widely as the acceptor in transamination reactions?
𝝰-ketoglutarate
* 𝝰-amino group of many AAs are funneled through Glu during AA catabolism
What are the most common aminotransferase enzymes for transamination?
- ALT: alanine amino transferase
- AST: Aspartate amino transferase
both are widespread in tissues and allow movement of amino groups between glutamate/𝝰-ketoglutarate & aspartate/oxaloacetate or alanine/pyruvate
Describe the ALT enzyme
alanine aminotransferase
* Found in the kidney, heart and muscle but in great concentration in liver compared with other tissues; also plays a different role depending on the tissue
* reversible reaction
* is a cytoplasmic enzyme catalyzing transamination reactions (so in the cell!)
Describe the AST enzyme
Aspartate aminotransferase
* found in high concentration in heart compared eith other tissues of the body such as the liver, skeletal muscle and kidney
* Can be cytosolic or found in the mito
What is ALT a good indicator of?
cell health
* should be in the cytoplasm of cells so if it is found in the circulation usually indicates that something is wrong, usually with the liver.
* liver disease the blood ALT is higher than AST and the AST/ALT ration will be low (less than 1)
What are exceptions where the AST/ALT ratio might be increased?
usually increased (>2) in alcoholic hepatitis, cirrhosis, and in the frist day or two of acute hepatitis or injury from bile duct obstruction.
* alcohol liver disease has vit B6 deficiency that decreases serum ALT activity
Describe the deamination reaction
Is the liberation of free ammonia from the amino acid coupled with oxidation
* main mechanism for how we get rid of ammonia
Where does deamination occur?
mainly in the mitochondria of liver and kidney
* substantial amounts of ammonia are generated in the liver from glutamate and in the kidney by deamination of glutamine
Purpose of deamination
Provide NH3 for urea synthesis and 𝝰-keto acids for a variety of reactions including energy production when needed.
What amino acid is most commonly seen in the deamination reactions?
Much of the oxidative deamination occurring in cells involves the amino acid glutamate (glutamic acid), which can be oxidatively deaminated by the enzyme glutamate dehydrogenase (GDH), using NAD or NADP as a coenzyme.
What is the metabolic fate of the NH3 released from amino acid deamination reactions?
Urea cycle
* supples the urea cycle with NH3 and excretes it through urine
What might increase the urea cycle?
- high protein diet
- during early starvation
How does high protein diet increase the urea cycle?
Tons of AAs coming in and eating more of this usually means eating less glucose, so start making glucose from AAs and need to get rid of the ammonia that is produced in the process
* increase enzyme activity/ concentration
* return to balanced diet: enzyme concentration goes down
* liver is the main regulator of this since it is the first bypass
How does early starvation increase the urea cycle?
increased activity
* proteins are degraded and the C skeleton is used for energy which increases the NH3 output
* If the N excretion fails there is an increase in free ammonia circulating the body, which is toxic
