Lecture 14 Protein-AA Metabolism

Question 1

How big is the protein pool?

Answer 1

very limited pool, high conservation, not stored so constant turnover
* ~150g (dietary protein ~100g and endogenous protein ~75g)
* flux 400-500g/day

Question 2

Major metabolic fates of amino acids in the body

Answer 2

• 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

Question 3

What are the major nitrogen movements in AAs?

Answer 3

• protein synthesis from AAs
• protein degradation to AAs
• NH3 from AAs
• urea from free AA
• urea from NH3
• urinary from urea

Question 4

Where is the start of AA metabolism?

Answer 4

The liver
* site of synthesis of many different proteins (both structural & plasma proteins) from AAs

Question 5

What are some non-protein nitrogenous molecules that come from AAs?

Answer 5

just for interest

Question 6

What are the two most important reactions of protein metabolism?

Answer 6

• transamination
• oxidative deamination

Question 7

Describe the transamination reaction

Answer 7

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

Question 8

What are the enzymes called that catalyze transamination?

Answer 8

aminotransferases
* pyridoxal 5’-phosphate [PLP] dependant

Question 9

What AAs are actively transaminated in human tissue?

Answer 9

• Ala
• Asp
• Glu
• Ser
• Val
• Ile
• Leu

Question 10

What AAs do not participate in transamination reactions?

Answer 10

• Lys
• Thr
• Pro

Question 11

What 𝝰-keto acid is used widely as the acceptor in transamination reactions?

Answer 11

𝝰-ketoglutarate
* 𝝰-amino group of many AAs are funneled through Glu during AA catabolism

Question 12

What are the most common aminotransferase enzymes for transamination?

Answer 12

• 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

Question 13

Describe the ALT enzyme

Answer 13

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!)

Question 14

Describe the AST enzyme

Answer 14

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

Question 15

What is ALT a good indicator of?

Answer 15

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)

Question 16

What are exceptions where the AST/ALT ratio might be increased?

Answer 16

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

Question 17

Describe the deamination reaction

Answer 17

Is the liberation of free ammonia from the amino acid coupled with oxidation
* main mechanism for how we get rid of ammonia

Question 18

Where does deamination occur?

Answer 18

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

Question 19

Purpose of deamination

Answer 19

Provide NH3 for urea synthesis and 𝝰-keto acids for a variety of reactions including energy production when needed.

Question 20

What amino acid is most commonly seen in the deamination reactions?

Answer 20

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.

Question 21

What is the metabolic fate of the NH3 released from amino acid deamination reactions?

Answer 21

Urea cycle
* supples the urea cycle with NH3 and excretes it through urine

Question 22

What might increase the urea cycle?

Answer 22

• high protein diet
• during early starvation

Question 23

How does high protein diet increase the urea cycle?

Answer 23

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

Question 24

How does early starvation increase the urea cycle?

Answer 24

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

Question 25

What patients need a low protein diet?

Answer 25

Patients with renal disease

Question 26

What patients need a low protein diet?

Answer 26

Patients with renal disease

Question 27

Describe ammonia toxicity

Answer 27

free ammonia produced during catabolism of AA and nucleic acids and if not rid of properly can build up and circulate and is toxic because it raises the pH to a damaging level, interfering with ETC and causing irreversible cell damage and neorotoxicity

Question 28

Describe glucogenic amino acids

Answer 28

C skeletons of AAs used to make pyruvate or CAC intermediates
* gluconeogenesis removes oxaloacetate from CAC therefore AAs that provide intermediates (anaplerosis) are glucogenic

Question 29

What amino acids are glucogenic

Answer 29

• Ala
• Gly
• Cys
• Asp
• Asn
• Glu
• Gln
• Arg
• Met
• Val
• His
• Pro

Question 30

What amino acids are not gluconeogenic?

Answer 30

• leusine
• isoleucine

these are ketongenic

Question 31

Describe ketogenic amino acids

Answer 31

CANNOT make glucose - C skeleton of AAs are used to make acetyl CoA or acetoacetate (which also makes acetyl CoA)
* includes Lys and Leu
* The acetyl CoA can enter the CAC only if sufficient oxaloacetate therefore they are ketogenic, because cannot make anythig new
* short on energy or glucose, some acetyl COA is pushed towards ketone synthesis

Question 32

What are the partially ketogenic & partially glucogenic AAs?

Answer 32

Part is used for glucose and a part is used for Acetyl-CoA
* Phe
* Ile
* Thr
* Trp
* Tyr

Question 33

Entry of the glucogenic and ketogenic AAs

Answer 33

Do not need to memorize

Question 34

Describe the glucose alanine cycle

Answer 34

1. Alanine forms in muscle from transamination with glutamate (generated from leucine transamination) and from pyruvate (generated from glucose oxidation via glycolysis)
2. alanine travels in the blood to the liver
3. In the liver, alanine is transaminated with 𝝰-ketoglutatrate to form pyruvate and glutamate (essentially opposite from muscle)
4. pyruvate can be converted back to glucose through gluconeogenesis
5. the glucose is released from the liver back into circulation to be used for energy
6. the glutamate goes through deamination reaction to release NH4 which is then excreted in the urea cycle.

Question 35

What is the most abundant circulating amino acid

Answer 35

glutamine?

Question 36

interrelationships between AAs, urea cycle and CAC

Answer 36

summary slide