Glucose/Alanine Cycle

What is the glucose/alanine cycle? How is it important to metabolism?

Related topics:

1. ENERGY SUPPLY DURING FASTING
2. GLUCONEOGENESIS 
3. PROTEINS & AMINO ACIDS AS ENERGY SOURCE 

Keywords: skeletal muscle; breakdown of muscle to supply energy during fasting; use of amino acids as a source of energy; alanine; ALA.


GLUCOSE/ALANINE CYCLE

(i) Summary of the glucose/alanine cycle:

1. During extended periods of fasting (eg baby not feeding well, Ramadan fasting/puasa bulan Ramadhan), skeletal muscle is degraded as an alternative source of energy.
2. Alanine is the major amino acid present when muscle (protein) is degraded.
3. The glucose-alanine cycle occurs in skeletal muscle to eliminate nitrogen while replenishing (renewing) the energy supply for muscle.
4. The amino group transported from the muscle to the liver in the form of alanine, is converted to urea in the urea cycle and excreted.

(ii) Reactions in skeletal muscle:

1. In muscle cells and other peripheral tissues, glycolysis produces pyruvate.
2. Pyruvate can be transaminated to  alanine.
3. The transamination reaction requires an α-amino acid as donor of the amino group, generating an α-keto acid in the process. 
4. This reaction is catalyzed by alanine transaminase, ALT.
5. The alanine then enters the blood stream and is transported to the liver.
6. Alanine is returned to the liver for gluconeogenesis.

(iii) Reactions in liver:

1. Within the liver, alanine is converted back to pyruvate by deamination.
2. Pyruvate is a source of carbon atoms for gluconeogenesis.
3. Gluconeogenesis converts pyruvate to form glucose.
4. The newly formed glucose can then enter the blood for delivery back to the muscle.
5. This pathway is termed the glucose-alanine cycle.

(iv) Related FAQs and comments:

1. Note: Liver is a hepatic tissue. Muscle is a non hepatic tissue or extrahepatic tissue, EHT.
2. Note: Skeletal muscle is protein.
3. Alanine is the major amino acid in protein (eg skeletal muscle). 
4. Transamination. The amino group is transferred to another molecule. What does transamination mean? Write down this reaction. What are the subsrates, products and enzyme?
5. Note: Urea cycle occurs in the liver.  What is the structure of urea?
6. What is the urea cycle? What is its importance?
7. What happens if the urea cycle does not function?
8. What is the condition when there is high urea present in blood?
9. What happens when there is ammonia build-up in the body?
10. What is the condition when there is high ammonia present in blood?
11. Is ALT an important enzyme? What does the presence of high ALT in serum indicate?
12. Please take note: ALT used to be referred to a serum glutamate-pyruvate transaminase, SGPT, as serum was used for the determination on the enzyme. The term SGPT is now obsolete and you will only see it in old textbooks.

(v) Images of the glucose/alanine cycle:

Eglobalmed.com:





The Medical Biochemistry Page.Org:
http://themedicalbiochemistrypage.org/gluconeogenesis.php

Andrea Hambly's White Tiger Natural Medicine:
http://www.whitetigernaturalmedicine.com/nutrition/proteins-amino-acids

PowerPoints

http://slideplayer.com/search/alanine+cycle/

http://slideplayer.com/slide/4214240/

http://slideplayer.com/slide/4213836/