This document summarizes several pathways involved in carbohydrate metabolism. It describes the hexose monophosphate pathway (HMP pathway), which provides an alternative route for glucose metabolism and is significant for biosynthesis of NADPH and pentose sugars. It does not consume or produce ATP. The document also summarizes the gamma amino butyrate (GABA) shunt, which converts glutamate to succinate via GABA. Finally, it provides an overview of glycogen metabolism, describing glycogen synthesis from glucose and glycogen breakdown into glucose.

1. CARBOHYDRATE
METABOLISM
M.Sc. I
Roll No. 12

2. Hexose Monophosphate Pathway
(HMP)
• Also called as HMP shunt or Pentose Phosphate
Pathway(PPP) or Phosphogluconate pathway.
• Alternative route for metabolism of glucose.
• Significance - Biosynthesis of NADPH and Pentose
sugar.
• NO ATP is consumed or produced.
• Location - In cytoplasm of liver and adipose tissue, etc.
• It helps in-
1. Formation of NADPH for synthesis of F.A. and
steroids.
2. Prevent cell from oxidative damage.
3.Synthesis of Ribose for nucleotide and nucleic acid
formation.

3. Steps of Pentose Phosphate Pathway
 Oxidative Phase
 This phase is Irreversible.
 Glucose-6-phospshate Ribulose-5-phosphate
+ 2 NADPH
 Non-oxidative Phase
 This phase is Reversible.
 it catalyze the conversion of pentose produced in
phase one into 2 molecules of Glyceraldehyde-3-
phosphate and Fructose-6-phoshate.

6. Gamma Amino Butyrate Shunt
• Converts glutamate to succinate via γ-Amino
Butyric acid (GABA)
• GABA has an amino group on the γ -carbon rather
than on the α-carbon.
• Glutamate is Decarboxylated to give GABA
• Catalyzed by pyridoxal phosphate dependant
Glutamate Decarboxylase.
• GABA undergoes transamination followed by
oxidation to form succinate which enters TCA cycle

7. Function of
GABA
• An inhibitory
neurotransmitter –
in brain.
• Regulates
activity of neuron.

8. Glycogen Metabolism
• Glycogen is the storage form
of glucose in animals
• Excess glucose is converted
into glycogen for storage.
• It is stored in cytosol of liver
and muscle cells in the form of
granules.
• Glycogen Granules- complex
aggregates of glycogen +
enzymes for synthesis and
degradation + machinery for
regulating these enzymes.

9.  Functions of Glycogen
1. Maintenance of blood glucose level
2. Muscle glycogen-
- Quick source of energy for metabolism
- Exhausted in less than one hour
3. Liver glycogen-
- Reservoir of glucose for other tissues
- When dietory glucose is not available
- Imp for neurons(which only use glucose as a fuel)
- Can be deployed in 12-24 hours.
 Why To Store Glycogen as a Fuel Reserve?
1. Rapidly mobilized
2. Can generate energy in absence of oxygen
3. Brain depends on continuous supply of glucose- mostly
comes from glycogen.

10. Overview of Metabolism
Glycogen Synthesis Glycogen Breakdown

11. Glycogenesis
Synthesis of glycogen from glucose
1. Synthesis of UDP-glucose-

12. 2. Requirements of Primer To Initiate Glycogenesis-
* Small fragment of preexisting glycogen- Primer
* Primer absent - glycogen initiator synthase- glycogenin
accept glucose from UDP-glucose.
3. Glycogen Synthesis By Glycogen Synthase-

13. 4. Formation Of Branches In Glycogen
Net reaction-
(Glucose)n + Glucose + 2 ATP (Glucose)n+1 + 2 ADP + Pi

14. Glycogenolysis
Degradation of stored glycogen
1. Action Of Glycogen Phosphorylase
• degrade α- 1,4- Glycosidic bond
• Phosphorylosis -
• Limit Dextrin

15. 2. Action Of De-branching Enzyme-
•Transferase activity -
α – 1,4 glycosidic bond
•Glucosidase activity-
α - 1,6 glycosidic bond

16. 3. Formation of glucose-6-phosphate to glucose-
Reaction catalyzed by phosphoglucomutase

17. • The catalytic site of glucose 6-phosphatase faces the lumen of the ER.
• A glucose 6-phosphate (G6P) transporter (T1) carries the substrate from the
cytosol to the lumen, and the products glucose and Pi pass to the cytosol on
specific transporters (T2 and T3).
• Glucose leaves the cell via the GLUT2 transporter in the plasma
membrane.

18. References
• Harper’s Biochemistry- 27th Edition
• Principle Of Biochemistry -Lehninger
• Biochemistry – U. Satyanarayana.