Metabolism Of Carbohydrates Biochemistry Notes-IV

Afza.Malik GDA

Carbohydrates Metabolism Biochemistry for Nurses

Metabolism Of Carbohydrates Biochemistry Notes-IV

Metabolism of glycogen can be discussed under two headings, Glycogenolysis, Hexose Monophosphate (HMP) Shunt,EM and HM Path way Comparison,Regulation of HMP Shunt, Uronic Acid Pathway, Gluconeogenesis, Hormones in Gluconeogenesis, Fates of Lactic Acid In The Body,Biosynthesis of Lactose,Metabolism of Fructose,Regulation of Blood Glucose,Hormonal Influences: (Endocrine Influences) On Carbohydrate Metabolism.

Metabolism of glycogen can be discussed under two headings:

A. Synthetic phase: Formation of glycogen

B. Catabolic phase: Breakdown of glycogen



It is the formation of glycogen from glucose. Sites: Principally it occurs in liver and skeletal muscles, but it can occur in every tissue to some extent.

Stimulation of Glycogenesis

Inhibition of Glycogenesis

1. Insulin: Insulin increases the Protein-phosphatase-1 activity

1. Increased concentration of glycogen inhibits glycogenesis, “Feedback” inhib

2. Glucocorticoids: Effects seen 2 to 3 hours after administration.

Enhances gluconeogenesis and glycogen synthesis in liver

Increases synthesis of the enzyme glycogen synthase

2. Increased concentration of cyclic-AMP ↑

stimulates inhibitor-1, to form ‘active’ inhibitor-1-P,. which in turn inhibits Protein Phosphatase-1

3. Glucose: High substrate concentration increases synthesis (allostery)



Relation of glycogenesis with K+ influx into the cell:

• Clinical importance in treatment of hyperkalaemia, when insulin and glucose are administered.

• Importance in treatment of diabetic ketoacidosis, with insulin and glucose, danger of “hypokalaemia” to precipitate. The patient should be monitored for potassium level in blood while treating diabetic ketoacidosis with insulin and glucose infusion.


Breakdown of glycogen to glucose is called as glycogenolysis.

Regulation of Glycogen Metabolism . Regulation of glycogen metabolism is achieved by a balance in activities between Glycogen synthase and Phosphorylase, which are as follows:

• Substrate control (through allostery) as well as

• Hormonal control and by

• End products.

Hexose Monophosphate (HMP) Shunt

An Alternate Pathway for Oxidation of Glucose

Synonyms: Variously called as:

• Hexose monophosphate pathway or shunt

• Pentose phosphate pathway (PP-pathway)

• Pentose cycle

• Phosphogluconate pathway

• Warburg-Dickens-Lipman Pathway.

Biomedical Importance

• Though it is oxidation of glucose, but it is not meant for energy.

• Provides NADPH which is required for various reductive synthesis in metabolic pathways.

• Provides pentoses required for nucleic acid synthesis.

• Deficiency of a particular enzyme leads to haemolytic anaemia, which is of great clinical importance.

EM and HM Path Way Comparison

EM Pathway

HM Pathway

2. Not a multicyclic process


1. Occurs in certain special tissues for special function

1. Occurs in all tissues


2. Multicyclic process

3. Oxidation by dehydrogenation but NAD is H-acceptor

3. Oxidation achieved by dehydrogenation but NADP* is used as H-acceptor

4. ATP is required and ATP is produced

4. Not meant for energy; ATP is not produced, ATP is required for glucose to glucose-6-P. (for phosphorylation) and for interconversion of Pentoses

5. CO2 is never formed

5. CO2 is produced

Regulation of HMP Shunt

1. Reaction catalysed by G-6-PD, the first reaction of the pathway constitutes the rate-limiting step. It is primarily regulated by cytoplasmic levels of NADP+ and NADPH, thus the ratio of the two, i.e. [NADP+]/ [NADPH]. If the cytoplasmic ratio is high, i.e. a rise in NADP+, enhances the “rate-limiting” reaction as well as the shunt pathway. 

    A decrease in the ratio, i.e. a rise in NADPH level, inhibits both G-6-PD and 6-phosphogluconate dehydrogenase by making less NADP+ available for their catalytic reactions and also by competing with NADP+ to occupy the enzyme binding site of G-6-PD.

2. Activities of both dehydrogenases and the rate of the pathway are enhanced on feeding high carbohydrate diets and are reduced in starvation and dibetes mellitus.

3. Increase in FA synthesis and steroid synthesis re-oxidises NADPH to NADP+ and cytoplasmic ratio of NADP+/NADPH increased which enhances the shunt pathway.

4. Hormones

• Insulin: Induces the synthesis of both the dehydrogenases and thus enhances the activity of the pathway.

• Thyroid hormones: Enhances the activity of G-6- PD and thus the shunt pathway.

Uronic Acid Pathway

It is an alternate pathway for oxidation of glucose.

Biomedical Importance

• In this pathway energy is not produced.

• Major function is to produce D-Glucuronic acid which is mainly utilised for detoxication of foreign chemicals (Xenobiotics). Also used for synthesis of MPS.

• Inherited deficiency of an enzyme in this pathway produces ‘essential pentosuria’.

• Total absence of one particular enzyme in primates, accounts for the fact that ascorbic acid (vitamin C) cannot be synthesised by humans, and requires to be provided in the diet.



The formation of glucose or glycogen from noncarbohydrate sources is called gluconeogenesis.

Biomedical Importance

Why Gluconeogenesis is Necessary in the Body?

1. Gluconeogenesis meets the requirements of glucose in the body when carbohydrates are not available in sufficient amounts from the diet. Even in conditions, where fat is utilized for energy still certain basal level of glucose is required to meet the need for glucose for special uses, e.g.

• Source of energy for nervous tissues and erythrocytes,

• Required for maintaining level of intermediates of TCA cycle,

• Source of glyceride-glycerol-P required for adipose tissue,

• It is a precursor of milk sugar (lactose) for lactating mammary gland,

• It serves as only fuel for skeletal muscles in anaerobic conditions.

2. Gluconeogenic mechanisms are required to clear the products of metabolism of other tissues from the blood, e.g.

• Lactic acid produced by muscles and erythrocytes,

 • Glycerol which is continuously produced by adipose tissue by lipolysis of TG (triacyl glycerol).

Hormones in Gluconeogenesis

• Glucagon: It increases gluconeogenesis from Lactic acid and amino acids.

• Glucocorticoids: They stimulate gluconeogenesis by increasing protein catabolism in the peripheral tissues and increasing hepatic uptake of amino acids and increases activity of transaminases and other enzymes concerned in gluconeogenesis.

Fates of Lactic Acid In The Body

1.Chief fate is conversion to Pyruvate and its utilisation as pyruvate which either undergoes oxidative decarboxylation to form Acetyl-CoA or it can be glucogenic.

2. What is “Cori Cycle”? Once formed Lactic acid can be further metabolised only by its reconversion to pyruvate as stated above. In contrast to the phosphorylated intermediates of glycolysis which are locked in the cells, lactates and pyruvates can readily diffuse out from the cells in which they are produced and pass into the circulation.

3. Lactate-Propanediol Pathway Miller and Olson observed that lactic acid utilisation by rat ventricle slices in vitro could not be completely inhibited by sodium fluoride and by anaerobic conditions.

Biosynthesis of Lactose

    In synthesis of Lactose in lactating mammary gland, UDP glucose is converted to UDP-galactose by the enzyme epimerase. UDP-Galactose condenses with one molecule of glucose to form Lactose, the reaction is catalysed by the enzyme Lactose synthetase. Lactose synthetase also called as galactosyl transferase.

Metabolism of Fructose

    Dietary Sources of Fructose: Fructose is present in fruit juices and honey. Chief dietary source is sucrose, a diasaccharide, taken as table sugar (cane sugar). Sucrose is hydrolysed in the intestine to one mol. of glucose and one mol. of fructose by the enzyme Sucrase. Fructose is absorbed by facilitated transport and taken by portal blood to liver, where it is mostly converted to glucose.

Biomedical Importance

• Fructose is easily metabolised and a good source of energy.

• Seminal fluid is rich in fructose and spermatozoa utilises fructose for energy. 

• Excess dietary fructose is harmful—leads to increased synthesis of TG.

• In diabetics, fructose metabolism through ‘sorbitol’ pathway may account for the development of cataract.

Regulation of Blood Glucose (Homeostasis) Blood glucose level is maintained within physiological limits 60 to 100 mg% (“true” glucose) in fasting state and 100 to 140 mg% following ingestion of a carbohydrate containing meal, by a balance between two sets of factors:

(A) Rate of glucose entrance into the blood stream

(B) Rate of its removal from the blood stream.

Hormonal Influences: (Endocrine Influences) On Carbohydrate Metabolism

Endocrine organs play an important key role in this homeostatic mechanism.


2. Adrenocortical hormones:

3. Anterior Pituitary Gland

4. Catecholamines


6.Thyroid Hormones

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