Master Gluconeogenesis: A Comprehensive Guide for all type of Medical Licensing and Biochemistry Exams and Tests

Gluconeogenesis, a critical component of energy metabolism, plays a vital role in maintaining the health and wellness of the human body. As Healthcare/Medical and Biochemistry professional, a comprehensive understanding of this process is crucial for success on exams and in practice. In this article, we will delve into the definition and overview of gluconeogenesis, exploring its importance in human physiology, the steps involved in the process, the comparison/differences between Glycolysis and Gluconeogenesis, and the enzymes and substrates required. We will also examine the role of gluconeogenesis in energy metabolism, its regulation, clinical significance, and any related disorders. Whether you are preparing for exams or looking to expand your knowledge, this article provides a clear and easy-to-understand guide to gluconeogenesis, ensuring you are equipped with the information necessary to succeed.

Comprehensive Biochemistry notes on Gluconeogenesis for medical professionals and general readers, important for BS Biochemistry, MBBS, MBDS, BS Nursing, Entry tests examination preparations

A detailed note on Gluconeogenesis for exams and self-study
From the Biochemistry Library of H.E.S (Health, Education, and Skills)

Gluconeogenesis | Definition and Overview

Gluconeogenesis is a metabolic process that involves the creation of glucose from non-carbohydrate precursors such as Lactate, Pyruvate, Glucogenic Amino acids, and Glycerol of Lipids.

This process serves as a complementary mechanism to glycolysis, which generates glucose from carbohydrate sources.
Gluconeogenesis starts during fasting, after the depletion of glycogen stores in the liver i.e. after 10-18 hours of fasting.
First few reactions till the formation of Oxaloacetate occur in the mitochondria.
Later on, reactions starting from Oxaloacetate till the synthesis of glucose, occur in the cytosol.
Gluconeogenesis occurs in several tissues, including the liver and kidneys.
The liver is the main organ, which carries out 90% of gluconeogenesis.
Kidneys carry out the remaining 10% of gluconeogenesis.
Skeletal, Cardiac, and Smooth muscles cannot start gluconeogenesis (because of a lack of the necessary enzymes required for gluconeogenesis, such as glucose-6-phosphatase).
This pathway maintains blood glucose levels during fasting.
Gluconeogenesis clears the metabolic substances from different tissues e.g. Lactate in RBCs (anaerobic glycolysis) and Glycerol from Adipose tissues (Lipolysis).

Importance of Glyconeogenesis in Human Physiology

I. Maintaining Glucose Homeostasis

Glucose is a primary source of energy for the body, and it is essential to ensure a consistent supply of glucose to the brain, muscles, and other organs. Gluconeogenesis allows the body to generate glucose when carbohydrate sources are limited, ensuring that energy needs are met even during periods of fasting, exercise, or stress.

II. Providing Energy to the Body

The end products of gluconeogenesis are used for a variety of purposes, including the synthesis of glycogen, the generation of ATP, and the provision of energy to the brain and other organs. The ability to generate glucose from non-carbohydrate precursors makes gluconeogenesis an important backup system when glucose from carbohydrate sources is not available.

III. Supplying Energy to the Brain

Gluconeogenesis ensures that the brain has a source of energy when carbohydrate sources are limited, maintaining optimal brain function even during periods of fasting or stress.

IV. Regulating Blood Sugar Levels:

When blood sugar levels drop, gluconeogenesis increases, generating glucose to bring blood sugar levels back to normal. On the other hand, when blood sugar levels are high, gluconeogenesis decreases, reducing the amount of glucose in the bloodstream.

Steps of Gluconeogenesis and their Significance

Here are the steps of gluconeogenesis and the enzymes involved in each step:

I. Conversion of Pyruvate to Phosphoenolpyruvate

The conversion of Pyruvate to PEP is not a direct reaction.
Pyruvate is first converted to Oxaloacetate by Pyruvate Carboxylase + Biotin (a co-factor).
Oxaloacetate is then converted to PEP by PEP Carboxykinase.
Pyruvate Carboxylase is found in liver and kidney cells but not in muscles. Therefore muscles cannot synthesize glucose by gluconeogenesis and have no role in maintaining blood glucose levels.
The conversion of Pyruvate to Oxaloacetate occurs in the mitochondria.
The mitochondrial membrane is impermeable to Oxaloacetate, therefore it must first be reduced to malate, which leaves the mitochondria, enters the cytosol, and then reoxidized to oxaloacetate. This interconversion of malate and oxaloacetate is called Malate Shuttle.
In the cytosol, oxaloacetate is converted to PEP by PEP Carboxykinase.

Pyruvate + ATP → PEP + ADP

Significance of Conversion of Pyruvate to Phosphoenolpyruvate

Pyruvate is a three-carbon molecule derived from glycolysis, the metabolic pathway that breaks down glucose to produce energy. In order to be used for gluconeogenesis, pyruvate must first be converted to PEP, which is a high-energy molecule that acts as a substrate for glucose production.
The conversion of pyruvate to PEP requires the transfer of a phosphate group from ATP to pyruvate, which is catalyzed by the enzyme pyruvate kinase. This reaction is energetically unfavorable and is driven by the high-energy state of PEP, which is stabilized by the presence of the phosphate group. The energy generated by this reaction is then used to drive the other reactions in gluconeogenesis.

II. Formation of Fructose-1,6-bisphosphate

PEP is converted to Fructose-1,6-bisphosphate by the enzyme Phosphoenolpyruvate Carboxykinase (PEPCK).

The mechanism of the reaction can be broken down into several steps:

Activation of PEP: PEPCK catalyzes the transfer of a phosphate group from ATP to PEP, resulting in the formation of the Phosphoenolpyruvate carboxykinase (PEP) intermediate.
Decarboxylation of oxaloacetate: The enzyme PEPCK catalyzes the decarboxylation of oxaloacetate to form phosphoenolpyruvate (PEP) and CO2.
Phosphorylation of PEP: Finally, a phosphate group from ATP is added to PEP, producing Fructose-1,6-bisphosphate.

PEP + ATP + H2O → Fructose-1,6-bisphosphate + ADP

Significance of Formation of Fructose-1,6-bisphosphate

Fructose-1,6-bisphosphate is an important intermediate in the metabolic pathway of glycolysis and gluconeogenesis and serves as a link between the two processes.
In gluconeogenesis, the enzyme fructose 1,6-bisphosphatase catalyzes the reverse reaction of fructose-2,6-bisphosphate synthase, converting Fructose-2,6-bisphosphate to Fructose-1,6-bisphosphate. This process allows the organism to recycle the energy stored in Fructose-2,6-bisphosphate and convert it into a form that can be used for energy production during periods of low glucose availability.
The concentration of Fructose-2,6-bisphosphate is positively correlated with the activity of glycolysis, while the concentration of Fructose-1,6-bisphosphate is positively correlated with the activity of gluconeogenesis. Therefore, by controlling the rate of Fructose-1,6-bisphosphate formation, the organism can regulate the balance between glycolysis and gluconeogenesis and coordinate its energy metabolism according to its needs.

III. Hydrolysis of Fructose-1,6-bisphosphate

The enzyme Aldolase B cleaves Fructose-1,6-bisphosphate into 2-C molecules i.e. Glyceraldehyde-3-phosphate, dihydroxyacetone phosphate.

Fructose-1,6-bisphosphate → Glyceraldehyde-3-phosphate + Dihydroxyacetone phosphate

The mechanism of the Aldolase B-catalyzed reaction can be broken down into several steps:

Enzyme-substrate complex formation: Aldolase B binds to Fructose-1,6-bisphosphate and forms an enzyme-substrate complex.
Dephosphorylation: The enzyme cleaves the phosphate ester bond in Fructose-1,6-bisphosphate, releasing inorganic phosphate (Pi) and producing Fructose-1-6-diphosphate.
Aldol reaction: Aldolase B then cleaves the Fructose-1-6-diphosphate molecule into two molecules of Glyceraldehyde-3-phosphate.

Similarity and differences of this step in Glycolysis and Gluconeogenesis of

The steps involved in the cleavage of Fructose-1,6-bisphosphate into two molecules of Glyceraldehyde-3-phosphate by the enzyme Aldolase B occur in both glycolysis and gluconeogenesis. However, the direction of the reaction is different in the two pathways.
In glycolysis, the reaction occurs as part of the process of generating energy from glucose. In this case, Fructose-1,6-bisphosphate is cleaved into two molecules of Glyceraldehyde-3-phosphate, which are then further processed to generate ATP.
In gluconeogenesis, the reverse reaction occurs, where two molecules of Glyceraldehyde-3-phosphate are combined to form Fructose-1,6-bisphosphate. This reaction is part of the process of generating glucose from non-carbohydrate precursors.

Significance of Hydrolysis of Fructose-1,6-bisphosphate

The hydrolysis of Fructose-1,6-bisphosphate is a crucial step in the pathway, as it results in the production of two important metabolites, glyceraldehyde-3-phosphate, and dihydroxyacetone phosphate.
Glyceraldehyde-3-phosphate can be converted into 1,3-bisphosphoglycerate, which is a substrate for the production of ATP.
Dihydroxyacetone phosphate can be converted into glyceraldehyde-3-phosphate, making it an important source of intermediates for the glycolysis pathway.

IV. Conversion of Glyceraldehyde-3-phosphate to Glucose-6-phosphate

The enzyme Phosphohexose Isomerase (PHI) converts Glyceraldehyde-3-phosphate to Glucose-6-phosphate.

Glyceraldehyde-3-phosphate → Glucose-6-phosphate

The basic mechanism of the reaction involves the isomerization of the aldehyde functional group in Glyceraldehyde-3-phosphate to the ketone functional group in Glucose-6-phosphate.
This isomerization is accomplished through a series of structural changes within the enzyme's active site, which involves the transfer of a phosphate group from Glyceraldehyde-3-phosphate to a histidine residue within the enzyme.
Once the isomerization is complete, the phosphate group is transferred back to the ketone functional group, forming Glucose-6-phosphate.
This reaction is an important step in the process of gluconeogenesis, as Glucose-6-phosphate is a key intermediate in the pathway that leads to the formation of glucose.

Significance of Conversion of Glyceraldehyde-3-phosphate to Glucose-6-phosphate

Glucose-6-phosphate is a key intermediate in the glycolysis and glycogen metabolism pathways, and it is necessary for the maintenance of glucose homeostasis in the body.
The conversion of glyceraldehyde-3-phosphate into glucose-6-phosphate involves a series of enzyme-catalyzed reactions, which are regulated by hormones such as insulin and glucagon.

V. Formation of Glucose

Glucose-6-phosphate is converted to Glucose by the enzyme Glucose-6-Phosphatase (also known as glucose 6-phosphate phosphatase), which cleaves the phosphate group.

Glucose-6-phosphate + H2O → Glucose + Pi

The enzyme glucose 6-phosphate is present only in the liver and kidney but not in the muscles.
Therefore muscles cannot form glucose 6-phosphate and thus cannot provide blood glucose through gluconeogenesis and also through glycogenolysis.
Thus muscles have no role in maintaining blood glucose levels.

Significance of Formation of Glucose

The release of glucose into the bloodstream is essential for maintaining glucose homeostasis, as it provides the necessary substrate for energy production and metabolism by various tissues in the body.

Regulation of gluconeogenesis

i. Glucagon

Glucagon acts only on liver gluconeogenesis.
It activates fructose 1,6-bisphosphatase (favors gluconeogenesis) and inhibits phosphofructokinase (stops glycolysis).
It increases the level of cAMP (cyclic AMP) and cAMP-dependent protein kinase activity, causing the inactivation of pyruvate kinase.
The inactivation of pyruvate kinase decreases the conversion of PEP to pyruvate, which results in the diversion of PEP toward glucose synthesis.

ii. Glucocorticoids

Glucocorticoids especially cortisol, cause an increase in circulating glucose, fatty acids, and amino acids.
In peripheral tissues, they decrease the utilization of glucose.
They increase total protein synthesis, gluconeogenesis, glycogen deposition, and amino acid conversion to carbon-di-oxide and urea.
Many of the gluconeogenic effects in the liver are caused by glycerol (from triacylglycerol) and amino acids mobilized from peripheral tissues.
Glucocorticoids increase the key enzymes in the regulation of gluconeogenesis i.e. pyruvate carboxylase, PEP-carboxykinase, fructose 1,6-bisphosphate, and glucose 6-phosphatase.

iii. Availability of substrate

Gluconeogenic precursors markedly influence the rate of gluconeogenesis.
Lack of insulin causes mobilization of triacylglycerol from adipose tissues and amino acids from muscle proteins which provide substrate for gluconeogenesis.

Differences between Glycolysis and Glyconeogenesis

GLYCOLYSIS

GLUCONEOGENESIS

· It is the breakdown of glucose up to the pyruvate or lactate.

· It occurs in the liver and muscles.

· It occurs solely in the cytosol.

· There are two subtypes i.e. aerobic, and anaerobic.

· It provides energy in the form of ATPs.

· There are 3 rate-limiting steps (irreversible reactions).

· The 3 rate-limiting enzymes are hexokinase/glucokinase, phosphofructokinase, and pyruvate kinase.

· It is the synthesis of glucose from non-carbohydrate substances e.g. pyruvate, lactate, alpha-ketoglutarate, gluconeogenic amino acids, and glycerol.

· It occurs in the liver and kidneys.

· It occurs initially in mitochondria and later on in cytosol.

· There are no sub-types.

· It provides glucose, not ATPs.

· There are 4 rate-limiting steps (irreversible reactions).

· The 4 rate-limiting enzymes are pyruvate carboxylase, phosphoenol pyruvate carboxylase, fructose 1,2-bisphosphatase, and glucose 6-phosphate phosphatase.

Clinical Significance of Glyconeogenesis

The clinical significance of gluconeogenesis lies in its role in maintaining glucose homeostasis in the body. Glucose homeostasis refers to the balance between glucose uptake and glucose production in the body, and it is essential for ensuring that the body has a sufficient supply of glucose for energy production and metabolism.
In times of low carbohydrate availability, such as during fasting or prolonged exercise, the body relies on gluconeogenesis to generate glucose from non-carbohydrate precursors and maintain glucose levels in the blood. This is crucial for maintaining the proper function of tissues that depend on glucose for energy, such as the brain and red blood cells.
The activity of gluconeogenesis is regulated by hormones such as insulin and glucagon, which play a critical role in controlling the balance between glucose uptake and glucose production. Insulin stimulates the uptake of glucose by peripheral tissues and inhibits gluconeogenesis, while glucagon stimulates gluconeogenesis and glucose release from the liver. This delicate balance between insulin and glucagon is essential for maintaining glucose homeostasis in the body.
Abnormalities in gluconeogenesis can have a significant impact on health and are associated with several metabolic disorders, including diabetes, hypoglycemia, and liver disease. For example, in diabetes, the body is unable to produce enough insulin, which leads to elevated blood glucose levels and a state of hyperglycemia. On the other hand, in hypoglycemia, the body produces too much insulin, leading to low blood glucose levels and symptoms such as weakness, fatigue, and confusion.

Disorders related to gluconeogenesis

There are several metabolic disorders that are related to gluconeogenesis, including:

Diabetes Mellitus: In type 1 diabetes, the body is unable to produce enough insulin, leading to elevated blood glucose levels and a state of hyperglycemia. In type 2 diabetes, the body becomes resistant to insulin, leading to elevated blood glucose levels. In both cases, gluconeogenesis is increased as a compensatory mechanism to maintain glucose homeostasis.
Hypoglycemia: Hypoglycemia is a condition in which blood glucose levels are too low. This can occur as a result of an overproduction of insulin, excessive consumption of glucose, or a lack of glucose production. Gluconeogenesis is increased in response to low blood glucose levels to maintain glucose homeostasis.
Liver disease: Liver disease can impact gluconeogenesis, leading to hypoglycemia and other metabolic abnormalities. For example, cirrhosis, a condition in which the liver is damaged and scarred, can result in a decreased ability to produce glucose, leading to hypoglycemia.
Alcoholism: Chronic alcoholism can result in liver damage, leading to decreased glucose production and hypoglycemia. Alcohol also affects the regulation of hormones that control glucose metabolism, such as insulin and glucagon.
Certain medications: Certain medications, such as sulphonylureas and insulin, can impact gluconeogenesis and lead to hypoglycemia.

Conclusion

In conclusion, gluconeogenesis is a critical biological process in the human body that plays a significant role in maintaining blood sugar levels. It is a metabolic pathway that allows the body to convert non-carbohydrate sources into glucose, which is the primary source of energy for the body's cells. The process is regulated by hormones such as insulin and glucagon, which control the balance between glucose utilization and glucose production.

The key difference between glycolysis and gluconeogenesis is that glycolysis breaks down glucose into pyruvate, while gluconeogenesis converts non-carbohydrate sources into glucose. The clinical significance of gluconeogenesis lies in its role in the management of diabetes and other metabolic disorders.

However, the regulation of gluconeogenesis can also be disrupted in certain disorders, leading to conditions such as hypoglycemia and hyperglycemia. Therefore, a proper understanding of the process and its regulation is crucial for the effective management of these conditions.

Exams Success Notes: A Complete Study Guide on Gluconeogenesis and its Steps"