As we all know that our body requires glucose as a source of energy. A supply of glucose is necessary especially for the nervous system and red blood cells. If our blood glucose level goes below or higher than the required level, it can create an imbalance. Therefore, it is extremely important for our system to maintain glucose balance. Our body knows various processes and pathways to keep glucose levels in check. Gluconeogenesis is one such pathway. The Gluconeogenesis pathway occurs in all animals, plants, fungi, and microorganisms. The reactions are essentially the same in all tissues and all species.
The gluconeogenic pathway converts pyruvate into glucose. Noncarbohydrate precursors of glucose will first convert into pyruvate or enter the pathway at later intermediates such as oxaloacetate and dihydroxyacetone phosphate.
Importance of Gluconeogensis
The Gluconeogenesis pathway takes place when our body glucose level falls below the required level. It helps in maintaining blood glucose levels during fasting and starvation. Hence, it is a very important process and its failure is usually fatal. Hypoglycemia (a condition when the blood glucose levels drop below the specified limits) is very fetal and it can cause coma and even death.
Glucose is also important in maintaining the level of intermediates of the krebs cycle even when fatty acids are the main source of acetyl-CoA in the tissues. Gluconeogenesis also removes lactate from muscle and erythrocytes, as well as glycerol from adipose tissue.
Glycolysis and Gluconeogenesis
Glycolysis: It is the major pathway for glucose metabolism in which glucose will convert to pyruvate (under aerobic condition) or lactate (anaerobic). Afterwards, Pyruvate can be completely oxidized to CO2 and H2O by enzymes present in the mitochondria. Basically, glycolysis occurs in the cytosol of all the cells and is also known as “Embden-Meyerhof-Parnas pathway”.
For more information, refer: Glycolysis Pathway
Gluconeogenesis: It is the process in which new glucose molecules will form from non-carbohydrate precursors. The major substrates are glucogenic amino acids, lactate, glycerol, and propionate. The liver and kidney are the major gluconeogenic tissues, but the small intestine may also be a source of glucose in the fasting state.
In glycolysis, glucose will convert into pyruvate whereas, in gluconeogenesis, pyruvate is converted into glucose. However, gluconeogenesis is not a reversal of glycolysis. The irreversible steps in glycolysis are bypassed by four enzymes which are the key enzymes of gluconeogenesis.
I have written a separate article on the similarities and differences between glycolysis and gluconeogenesis. If you want to read it, please see this article: Glycolysis VS gluconeogenesis
Where does gluconeogenesis occur?
Gluconeogenesis takes place primarily in the liver, and to a lesser extent in the renal cortex. A small amount of gluconeogenesis takes place in the brain, skeletal muscle, or heart muscle. The liver preferentially uses lactate, glycerol, and glucogenic amino acids while the kidney preferentially uses lactate, glutamine and glycerol. The pathway is partly mitochondrial and partly cytoplasmic.
Gluconeogenesis Pathway
Gluconeogenesis requires both a source of energy for biosynthesis as well as a source of carbon for the formation of the backbone of the glucose molecule. The carbon skeletons are provided from lactate (derived from tissues), amino acids (from muscle) and glycerol (from triglycerides). Krebs cycle intermediates can also function as substrates for gluconeogenesis. Among these, muscle protein is the major precursor of blood glucose during fasting and starvation.
Irreversible steps of glycolysis
Before we proceed to gluconeogenesis, I want to give you a quick brief of glycolysis. Overall, glycolysis has ten steps. Out of which, three are irreversible. For the gluconeogenesis pathway, we have to bypass these irreversible steps. In order to reverse these steps, it requires a high amount of energy. The irreversible steps in glycolysis are as follows:
- Phosphorylation of glucose to glucose-6-phosphate (G6P): In this step glucose will convert to G6P by the enzyme hexokinase( in all tissues) and glucokinase (in liver and pancreas). This reaction is step 1 of glycolysis.
- Phosphorylation of F6P to fructose-1,6-bisphosphate (F-1,6-bisP): fructose-6-phosphate phosphorylate to fructose-1,6-bisphosphate by the enzyme phosphofructokinase. (step 3)
- Dephosphorylation of PEP (phosphoenol pyruvate) to pyruvate: this reaction completes in two steps. Firstly, PEP converts into enol pyruvate intermediate. After that, it will spontaneously isomerize into keto pyruvate, the stable form of pyruvate. (step 9)
The above three steps (1,3,9) are irreversible steps of the glycolytic pathway.
There are slight variations in the gluconeogenesis pathway depending upon the substrate. The pathway uses lactate, amino acids, glycerol and propionate as its major substrate. We will discuss these substrate-specific pathways later.
Gluconeogenesis enzymes
Gluconeogenesis utilizes several enzymes of glycolysis. Apart from glycolytic enzymes, it uses four other enzymes and these are:
- Pyruvate carboxylase
- Phosphoenol pyruvate carboxy kinase
- Fructose-1-6-bisphosphatase
- Glucose-6-phosphatase
Gluconeogenesis steps (irreversible steps)
STEP 1: Carboxylation of pyruvate to oxaloacetate
Firstly, pyruvate in the cytoplasm enters the mitochondria. After that, mitochondrial enzymes pyruvate carboxylase catalyzes the carboxylation reaction of pyruvate to oxaloacetate. This reaction requires biotin and ATP molecules to proceed.
The oxaloacetate, now, has to be transported from mitochondria to cytosol, because further reactions of gluconeogenesis will take place in the cytosol. For this, oxaloacetate will first convert to malate by malate dehydrogenase, which traverses the membrane and reaches the cytoplasm. Malate is then re-converted to oxaloacetate.
STEP 2: Conversion of oxaloacetate to phosphoenol pyruvate (PEP)
Oxaloacetate will convert to phosphoenol pyruvate by the enzyme phosphoenol pyruvate carboxykinase (PEPCK). This reaction removes a CO2 molecule. GTP or ITP donates the phosphate Group to the reaction.
The phosphoenol pyruvate undergoes further reactions catalyzed by the glycolytic enzymes to form fructose-1,6-bisphosphate.
For further information, see: Glycolysis steps
STEP 3: Conversion of Fructose 1,6-bis-phosphate to fructose-6-phosphate
Fructose 1,6-bisphosphate is then converted to fructose 6-phosphate by the enzyme fructose 1,6-bisphosphatase. After that, fructose-6-phosphate is isomerized to glucose-6-phosphate by the freely reversible reaction catalyzed by hexose phosphate isomerase.
This enzyme is present in the liver, kidney, and skeletal muscle, but is probably absent from the heart and smooth muscle.
STEP 4: Hydrolysation of glucose-6-phospaphate to glucose
Glucose-6-phosphate hydrolyses to free glucose by the enzyme glucose-6-phosphatase. This enzyme is active in the liver. It is also present in kidney and intestinal mucosa to a lesser extent but absent in muscle and adipose tissue.
This final step does not complete in the cytoplasm. Rather, glucose 6-phosphate moved into the lumen of the endoplasmic reticulum for the reaction to occur.
Regulation of Gluconeogenesis
Gluconeogenesis and glycolysis are reciprocally regulated so that one pathway is relatively inactive when the other is active. When there is a need for energy, glycolysis will predominate while there is an excess of energy, gluconeogenesis will take charge. There are the following steps in the regulation of gluconeogenesis:
Activators and Inhibitors
Pyruvate carboxylase (catalyze 1st irreversible reaction of gluconeogenesis) is an allosteric enzyme.
It requires acetyl-CoA as an allosteric activator. A higher level of acetyl CoA will favour the activity of pyruvate carboxylase which in turn, favours the production of oxaloacetate.
Similarly, citrate is an activator while fructose-2,6-bisphosphate and AMP are inhibitors of Fructose-1,6-bisphosphatase.
Both acetyl CoA and citrate activate the gluconeogenesis enzymes while inhibiting the glycolysis enzyme pyruvate kinase.
Hormonal Regulation of Gluconeogensis
Glucagon and epinephrine (responsible for decreasing blood glucose levels) inhibit glycolysis and
stimulate gluconeogenesis in the liver by increasing the concentration of cAMP. This, in turn, activates cAMP-dependent protein kinase, leading to the phosphorylation and inactivation of pyruvate kinase.
Insulin enhances the synthesis of the key enzymes in glycolysis. It also antagonizes the effect of the glucocorticoids and glucagon-stimulated cAMP, which induce synthesis of the key enzymes of gluconeogenesis (PEPCK, Fructose-1,6-bisphosphatase and glucose-6-phosphatase).
ATP
Gluconeogenesis is also increased by ATP.
Substrate of Gluconeogenesis
The major substrates for the gluconeogenic pathway are glucogenic amino acids, lactate, glycerol, and propionate. Gluconeogenesis can be obtained from the following above substrate.
1. Lactate
Gluconeogenesis from lactate is conceptually the opposite of anaerobic glycolysis but proceeds by a slightly different pathway, involving both mitochondrial and cytosolic enzymes. In the liver cell, lactate dehydrogenase converts lactate to pyruvate. The pyruvate enters the pathway to form glucose.
2.Glycerol
The glycerol enters the gluconeogenesis pathway at the level of triose phosphates. The glycerol phosphorylates in the liver cytosol by the enzyme glycerol kinase. After that, it will oxidize to dihydroxy acetone phosphate by an NAD+ dependent dehydrogenase.
3. Amino Acids
Most amino acids can act as a substrate for the gluconeogenic pathway. After deamination, their carbon skeletons can be converted into glucose. When the level of glucose falls below the required level, the glucogenic amino acids are transaminated to corresponding carbon skeletons. After that, these amino acids can enter the TCA cycle and form oxaloacetate or pyruvate.
All amino acids except leucine and lysine can supply carbon for the net synthesis of glucose by gluconeogenesis. Alanine and glutamine are the major amino acids that are present in the muscles and act as a substrate of gluconeogenesis.
4. Propionyl CoA (Propionate)
Propionate is a good precursor for gluconeogenesis, generating oxaloacetate by the anaplerotic pathway. Propionyl CoA can form from odd chain fatty acids and the carbon skeleton of some amino acids. It will convert to succinyl CoA.
Please note that it is impossible to synthesize glucose from even chain fatty acids. It is because acetyl CoA and other intermediates of even chain fatty acid oxidation cannot convert to oxaloacetate or any other intermediate of gluconeogenesis.
Sources and External links
Textbook of biochemistry for medical students 7th edition by DM Vasudevan; Chapter 9: Major Metabolic Pathways of Glucose
Additionally, BRS Biochemistry 6th edition, molecular biology, and genetics by Michael A. Lieberman, PhD and Rick Ricer; chapter no. 6: carbohydrate metabolism.
Also, Lippincotts illustrated review biochemistry 6th edition; chapter 8: introduction to metabolism and glycolysis page no. 187 to 199 and chapter 10: gluconeogenesis.
Harper’s Illustrated Biochemistry 28th edition; chapter 18: glycolysis and the oxidation of pyruvate, page no. 317 to 327. Also, chapter 10: gluconeogenesis and the control of blood sugar
Textbook of biochemistry with Clinical Correlations 4th edition by Thomas L Delvin page no. 274 to 278
Lehninger: Principles of Biochemistry, 7th edition, chapter 14 Glycolysis, Gluconeogenesis,
and the Pentose Phosphate Pathway
https://en.wikipedia.org/wiki/Gluconeogenesis