Glycolysis, steps, scheme, regulation - The virtual notebook (2023)

WHAT IS GLYCOLYSIS?

Glycolysis is the main pathway for glucose metabolism, in which glucose will be converted to pyruvate (under aerobic conditions) or lactate (anaerobic). Subsequently, pyruvate can be fully oxidized to CO²y H₂Or by enzymes present in the mitochondria. Glycolysis takes place in the cytoplasm of the cell and is completed in a series of ten reactions. Furthermore, it is a central metabolic pathway with many of its intermediates providing branch points for other pathways. Thus, the intermediates of glycolysis are useful for the synthesis of amino acids and fats.

The word glycolysis, derived from the Greek words, 'glycine'which means sweet; It is'lise' which means to divide. So glycolysis technically means "splitting of sugar" or "breaking down of sugar". Basically, it occurs in the cytosol of all cells and is also known as “Via Embden-Meyerhof-Parnas“.

Is glycolysis aerobic or anaerobic?

Glycolysis can occur as both aerobic and anaerobic, depending on the availability of oxygen and the electron transport chain. Indeed, glycolysis is an oxygen-independent metabolic pathway. Although, it can also occur under poor oxygen concentration. Pyruvate is the end product of aerobic glycolysis whereas lactate will be produced in anaerobic glycolysis. Anaerobic glycolysis occurs in erythrocytes, cornea, lens, and retinal regions.

MEANING OF GLYCOLYSIS

1. It occurs in every cell in the body. Therefore, it is the only pathway that occurs in all cells.
2. Glycolysis is the main pathway for the metabolism of glucose and the main pathway for the metabolism of fructose, galactose and other carbohydrates derived from the diet.
3. In addition, anaerobic glycolysis constitutes the main source of energy for the muscles when there is a lack or insufficient amount of oxygen.
4. Most reactions in the glycolytic pathway are reversible, which are also used togluconeogenesis.
5. Glycolysis is the only source of energy in cells that lack mitochondria. For example, erythrocytes, cornea, lens and retinal regions.
6. Furthermore, it is a pathway through which the greatest flux of carbon occurs in most cells. It provides carbon skeletons for the synthesis of non-essential amino acids, as well as part of the glycerol in fat.

Glycolysis VS Gluconeogenesis

glycolysis andgluconeogenesisare two pathways of glucose metabolism. One is glucose breakdown, while the other is glucose synthesis. Gluconeogenesis closely resembles the reverse pathway of glycolysis, although it is not a complete reversal of glycolysis.

I have written a separate article on glycolysis versus gluconeogenesis in which I discuss the similarities and differences between glycolysis and gluconeogenesis. If you want to check it out, see:Glycolysis vs gluconeogenesis

GLYCOLYSIS CYCLE

Glucose transporter-4 (GluT4) transports glucose from extracellular fluid into muscle cells and adipocytes. This translocase is under the influence of insulin. In diabetes mellitus, insulin deficiency makes it difficult for glucose to enter peripheral cells. Insulin does not significantly stimulate glucose transport to tissues such as the liver, brain and red blood cells. In these cases, GluT2 is the transporter.

Mainly, glycolysis is completed in a series of ten steps in total. The conversion of glucose to pyruvate generally takes place in two stages:

1. Energy investment phase
   1. setup phrase
   2. split phase
2. power generation phase
   1. Oxidation-reduction phase

STAGES OF GLYCOLYSIS

1. Energy investment phase

These are the first four reactions of glycolysis in which (phosphorylated) intermediates are synthesized at the expense of ATP. In general, this phase requires two molecules of ATP.

1.1 Preparatory phase

Basically, it involves the first 3 reactions of the glycolytic cycle and requires two molecules of ATP.

STEP 1: Phosphorylation of glucose to glucose-6-phosphate (G6P)

The enzyme involved: hexokinase (in all tissues) and glucokinase (in liver and pancreas).

Reaction: Glucose is phosphorylated to glucose-6-phosphate in the presence of hexokinase which splits ATP into ADP. Eventually, the ATP molecule acts as a phosphate donor in this reaction.

PAGpoints to remember:Hexokinase is allosterically inhibited by its product, glucose 6-phosphate. So this reaction isirreversible.

STEP 2: Isomerization of G6P to fructose-6-phosphate (F6P)

The enzyme involved: phosphohexose isomerase or you can say phosphoglucose isomerase.

Reaction: glucose-6-phosphate will be isomerized into fructose-6-phosphate by phosphohexose isomerase.

PAGpoints to remember:this reaction is easily reversible.

STEP 3: Phosphorylation of F6P to fructose-1,6-bisphosphate (F-1,6-bisP)

The enzyme involved: Phosphofrutokinase (PFK).

Reaction: fructose-6-phosphate will be further phosphorylated into fructose-1,6-bisphosphate. The enzyme phosphofructokinase catalyzes the second phosphorylation step of glycolysis using a second ATP molecule.

PAGpoints to remember:PFK is an inducible, allosteric and regulatory enzyme. obviously it isthe most importantcheckpoint and compromised stage of glycolysis. Again, the reaction isirreversible.

1.2 Division phase

STAGE 4: Cleavage of fructose-1,6-bisphosphate (F-1,6-bisP) to triose phosphate

The enzyme involved: aldolasa.

Reaction: fructose-1,6-bisphosphate (6C) is cleaved into glyceraldehyde-3-phosphate (3C) and dihydroxyacetone phosphate (DHAP) (3C).

Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are interconverted by the enzyme phosphotriose isomerase.

PAGpoints to remember:the reaction is reversible. This reaction is carried out in two steps. First, F-1,6-bisP is cleaved into two phosphate trioses. Thereafter, DHAP is isomerized to glyceraldehyde-3-phosphate. Thus, up to this point, we have obtained two molecules of glyceraldehyde-3-phosphate from one molecule of glucose at the expense of 2 molecules of ATP.

2.power generation phase

The other reactions will enter this phase in which a network of two ATP molecules will be formed by phosphorylation at the substrate level per metabolized glucose molecule.

2.1 Oxidation-reduction phase

STEP 5: Oxidation and phosphorylation of glyceraldehyde-3-phosphate to (1,3-BPG)

The enzyme involved: glyceraldehyde-3-phosphate dehydrogenase.

Reaction: Glyceraldehyde-3-phosphate is simultaneously oxidized and phosphorylated to 1,3-bisphosphoglycerate (1,3-BPG) with the help of NAD+.

PAGpoints to remember:the reaction is reversible and the product contains a high-energy bond. During this reaction, an aldehyde (glyceraldehyde 3-phosphate) will be oxidized to a carboxylic acid with the reduction of NAD+ to NADH.

STEP 6: 1,3-BPG is converted to 3-phosphoglycerate (3PG)

The enzyme involved: phosphoglycerate kinase.

Reaction:1,3-Bisphosphoglycerate reacts with ADP to produce 3-phosphoglycerate and ATP.

PAGpoints to remember:is an example ofsubstrate-level phosphorylation.Meanwhile, two ATP molecules will be formed at this stage for every glucose molecule that undergoes glycolysis.

STEP 7: Isomerization of 3PG to 2-phosphoglycerate (2PG)

The enzyme involved: phosphogliceromutasa.

(Video) BIO 211 Lab 1.1

Reaction: el 3-phosphoglycerate is isomerized to 2-phosphoglycerate.

PAGpoints to remember:the reaction is reversible.

STEP 8: Dehydration of 2PG to Phosphoenol Pyruvate (PEP)

The enzyme involved: enolasa

Reaction: the dehydration of 2-phosphoglycerate results in the formation of phosphoenolpyruvate with theremoval of water molecules. As a result, a high energy phosphate bond will be formed.

PAGpoints to remember:enolase requer Mg²⁺the manganese²⁺and if we remove magnesium ions, fluorine will irreversibly inhibit this enzyme. Therefore, fluoride will stop all glycolysis. Therefore, fluoride is added to the blood while the blood is drawn for sugar estimation. Otherwise, glucose is metabolized by blood cells, resulting in lower blood sugar values.

STEP 9: Dephosphorylation of PEP to pyruvate

The enzyme involved: pyruvate kinase.

Reaction: This reaction is completed in two steps. First, PEP is converted to an enol pyruvate intermediate. Thereafter, it will spontaneously isomerize to ketopyruvate, the stable form of pyruvate.

PAGpoints to remember: Pyruvate kinase is a key glycolytic enzyme and is thethird irreversible reaction. Also, this is another example of phosphorylation at the substrate level.

STEP 10: Reduction of pyruvate to lactate

The enzyme involved: lactate dehydrogenase.

Reaction: under anaerobic conditions, pyruvate is reduced to lactate.

PAGpoints to remember: Under aerobic conditions, pyruvate enters the Krebs cycle for complete oxidation.

For more information:verKrebs Cycle/Citric Acid Cycle/TCA Cycle

FINAL PRODUCT OF GLYCOLYSIS

In one cycle, glycolysis produces two pyruvate molecules, two ATP, two NADH, and two water molecules.

liquid reaction of glycolysis

glucose + 2NAD⁺+ 2 Pi + 2 ADP →2piruvato + 2 NADH + 4 H⁺+ 2 ATP + 2 H2O.

Product of glycolysis under aanaerobicillness

In general, when 1 mole of glucose will be converted into 2 moles of pyruvate. 2 moles of ATP will be used in the process and 4 moles of ATP will be produced in the subsequent reaction, for a net yield of 2 moles of ATP. There is no net production or consumption of NADH.

Product of glycolysis under aerobic conditions.conditions

The direct consumption and formation of ATP are the same as in anaerobic glycolysis. Two NADH molecules are also produced per glucose molecule. Continuous aerobic glycolysis requires most of this NADH to be oxidized by the electron transport chain, producing approximately three ATPs for every molecule of NADH that enters the chain.

GLYCOLYSIS DIAGRAMS

REGULATION OF GLYCOLYSIS

The regulatory enzymes or key enzymes of glycolysis are:

1. Hexokinase (glucokinase, in the liver),
2. phosphofructokinase and
3. pyruvate kinase.

Although most reactions in glycolysis are reversible, three are markedly exothermic and therefore must be considered physiologically irreversible. Indeed, these reactions are the main sites of regulation of glycolysis.

Hexoquinasa

Phosphorylates glucose to G6P for ATP production, even when blood glucose levels are low. Comparatively, hexokinase has a low Km for glucose (about 0.1 mM). So you are working close to your maximum rate (Vmax) even with fasting blood glucose levels.

The enzyme is inhibited by its product (glucose-6-phosphate). Therefore, it is most active when glucose-6-phosphate is used up quickly.

Phosphofructokinase

PFK is regulated by several factors. It is the most prominent regulatory enzyme in glycolysis. In general, PFK-1 is allosterically inhibited by high levels of ATP, which acts as a "high energy" signal, indicating an abundance of high-energy compounds.

Furthermore, high levels of citrate also inhibit PFK-1. In contrast, PFK-1 is activated allosterically by high concentrations of AMP, indicating that the cell's energy reserves are depleted.

pyruvate kinase

Pyruvate kinase catalyzes an irreversible step and is a regulator
glycolysis enzyme. It is activated by fructose 1,6-bisphosphate and inhibited by alanine and phosphorylation in the liver during fasting when glucagon levels are high. Furthermore, insulin increases its activity while glucagon inhibits it.

EXTERNAL SOURCES AND LINKS

To use:page numbers may vary by edition.

Biochemistry Textbook for Medical Students, Seventh Edition, by DM Vasudevan; Chapter 9: Major Glucose Metabolic Pathways, page no. 108 to 120

In addition, BRS Biochemistry 6th Edition, Molecular Biology and Genetics by Michael A. Lieberman, PhD and Rick Ricer; chapter #6: carbohydrate metabolism.

Also, Lippincott's Illustrated Review of Biochemistry, Sixth Edition; chapter 8: introduction to metabolism and glycolysis page no. 187 to 199.

Harper's Illustrated Biochemistry, 28th edition; chapter 18: glycolysis and pyruvate oxidation, page no. 317 and

Finally, Textbook of Biochemistry with Clinical Correlations 4th Edition by Thomas L Delvin Page No. 274 and