Glycolysis: Definition, Process, Regulation

Glycolysis, which is a component of cellular respiration, is the initial phase of most carbohydrate catabolism, which is defined as the breakdown of bigger molecules into smaller ones. The term glycolysis comes from two Greek words that imply “to break down anything sweet.” Glycolysis produces two molecules of ATP while breaking down glucose and forming pyruvate. If no oxygen is available, the pyruvate end product of glycolysis can be utilised in anaerobic respiration or aerobic respiration via the TCA cycle, which generates substantially more useable energy for the cell. This article gives you a brief idea about glycolysis. Read further to find more.

Glycolysis Definition

Glycolysis is the series or sequence of reactions or pathways by which glucose is broken down anaerobically to form pyruvic acid. During glycolysis, one glucose molecule makes two molecules of pyruvate.

Glycolysis means (Greek: Glycos – sugar and lysis – breaking or dissolution) the splitting up of sugar. Glycolysis is a primary step of cellular respiration.

This process occurs in both aerobic and anaerobic respiration. In addition, glycolysis takes place in the cytoplasm of all living organisms.

This process, also known as EMP Pathway, was discovered by three German scientists, Embden, Meyerhof, and Parnas.

1. Equation: \({{\rm{C}}_{\rm{6}}}{{\rm{H}}_{{\rm{12}}}}{{\rm{O}}_{\rm{6}}}{\rm{\; +  2}}\,{\rm{ADP  +  2}}\left[ {\rm{P}} \right]{\rm{i  +  2}}\,{\rm{NAD}} \to {\rm{2}}\,{\rm{Pyruvate  +  2}}\,{\rm{ATP  +  2}}\,{\rm{NADH  +  2}}\,{{\rm{H}}^{\rm{ + }}}\)

Glycolysis Diagram

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Fig. Diagram of Glycolysis

Glycolysis Process

Glycolysis is an anaerobic oxidative process because it occurs in the absence of oxygen, and there is a loss of hydrogen. In this process, one molecule of glucose is broken down into two molecules of pyruvic acid. In this process, two molecules of ATP are used to produce four molecules of ATP. This process takes place in two phases:

1. Preparatory or Energy Investment Phase- In this phase, glucose is converted to glyceraldehyde-\(3\)-phosphate, and energy or ATP is consumed.
2. Pay-off or Energy Harvesting Phase- In this phase, triose phosphates are converted to pyruvate, and energy or ATP synthesized.

Fig. Two Phases of Glycolysis

Glycolysis Cycle

Glycolysis is a series of reactions in which one glucose molecule breaks down partially to produce two pyruvate molecules. Plants obtain glucose from sucrose or stored carbohydrates. In plants, sucrose breaks down into glucose and fructose with enzyme invertase, and then these two monosaccharides readily enter the glycolytic pathway. This glycolytic pathway consists of ten steps. A specific enzyme catalyzes each step. The ten steps of glycolysis are:

1. Phosphorylation of Glucose: In this step, phosphorylation of glucose by ATP occurs in the presence of \({{\mathop{\rm Mg}\nolimits} ^{2 + }}\) and an enzyme hexokinase to form Glucose-\(6\)-phosphate.
2. Synthesis of Fructose-6-Phosphate: This is a reversible reaction in which isomerization of phosphorylated glucose (Glucose-\(6\)-phosphate) takes place by enzyme phosphohexose isomerase to develop Fructose-\(6\)-phosphate.
3. Formation of Fructose-\(1,6\)-bisphosphate: This fructose-\(6\)-phosphate is converted into fructose-\(1,6\)-bisphosphate and ADP through phosphorylation with the help of enzyme phosphofructokinase and ATP.
4. Splitting: Fructose-\(1,6\)-bisphosphate splits into two \(3\)-carbon compounds, Dihydroxyacetone Phosphate (DHAP) and \(3\)-Phosphoglyceraldehyde (\(3\)-PGAld) by the action of the enzyme aldolase.
5. Isomerization: The \(3\)-carbon containing triosephosphate molecule Dihydroxyacetone Phosphate may be converted to \(3\)-Phosphoglyceraldehyde and vice versa with the help of the enzyme triosephosphate isomerase (reversible reaction).
6. Oxidation and Phosphorylation: \(3\)-Phosphoglyceraldehyde is converted to \(1,3\)-Bisphosphoglyceric acid and \({\rm{NADH + }}{{\rm{H}}^ + }\) is formed in this reaction. The enzyme phosphoglyceraldehyde dehydrogenase (reversible reaction) helps in this reaction.
7. Substrate Level Phosphorylation: \(1,3\)-Bisphosphoglyceric acid by the action of enzyme. phosphoglycerokinase converted to \(3\)-Phosphoglyceric Acid and ATP. This process is called substrate-level phosphorylation because this constitutes a direct synthesis of ATP from metabolites (reversible reaction).
8. Isomerization II: \(3\)-Phosphoglyceric acid is subsequently isomerized by enzymes phosphoglyceromutase to form 2-Phosphoglyceric acid (reversible reaction).
9. Dehydration: In the presence of the enzyme enolase and \({\rm{M}}{{\rm{g}}^{2 + }}, 2\)-Phosphoglyceric acid gives out one molecule of water to form \(2\)-Phosphoenol pyruvic acid (reversible action).
10. Formation of Pyruvate: In the last step \(2\)-Phosphoenol pyruvic acid is converted to Pyruvic acid by the removal of phosphorus, thus one molecule of ATP is synthesized from ADP by substrate-level phosphorylation with the action of enzyme pyruvic Kinase. Pyruvic acid is the end product of glycolysis.

Fig. Steps of Glycolysis

Glycolysis – At a Glance

Glycolysis – ATP Balance Sheet

1. ATP consumed in Glycolysis
   a. Glucose \(\left( {1\;{\rm{ mole}}} \right){\rm{ }} \to \) Glucose  phosphate \(\left( {1\;{\rm{mole}}} \right){\rm{ }} = {\rm{ }}1\,{\rm{ATP}}\)
   b. Fructose \(6\) phosphate \(\left( {1\;{\rm{ mole}}} \right){\rm{ }} \to \) Fructose \(1,6\)-diphosphate \(\left( {1\;{\rm{ mole}}} \right){\rm{ }} = {\rm{ }}1\;{\rm{ ATP}}\)
2. ATP formation by substrate phosphorylation
   a. \(1,3\)-diphosphoglyceric acid \(\left( {2\;{\rm{ moles}}} \right){\rm{ }} \to {\rm{ }}\;3\) phosphoglyceric acid \(\left( {2\;{\rm{ moles}}} \right){\rm{ }} = {\rm{ }}2\;{\rm{ ATP}}\)
   b. Phosphoenolpyruvic acid \(\left( {2\;{\rm{ moles}}} \right){\rm{ }} \to \) Pyruvic acid \(\left( {2\;{\rm{ moles}}} \right){\rm{ }} = {\rm{ }}2\;{\rm{ ATP}}\)

Glycolysis Regulation

Glycolysis is regulated by three regulatory enzymes hexokinase or glucokinase, phosphofructokinase, and pyruvate kinase, and glucose concentration in the blood and by a certain hormone level in the blood.

1. Hexokinase impeded by glucose-\(6\)-phosphate. This enzyme prevents the accumulation of glucose-6-phosphate due to product inhibition.
2. Phosphofructokinase (PFK) is the most important regulatory enzyme in glycolysis. It is an allosteric enzyme regulated by allosteric effectors ATP.
3. Pyruvate kinase (PK) is inhibited by ATP and activated by fructose-\(1,6\)-bisphosphate. Pyruvate kinase is active in a dephosphorylated state and inactive in a phosphorylated form. Inactivation of pyruvate kinase by cAMP-dependent protein kinase. The Hormone glucagon hepatic glycolysis by this mechanism.

Fig: Regulation of Glycolysis

Fate of Pyruvic Acid

1. Glucose, a six-carbon compound, is broken into two moles of three-carbon keto acid or pyruvic acid.
2. When oxygen is present and the cell proceeds to aerobic respiration, the pyruvic acid is converted into Acetyl Coenzyme A by oxidative decarboxylation.
   a. Each pyruvic acid molecule enters the mitochondria where oxidation is completed by oxidative decarboxylation and Krebs cycle.
3. When oxygen is not available, or the cell is performing anaerobic respiration only, this acid acts as the substrate for the fermentation process.
   a. Yeast and bacteria can convert this acid into ethanol and carbon dioxide.
   b. In animal cells, lactic acid is fermented from the pyruvic acid under anaerobic conditions.

Summary

Glucose is one of the key metabolites in human metabolism. Glycolysis is the first metabolic pathway of cellular respiration. In this process, a sequence of biochemical reactions converts glucose into pyruvate and generates energy (ATP). Glycolysis is the pathway that converts glucose into pyruvic acid and yields energy in the form of ATP and \({\rm{NADH}}.{{\rm{H}}^ + }.\) In this pathway oxygen is not needed and it is a common pathway for aerobic and anaerobic respiration. This process takes place in the cytoplasm of all living cells. The end product is pyruvic acid which is a three-carbon compound.

Frequently Asked Questions (FAQs) on Glycolysis

Q.1. What is Glycolysis?
Ans: Glycolysis is a series of reactions in which one glucose molecule partially breaks down to produce energy. It produces two molecules of pyruvate.

Q.2. Explain the process of Glycolysis?
Ans: Glycolysis is an anaerobic oxidative process because it occurs in the absence of oxygen, and there is a loss of hydrogen. In this process, one molecule of glucose is broken down into two molecules of pyruvic acid. In this process, two molecules of ATP are used to produce four molecules of ATP.

Q.3. What is the formula of Glycolysis?
Ans: The formula of Glycolysis:
\({{\rm{C}}_{\rm{6}}}{{\rm{H}}_{{\rm{12}}}}{{\rm{O}}_{\rm{6}}}\;{\rm{ + 2}}{\mkern 1mu} {\rm{ADP + 2}}\left[ {\rm{P}} \right]{\rm{i + 2}}{\mkern 1mu} {\rm{NAD}} \to \;{\rm{2}}{\mkern 1mu} {\rm{Pyruvate + 2}}{\mkern 1mu} {\rm{ATP + 2}}{\mkern 1mu} {\rm{NADH + 2}}{\mkern 1mu} {{\rm{H}}^{\rm{ + }}}\)

Q.4. What are the two phases of Glycolysis?
Ans: The two phases of glycolysis are:
a. Preparatory or Energy investment phase.
b. Pay-off or Energy harvesting phase.

Q.5. What are the three stages of Glycolysis?
Ans: The three stages of glycolysis are
a. In the first stage, glucose is trapped and destabilized.
b. The second stage involves the breakdown of glucose into three-carbon molecules.
c. In the third stage, ATP is generated.

Q.6. Does Glycolysis occur in humans?
Ans: Yes, Glycolysis occurs in humans during cellular respiration.

Q.7. What is the equation of Glycolysis?
Ans: The overall equation of Glycolysis:
\({{\rm{C}}_{\rm{6}}}{{\rm{H}}_{{\rm{12}}}}{{\rm{O}}_{\rm{6}}}\;{\rm{ + 2}}{\mkern 1mu} {\rm{ADP + 2}}\left[ {\rm{P}} \right]{\rm{i + 2}}{\mkern 1mu} {\rm{NAD}} \to {\rm{2}}{\mkern 1mu} {\rm{Pyruvate + 2}}{\mkern 1mu} {\rm{ATP + 2}}{\mkern 1mu} {\rm{NADH + 2}}{\mkern 1mu} {{\rm{H}}^{\rm{ + }}}\)

Q.8. How many ATPs are consumed in glycolysis for 1 molecule of glucose?
Ans: In the initial reactions, \(2\;{\rm{ ATP}}\) molecules are consumed for each molecule of glucose in glycolysis.

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