Glycolysis and TCA cycle
M.Sc. Microbiology 1st semester
Department of Microbiology
Kanya Gurukul Campus
Gurukul Kangri Vishwavidyalaya, Haridwar
• Glycolysis is a universal pathway in all the living cells.
• The complete pathway of glycolysis was elucidated in 1940. This pathway is often referred to as EMP pathway as
it was given by Embden-Meyerhof who made a major contribution in the knowledge of glycolysis.
• Glycolysis is a major pathway for ATP synthesis in tissues lacking mitochondria (erythrocytes, cornea, lens etc.).
• It is very essential for brain which dependent on glucose for the energy. Glucose in brain has to undergo in
glycolysis before it is oxidized to carbon dioxide and water.
• Glycolysis is a series of enzyme catalyzed reactions in which one molecule of glucose yield to two molecules of
pyruvate with the production of ATP. It takes place in cytosol of the cells.
• It occurs in the absence of oxygen (anaerobic) or in the presence of oxygen (aerobic).
• Lactate is the end product under anaerobic condition. In the aerobic condition pyruvate
is formed, which is then oxidized to carbon dioxide and water.
Two phases in glycolysis
The pathway can be divided into two phases:
A. Energy investment phase or preparatory phase:
• It consist of first five steps of glycolysis in which the glucose is enzymatically phosphorylated by ATP to yield
• This fructose 1,6-bisphosphate is then split into two 3C containing molecules, glyceraldehyde 3-phosphate
& dihydroxyacetone phosphate.
• Thus the first phase results in cleavage of the hexose chain. This cleavage requires an investment of 2ATP
molecules to activate the glucose molecule & prepare it for its cleavage into three carbon compound.
B. Energy generation phase or payoff phase:
• This phase constitutes the last five reactions of glycolysis.
• This phase marks the release of ATP molecule during the conversion of glyceraldehyde 3-phosphate to two
molecules of pyruvate.
• Here 4ADP are phosphorylated into 4ATP. Although 4ATP are formed, the net result id only 2ATP per
molecule of glucose oxidized.
Series of reactions in glycolysis
Step 1: In step 1 glucose is phosphorylated to glucose 6-phosphate by the enzyme hexokinase. This is
an irreversible reaction dependent on ATP & magnesium ion.
Step 2: Glucose 6-phosphate undergoes isomerization to give fructose 6-phosphate in the presence of
enzyme phosphofructohexose isomerase. It is a reversible reaction.
Step 3: Fructose 6-phosphate is phosphorylated to fructose 1,6-bisphosphate by
phosphofructokinase. This is an irreversible and regulatory step in glycolysis.
Step 4: fructose 1,6-bisphosphate (6C) is split into two (3C) compound, glyceraldehyde 3-
phosphate & dihydroxy acetone phosphate by the enzyme aldolase.
Step 5: The enzyme phosphotriose isomerase catalyzes the reversible interconversion of
glyceraldehyde 3-phosphate & dihydroxyacetone phosphate. Thus two molecules of glyceraldehyde
3-phosphate are obtained from one molecule of glucose.
Step 6: Glyceraldehyde 3-phosphate converted into 1,3-bisphosphoglycerate in the presence of
glyceraldehyde 3-phosphate dehydrogenase. In this step there is the formation of NADH.
Step 7: The enzyme phosphoglycerate kinase acts on 1,3-bisphosphoglcerate resulting in
synthesis of ATP and formation of 3-phosphoglycerate. This step is a good example of substrate
level phosphorylation, since ATP is synthesized from the substrate. This is a reversible reaction.
Step 8: 3-pjosphoglycerate is converted to 2-phosphoglycerate by the enzyme
phosphoglycerate mutase. This is an isomerization & reversible reaction.
Step 9: Phosphoenolpyruvate is generated from 2-phosphoglycerate by the enzyme
enolase. This reaction is reversible.
Step 10: Pyruvate is formed from phosphoenolpyruvate in which the enzyme
pyruvate kinase catalyzes the transfer of phosphate group from phosphoenolpyruvate
to ADP, leading to the formation of ATP. This is an irreversible reaction.
CITRIC ACID CYCLE
• The citric acid cycle is the final common oxidative pathway for carbohydrates, fats & amino
• The citric acid cycle was proposed by Hans Adolf Krebs ( Biochemist born in Germany) in 1937,
based on the studies of oxygen consumption in pigeon breast muscle. He got the Nobel prize in
1953 for physiology of medicine.
• Citric acid cycle basically involves the combination of a two carbon acetyl CoA with a four
carbon oxaloacetate produce a six carbon tricarboxylic acid.
• In the reactions that follow, the two carbons are oxidized to carbon dioxide and oxaloacetate is
regenerated & recycled. Oxaloacetate is considered to play a catalytic role in citric acid cycle.
• The cycle is divided into two phases I.e. oxidation of pyruvate and citric acid cycle.
Two phases in citric acid cycle
A. Pyruvic acid oxidation:
• Pyruvate which is the end product of glycolysis does not enter the citric acid cycle directly, the 3C
molecule (pyruvate) is first converted into 2C molecule (acetyl CoA).
• One carbon is released as carbon dioxide and acetic acid on entering the mitochondria units with
CoA to form acetyl CoA. This reaction is catalyzed by several enzymes collectively known as pyruvate
• This step is a connecting link between glycolysis and citric acid cycle.
B. Citric acid cycle or Krebs cycle:
• Citric acid cycle takes place in mitochondrial matrix. Pyruvate is oxidized to acetyl CoA which then
undergoes a series of reaction referred to as citric acid cycle.
• The citric acid cycle is also called the tricarboxylic acid (TCA) cycle since citric acid is a tricarboxylic
acid I.e. three carboxylic groups are present
Reactions of citric acid cycle
Step 1: Formation of citrate-
The first step in the TCA cycle is the condensation of the two carbon molecule acetyl CoA with
four carbon molecule oxaloacetate to form a six carbon molecule citrate.
The linkage between the acetyl group & CoA in acetyl CoA is hydrolysed by a molecule of water.
The catalytic enzyme is citrate synthase.
Step 2: Dehydration- Citrate undergoes dehydration under the catalytic action of enzyme
aconitase to form cis-aconitase.
Step 3: Hydration 1- Cis aconitase is rehydrated to iso-citrate under the inference of
Step 4: Dehydrogenation 1- Iso citrate undergoes dehydrogenation in the presence of the enzyme
iso citrate dehydrogenase to form oxalo succinate.
Step 5: Decarboxylation 1- Oxalo succinate undergoes decarboxylation to form alpha keto
glutarate, in this reaction one carbon dioxide is removed because of the loss of one carbon atom
during carboxylation. The alpha keto glutarate molecule have five carbon atoms. The reaction is
catalyzed by succinate decarboxylase.
Step 6: Dehydrogenation 1 & carboxylation 2- Alpha ketoglutarate undergoes simultaneous
decarboxylation & carboxylation & joins with CoA to form succinyl CoA, a four carbon atom
derivative of CoA. The reaction is catalyzed by alpha ketoglutarate dehydrogenase. The hydrogen
atom released are accepted by NAD to form NADH.
Step 7: Phosphorylation of ATP- Succinate is released from succinyl CoA under the
catalytic form. The action of Succinate thio kinase CoA is released in this reaction. The
reaction requires GDP ( guanosine diphosphate ).
Step 8: Dehydrogenation 3- Succinate undergoes under the catalytic action of succinate
dehydrogenase to form fumarate. The pair of hydrogen atoms form is transferred
directly to flavo protein. Flavo protein contains FAP (flavin adenine dinucleotide). FAD
accepts hydrogen atom form FADH.
Step 9: Hydration 2-Fumarate is hydrated to form malate. The reaction is catalyzed by the
Step 10: Dehydrogenation 4- In the final reaction malate is converted into oxaloacetate. The
catalytic enzyme is malate dehydrogenase. Dehydrogenation takes place during the process
in which NAD accepts the hydrogen ion to form NADH. It should be noted that two
molecules of pyruvate are formed per molecule of glucose during glycolysis, therefore two
molecules of oxaloacetate per molecule per molecule of glucose are formed at the end of
• Class Notes
• Google https://www.google.com
• Dr. U Satyanarayana https://logicwork.in/pdf-download-biochemistry-
• Lehninger https://logicwork.in/pdf-download-lehninger-principles-of-