Carbohydrate Metabolism
M.F.Ullah, Ph.D
PLACEMENT/YEAR/LEVEL: 2nd Year/Level 4, 2nd Semester
Cellular Respiration
The living cells obtain energy by oxidizing the nutrients such as glucose
through a series of stepwise metabolic reactions. This process is called
as cellular respiration.
The cellular respiration can be anaerobic (occuring in the absence of oxygen)
Or aerobic (occuring in the presence of oxygen).
Anaerobic respiration involves two major processes:
1. Glycolysis
2. Lactic acid fermentation
Aerobic respiration involves three major processes:
2. TCA cycle
3. Oxidative phosphorylation
Aerobic respiration yields more energy than anaerobic respiration
What is Glycolysis ?
In glycolysis (from the Greek glykys, meaning “sweet,” and lysis, meaning “splitting”), a
molecule of glucose is oxidatively degraded in a series of enzyme-catalyzed reactions to
yield two molecules of the three-carbon compound pyruvate. During the sequential
reactions of glycolysis, some of the free energy released from glucose is conserved in the
form of ATP and NADH.
Glycolytic pathway operates in the cytosol
Glycolysis was the first metabolic pathway to be elucidated. The complete glycolytic
pathway was elucidated by 1940, largely through the pioneering contributions of Gustav
Embden, Otto Meyerhof, Carl Neuberg, Jacob Parnas, Otto Warburg, Gerty Cori, and Carl
Glycolysis is also known as the Embden-Meyerhof pathway.
The overall equation for glycolysis is:
Glucose + 2NAD+ + 2ADP + 2Pi
2 pyruvate + 2NADH + 2H+ + 2ATP + 2H2O
Significance / Purpose
1.Glycolysis is an almost universal central pathway of glucose catabolism. The
glycolytic breakdown of glucose is the sole source of metabolic energy in some
mammalian tissues and cell types (erythrocytes, renal medulla, brain, and sperm,
for example) and partially contributes to the energy requirements of most cell types
along with other metabolic pathways.
2. In addition to serving as an anaerobic and aerobic source of ATP, glycolysis is
linked to anabolic pathways as it provides biosynthetic precursors. For example, in
liver and adipose tissue, this pathway generates pyruvate as a precursor for fatty
acid biosynthesis.
The Reactions of Glycolysis
The glycolytic pathway, which cleaves 1 mole of glucose (6C) to 2 moles of the 3-carbon
compound pyruvate, consists of a preparative phase and an ATP-generating phase.
Preparative Phase (requires energy)
In the initial preparative phase of glycolysis, glucose is phosphorylated twice by
ATP and cleaved into two triose phosphates. The preparative phase thus proceeds with
the investment of 2 ATP.
Energy payoff Phase (releases energy)
In the ATP-generating phase, glyceraldehyde 3-phosphate (a triose phosphate) is
oxidized by NAD+ and phosphorylated using inorganic phosphate. The high energy
phosphate bond generated in this step is transferred to ADP to form ATP. The
remaining phosphate is also rearranged to form another high-energy phosphate
bond that is transferred to ADP. Since there are 2 moles of triose phosphate formed,
the yield from the ATP-generating phase is 4 ATP and 2 NADH. The result is a net yield
of 2 moles of ATP, 2 moles of NADH, and 2 moles of pyruvate per mole of glucose.
Glucose metabolism begins with transfer of a phosphate from ATP to glucose to form glucose6-P.
This step is notable for two reasons: (1) glucose 6-phosphate cannot diffuse through the
membrane, because of its negative charges, and (2) the addition of the phosphoryl group
begins to destabilize glucose, thus facilitating its further metabolism.
The transfer of the phosphoryl group from ATP to the hydroxyl group on carbon 6 of glucose is
catalyzed by hexokinase (In liver this reaction is catalyzed by glucokinase). Kinases are
enzymes that catalyze the transfer of a phosphoryl group from ATP to an acceptor.
The second step in glycolysis is the isomerization of glucose 6-phosphate to fructose
Recall that the open chain form of glucose has an aldehyde group at carbon 1,
whereas the open-chain form of fructose has a keto group at carbon 2. Thus, the
isomerization of glucose 6-phosphate to fructose 6-phosphate is a conversion of an
aldose into a ketose. The reaction is catalyzed by phosphoglucose isomerase.
3.The Formation of Fructose 1,6-bisphosphate from Fructose 6-phosphate
A second phosphorylation reaction follows the isomerization step. Fructose 6-phosphate
is phosphorylated by ATP to fructose 1,6-bisphosphate (F-1,6-BP). This reaction is
catalyzed by phosphofructokinase (PFK).
The prefix bis- in bisphosphate means that two separate monophosphate groups are
present, whereas the prefix di- in diphosphate (as in adenosine diphosphate) means that
two phosphate groups are present and are connected by an anhydride bond.
4. Catalytic Cleavage Six-Carbon Sugar (Fructose 1,6-bisphosphate ) into Two
Three-Carbon Fragments (GAP & DHAP)
This reaction takes place with the splitting of fructose 1,6-bisphosphate into
glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP) by the enzyme
aldolase. The products of the remaining steps in glycolysis consist of three carbon units
rather than six-carbon units
5. Triose phosphate isomerase Salvages a Three-Carbon Fragment
Glyceraldehyde 3-phosphate is on the direct pathway of glycolysis, whereas dihydroxyacetone
phosphate is not. Unless a means exists to convert dihydroxyacetone phosphate into
glyceraldehyde 3-phosphate, a three-carbon fragment useful for generating ATP will be lost. GAP
and DHAP are isomers that can be readily interconverted: dihydroxyacetone phosphate is a
ketose, whereas glyceraldehyde 3-phosphate is an aldose. The isomerization of these threecarbon phosphorylated sugars is catalyzed by triose phosphate isomerase
6. Conversion of glyceraldehyde 3-phosphate into 1,3-bisphosphoglycerate (1,3BPG)
The preceding steps in glycolysis have transformed one molecule of glucose into two
molecules of glyceraldehyde 3- phosphate, but no energy has yet been extracted.
On the contrary, thus far two molecules of ATP have been invested. We come now to a
series of steps that harvest some of the energy contained in glyceraldehyde 3-phosphate.
The initial reaction in this sequence is the conversion of glyceraldehyde 3-phosphate into
1,3-bisphosphoglycerate (1,3-BPG), a reaction catalyzed by glyceraldehyde 3-phosphate
7.The Formation of ATP from 1,3-Bisphosphoglycerate (substrate level
The final stage in glycolysis is the generation of ATP from the phosphorylated threecarbon metabolites of glucose. Phosphoglycerate kinase catalyzes the transfer of the
phosphoryl group from the acyl phosphate of 1,3- bisphosphoglycerate to ADP. ATP and 3phosphoglycerate are the products.
The formation of ATP in this manner is referred to as substrate-level phosphorylation
because the phosphate donor, 1,3- BPG, is a substrate with high phosphoryl-transfer
Keep in mind that, because of the actions of aldolase and triose phosphate isomerase,
two molecules of glyceraldehyde 3- phosphate were formed and hence two molecules
of ATP were generated. These ATP molecules make up for the two molecules of ATP
consumed in the first stage of glycolysis.
8. Rearrangement of 3-phosphoglycerate to 2-phosphoglycerate
The position of the phosphoryl group shifts in the conversion of 3- phosphoglycerate into 2phosphoglycerate, a reaction catalyzed by phosphoglycerate mutase. In general, a mutase is
an enzyme that catalyzes the intramolecular shift of a chemical group, such as a phosphoryl
9. Dehydration of 2-phosphogycerate to Phosphoenolpyruvate
Enolase catalyzes the formation of phosphoenolpyruvate (PEP) by the dehydration of
2-phosphoglycerate. This dehydration markedly elevates the transfer potential of the
phosphoryl group.
10. Conversion of Phosphoenolpyruvate to Pyruvate
The last reaction in the glycolytic pathway is the transfer of a phosphoryl group from
phosphoenolpyruvate to ADP catalyzed by pyruvate kinase forming the pyruvate .
This is the second substrate level phosphorylation of the pathway yielding ATP. Because
the molecules of ATP used in forming fructose 1,6-bisphosphate have already been
regenerated, the two molecules of ATP generated from phosphoenolpyruvate are
Substrate level Phosphorylation & the ATP yield in the transformation of
glucose into pyruvate :
Substrate level phosphorylation is the mechanism of ATP formation by the transfer of
phosphoryl group from the metabolic substrates carrying “high transfer potential
phosphoryl group” to ADP. The formation of ATP in the gycolytic pathway is the
example of Substrate level phosphorylation.
1. The Formation of ATP from 1,3-Bisphosphoglycerate
2. The Formation of ATP from Phosphoenol Pyruvate
Total = 4 ATP
Total of 4 ATP is formed when glucose is oxidized to pyruvate in the glycolytic pathway.
However the net yield of ATP is 2 after compensating for the 2 ATP utilized in the preparative
phase for the conversion of glucose to Fructose 1,6-bisphosphate.
4 ATP formed – 2 ATP utilized = 2 ATP profit
Oxidative Fate of NADH Produced from Glycolysis
The NADH produced from glycolysis must be continuously reoxidized back to NAD+
to provide an electron acceptor for the glyceraldehyde-3-P dehydrogenase reaction.
If NADH is not reoxidized to NAD+ glycolysis will stop due to the shortage of NAD+
Without oxidation of this NADH, glycolysis cannot continue. There are two alternate
routes for oxidation of cytosolic NADH to NAD+ depending upon aerobic or
anaerobic respiration pathways.
One route is aerobic (in the presence of oxygen) , in which oxidation of NADH to
NAD+ takes palce across the mitochondrial membrane by the electron transport
chain (ETC) and oxygen.
The other route is anaerobic (without the use of oxygen). In anaerobic respiration,
NADH is reoxidized in the cytosol by lactate dehydrogenase, which reduces pyruvate
to lactate (Lactic acid fermentation)
Oxidative Fate of Pyruvate Produced from Glycolysis
The fate of pyruvate depends on the route used for NADH oxidation.
1. If NADH is reoxidized aerobically by ETC, pyruvate can be used for other pathways,
one of which is oxidation to acetyl-CoA and entry into the TCA cycle for complete
oxidation yielding more energy.
2. Alternatively, in anaerobic glycolysis, pyruvate is reduced to lactate and diverted
away from other potential pathways.
Thus, the use of the shuttle systems allows for more ATP to be generated than by
anaerobic glycolysis by both oxidizing the cytoplasmically derived NADH in the
electron transport chain and by allowing pyruvate to be oxidized completely to CO2.
Anaerobic Glycolysis (Occurs in red blood cells and in exercising muscle cell)
When the oxidative capacity of a cell is limited (e.g., the red blood cell, which has no
mitochondria and ETC), the pyruvate and NADH produced from glycolysis cannot be
oxidized aerobically. The NADH is therefore oxidized to NAD in the cytosol by reduction of
pyruvate to lactate. This reaction is catalyzed by lactate dehydrogenase (LDH) and called
lactic acid fermentation.
In both aerobic and anaerobic respiration, each mole of glucose generates 2 moles of ATP, 2 of
NADH and 2 of pyruvate by GLYCOLYSIS.
1. The energy yield from anaerobic respiration (glucose to 2 lactate) is only 2 moles of ATP
per mole of glucose generated by glycolysis . This is because NADH is recycled to NAD+ by
reducing pyruvate to lactate in LDH reaction of lactic acid fermentation. In this reaction
neither the NADH nor pyruvate produce energy.
Glucose to 2 pyruvate (Glycolysis)= 2ATP (directly)
2 NADH to 2 NAD+ (Lactic acid fermentation ) = 0 ATP
2 Pyruvate to 2 Lactate (Lactic acid fermentation) = 0 ATP
Total yield in anaerobic respiration = 2 ATP
2. However, when oxygen is available (aerobic respiration), and cytosolic NADH can
be oxidized by ETC generating ATP and pyruvate can also enter the mitochondria
and be completely oxidized to CO2 via Pyruvate dehydrogenase reaction and the
TCA cycle giving more ATP.
The oxidation of pyruvate by PDH & TCA cycle route generates roughly 12.5 moles of
ATP per mole of pyruvate.
The cytosolic NADH is oxidized by mitochondrial ETC generates approximately 1.52.5 moles of ATP per NADH depending upon the transport system that carries
NADH electrons to ETC. (glycerol 3–phosphate shuttle (1.5 ATP/NADH) or malate–
aspartate shuttle (2.5 ATP/NADH )
Thus, the 2 NADH molecules produced during glycolysis can lead to 3 to 5
molecules of ATP being produced, depending on which transport system is used.
Each pyruvate produced can give rise to 12.5 molecules of ATP, altogether 30 to
32 molecules of ATP can be produced from one mole of glucose oxidized to
carbon dioxide.
Glucose to 2pyruvate (Glycolysis) = 2ATP (directly)
2 NADH to 2 NAD+ (ETC) = 2x1.5 or 2.5= 3 or 5 ATP
2 Pyruvate to CO2 (PDH & TCA) = 2x12.5 = 25 ATP
Total yield= 30-32 ATP
Anaerobic glycolysis results in acid production in the form of H+ ions. Glycolysis forms
pyruvic acid, which is reduced to lactic acid in the absence of oxygen. At an intracellular
pH of 7.35, lactic acid dissociates to form the carboxylate anion, lactate, and H+ (the pKa
for lactic acid is 3.85).
Lactate and the H+ are both transported out of the cell into interstitial fluid by a
transporter on the plasma membrane and eventually diffuse into the blood. If the
amount of lactate generated exceeds the buffering capacity of the blood, the pH drops
below the normal range (7.4), resulting in lactic acidosis
Many tissues, including red blood cells, the kidney medulla, the tissues of the eye,
and skeletal muscles, rely on anaerobic glycolysis for at least a portion of their ATP
requirements .
The lack of mitochondria, or the increased rate of glycolysis, is often related to some
aspect of cell function.
For example, the mature red blood cell has no mitochondria because oxidative
metabolism might interfere with its function in transporting oxygen bound to
Some of the lactic acid generated by anaerobic glycolysis in skin is secreted in sweat,
where it acts as an antibacterial agent.
Many large tumors use anaerobic glycolysis for ATP production, as they lack blood
capillaries for oxygen supply in their core.
In yeast and other microorganisms under anaerobic conditions, the NAD+ required for
the continuation of glycolysis is regenerated by a process called alcoholic fermentation.
In this process pyruvate is converted to acetaldehyde (by pyruvate decarboxylase ) and
then to ethanol (by alcohol dehydrogenase) leading to the reoxidation of NADH to NAD+.
The energy yeild from alcoholic fermentation is similar to the anaerobic respiration
where pyruvate is converted to lactic acid to regenerate NAD+.
Regulation of Glycolysis
The Glycolytic Pathway Is tightly controlled. The rate of conversion of glucose into pyruvate
is regulated to meet two major cellular needs: (1) the production of ATP, generated by the
degradation of glucose, and (2) the provision of building blocks for synthetic reactions, such
as the formation of fatty acids.
In metabolic pathways, enzymes catalyzing essentially irreversible reactions are potential
sites of control. In glycolysis, the reactions catalyzed by hexokinase, phosphofructokinase,
and pyruvate kinase are virtually irreversible; hence, these enzymes would be expected to
have regulatory as well as catalytic roles.
Bisoynthetic function of Glycolysis
Glycolysis, in addition to providing ATP,
generates precursors for biosynthetic pathways
(Fig. 22.11). Intermediates of the pathway can
be converted to ribose 5phosphate, the sugar incorporated into
nucleotides such as ATP. Other sugars, such
as UDP-glucose, mannose, and sialic acid, are
also formed from intermediates of
glycolysis. Serine is synthesized from 3phosphoglycerate, and alanine from pyruvate.
The backbone of triacylglycerols, glycerol 3phosphate, is derived from dihydroxyacetone
phosphate in the glycolytic pathway.
Amino acids /
Mrs. Williams recently complaint of nausea, weakness and pain . Her clinical
history showed that she suffers from COPD (chronic obstructive pulmonary
disease). After a number of diagnostic tests, her family doctor told her that
she has lactic acidosis.
Explain the relationship between COPD and lactic acidosis
Animation video:

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