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: 1.Glycolysis 2. TCA cycle 3.
Oxidative phosphorylation Aerobic respiration yields more energy
than anaerobic respiration
Slide 3
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 Cori. Glycolysis is also known as the Embden-Meyerhof pathway.
What is Glycolysis ? The overall equation for glycolysis is:
Glucose + 2NAD + + 2ADP + 2Pi 2 pyruvate + 2NADH + 2H + + 2ATP +
2H2O
Slide 4
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. Significance /
Purpose
Slide 5
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.
Slide 6
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Glucose metabolism begins with transfer of a phosphate from ATP
to glucose to form glucose- 6-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. 1. CONVERSION OF GLUCOSE
TO GLUCOSE-6 PHOSPHATE
Slide 9
The second step in glycolysis is the isomerization of glucose
6-phosphate to fructose 6-phosphate. 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. 2. ISOMERIZATION OF GLUCOSE 6-PHOSPHATE
TO FRUCTOSE 6-PHOSPHATE
Slide 10
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. 3.The Formation of Fructose 1,6-bisphosphate from Fructose
6-phosphate
Slide 11
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
Slide 12
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 three- carbon
phosphorylated sugars is catalyzed by triose phosphate
isomerase
Slide 13
6. Conversion of glyceraldehyde 3-phosphate into
1,3-bisphosphoglycerate (1,3- BPG) 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 dehydrogenase
Slide 14
7.The Formation of ATP from 1,3-Bisphosphoglycerate (substrate
level phosphorylation) The final stage in glycolysis is the
generation of ATP from the phosphorylated three- carbon metabolites
of glucose. Phosphoglycerate kinase catalyzes the transfer of the
phosphoryl group from the acyl phosphate of 1,3-
bisphosphoglycerate to ADP. ATP and 3- phosphoglycerate 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 potential. 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.
Slide 15
The position of the phosphoryl group shifts in the conversion
of 3- phosphoglycerate into 2- phosphoglycerate, 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 group. 8. Rearrangement of 3-phosphoglycerate
to 2-phosphoglycerate
Slide 16
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.
Slide 17
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 "profit."
Slide 18
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. 22 ATP 2. The Formation of ATP from Phosphoenol
Pyruvate 22 ATP Total = 4 ATP 1.The Formation of ATP from
1,3-Bisphosphoglycerate
Slide 19
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
Slide 20
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)
Slide 21
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. Oxidative Fate of Pyruvate Produced from Glycolysis
Slide 22
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.
Slide 23
ENERGY YIELD OF AEROBIC VERSUS ANAEROBIC RESPIRATION 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
Slide 24
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 1.5- 2.5 moles of ATP per NADH depending upon the
transport system that carries NADH electrons to ETC. (glycerol
3phosphate 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
Slide 25
ACID PRODUCTION IN ANAEROBIC GLYCOLYSIS 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
Slide 26
TISSUES DEPENDENT ON ANAEROBIC GLYCOLYSIS 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
hemoglobin. 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.
Slide 27
Fermentation 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+.
Slide 28
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.
Regulation of Glycolysis
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Glycolysis, in addition to providing ATP, generates precursors
for biosynthetic pathways (Fig. 22.11). Intermediates of the
pathway can be converted to ribose 5- phosphate, 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 3-
phosphoglycerate, and alanine from pyruvate. The backbone of
triacylglycerols, glycerol 3- phosphate, is derived from
dihydroxyacetone phosphate in the glycolytic pathway. Bisoynthetic
function of Glycolysis DNA /RNA Amino acids / Proteins Lipids
Slide 31
A CASE STUDY 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