Bio 211 Intro Molecular and Cell Biology
Lecture 14 "Krebs Cycle"
Reading: Campbell Chap. 9 pp. 156-166
Homework: Write out sequence of steps in glycolysis for class
In the last lecture, we introduced you to some of the processes of energy
metabolism. Today we will look at two of these processes in some depth, glycolysis and
the Krebs cycle. In these pathways, glucose is broken down into pyruvate, then further
into carbon dioxide and water. If you have your homework with you, please have at hand
for the first part of today’s class.
Outline:
1. Glycolysis (full pathway)
2. Fermentation
3. Krebs cycle
1. Glycolysis: In the first part of the period today, we will review the process of
glycolysis.
Let’s try to summarize some of the key features of glycolysis.
a. What is the starting material? Glucose
b. Where does glycolysis occur? Cytosol
c. What is glucose broken down into? 2 Pyruvate
d. What are the other products formed from glycolysis? ATP and NADH + H+
e. What is meant by the energy-requiring and energy pay-off phases? In the first
phase ATP is used up, in the second phase ATP is produced.
f. What is substrate-level phosphorylation? Production of ATP from ADP by
transfer of a phosphate from a glycolytic intermediate.
g. What happens next to the pyruvate? In the presence of oxygen, it is oxidized
further and enters the Krebs cycle.
h. What happens next to the NADH? In the presence of oxygen, it passes its
electrons to the electron transport chain, ultimately yielding additional ATP.
Remember that the end result of glycolysis is:
Glucose
----> 2 Pyruvate + 2 H2O
2 ADP + 2 Pi ----> 2 ATP
2 NAD+
----> 2 NADH + H+
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Using Fig. 9.9 or your homework notes, we will next take a closer look at
glycolysis. On the exam, you won’t need to know all the details such as all of the
chemical structures and the names of the enzymes. However, I do want you take at least
one look at what’s really happening.
Step 1: Glucose + ATP ----> glucose-6-phosphate + ADP
Glucose enters the cell and is phosphorylated by the enzyme hexokinase, which
transfers a phosphate group from ATP to the sugar. You may remember that I used this
example in the lecture on thermodynamics. Adding phosphate to glucose is an
energetically unfavorable reaction. By coupling the breakdown of ATP to the
phosphorylation of glucose, the reaction now becomes energetically favorable.
Step 2: glucose-6-phosphate ----> fructose-6-phosphate
In this reaction catalyzed by an isomerase, one sugar is converted into its isomer.
Remember an isomer has the same chemical formula, but a different chemical structure.
Step 3: fructose-6-phosphate + ATP ----> fructose 1,6-bisphosphate + ADP
This is another step requiring ATP. Phosphate groups are now added to both ends
of the 6 C sugar fructose and the molecule is ready to be split in half.
Step 4: fructose 1,6-bisphosphate ----> dihydroxyacetone phosphate +
glyceraldehyde phosphate (PGAL)
This is the “lysis” reaction where the 6 C sugar is cleaved into two 3 C sugars.
Step 5: dihydroxyacetone phosphate ----> glyceraldehyde phosphate (PGAL)
<---This step is a conversion of one 3 C sugar isomer into another. In a cell, this
reaction is reversible, although the reverse reaction seldom happens because the PGAL is
immediately used in the next step.
Steps 1-5 comprise the energy requiring steps in which 2 ATP are used as glucose
(6 C) is converted to two 3 C sugars (PGAL).
Step 6: glyceraldehyde phosphate + NAD+ + Pi ----> 1,3-diphosphoglycerate +
NADH + H+ (happens twice for the two 3 C sugars)
This step is an oxidation/reduction reaction catalyzed by a dehydrogenase enzyme.
Glyceraldehyde phosphate is oxidized and NAD+ is reduced.
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Step 7: 1,3-diphosphoglycerate + ADP ----> 3-phosphoglycerate + ATP
This step produces ATP by substrate-level phosphorylation. A kinase adds the
phosphate to ADP to make ATP.
Step 8: 3-phosphoglycerate ----> 2-phosphoglycerate
In this step an enzyme shifts the position of the phosphate group.
Step 9: 2-phosphoglycerate ----> phosphoenolpyruvate + H2O
Water is removed to create a double bond in the 3 C sugar.
Step 10: phosphoenolpyruvate + ADP ----> pyruvate + ATP
The last step of glycolysis is the second energy yielding step. A kinase transfers
the phosphate group from the sugar to ADP to make more ATP.
Steps 6-10 result in the net synthesis of ATP and the production of NADH.
Remember that the end result of glycolysis is:
Glucose
----> 2 Pyruvate + 2 H2O
2 ADP + 2 Pi ----> 2 ATP
2 NAD+
----> 2 NADH + H+
Help! I’m lost in glycolysis and I can’t find my way out!
“Escape the maze” through to exam II:
Know key features of glycolysis.
Know major products of glycolysis.
Understand “energy coupling”.
Structures to know: glucose (6C sugar)
know pyruvate is 3C sugar
Enzymes to know:
Know about regulation of glycolysis by enzyme phosphofructokinase.
What is an allosteric inhibitor? How does it work?
Any questions on the process of glycolysis? Next we will take a look at what happens
next to the pyruvate in the cell.
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2. Fermentation
Not all organisms are able to use oxygen in metabolism. Even organisms such as
ourselves that are highly evolved still have some means to extract energy, even if oxygen
is in short supply. These processes which derive energy in the absence of oxygen are
known as fermentation. We will look at two major pathways for fermentation
a. Alcohol fermentation in microorganisms--> produces ethanol
b. Lactic acid fermentation in muscle --> produces lactate
How do organisms such as yeast and bacteria ferment sugars in the absence of
oxygen? Fermentation is essentially a modification of glycolysis that enables the reduced
NADH to be reoxidized to NAD+, so it can be used again. Figure 9.17a summarizes the
process of alcohol fermentation.
Glucose is broken down in glycolysis producing pyruvate and a net yield of 2
ATP. In two additional steps, pyruvate is converted to acetaldehyde and then to ethanol
while the NADH formed during glycolysis is reoxidized to regenerate more NAD+, so
fermentation can continue.
glucose ----> 2 pyruvate ----> 2 acetaldehyde + 2CO2 --> 2 ethanol
Fermentation is essential for the brewing industry in the production of beer, wine,
and other alcoholic beverages. Yeast, which are simple eukaryotic cells can carry out the
fermentation of sugars such as glucose to yield alcohol. Other microbes such as some
bacteria are also able to carry out fermentation.
We are also able to regenerate more NAD+ from NADH in the absence of oxygen during
oxygen deprivation of muscle cells. This process is called lactic acid fermentation and is
shown in Fig. 9.17b. In this pathways the products of glycolysis (2 pyruvate) are reduced
to give 2 lactate with regeneration of NAD+ from NADH.
glucose --> 2 pyruvate --> 2 lactate
During the early stages of strenuous exercise when sugar breakdown to produce
ATP is occuring at a faster rate than oxygen can arrive in the blood, much of the pyruvate
is converted to lactate in muscle.
3. Krebs cycle
Glycolysis only releases 1/4 of the chemical energy stored in glucose. If oxygen is
present, the pyruvate produced from glycolysis enters the mitochondrion where the
enzymes of the Krebs cycle complete the oxidation.
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a. Preparation for the Krebs cycle
Pyruvate doesn’t enter the Krebs cycle directly. First it is oxidized and combined
with a carrier molecule, coenzyme A. Fig. 9.10.
pyruvate + NAD+ + coenzyme A ----> acetyl CoA + CO2 + NADH + H+
(1) pyruvate is transported into the mitochondrion
(2) pyruvate is oxidized to a 2 C compound (acetate) with loss of CO2
(3) the acetate is linked to coenzyme A, forming acetyl CoA
(4) NAD+ is reduced in the reaction to form NADH + H+
Fig. 9.12 summarizes what happens during the Krebs cycle.
The top part of the figure summarizes the conversion of pyruvate (3 C) to acetyl
CoA (2 C). In the Krebs cycle, the two carbons from acetyl CoA are passed to a 4C
compound oxaloacetate and then through a series of intermediates. These reactions take
place in the matrix of the mitochondrion. Ultimately, two carbons are released as carbon
dioxide by the reactions of the Krebs cycle.
Other key products of the Krebs cycle result in capturing the chemical energy of
the oxidized sugars in forms that the cell can use.
One “turn” of the Krebs cycle produces
(1)
(2)
(3)
vitamin,
(4)
two molecules of carbon dioxide
3 NADH + H+
1 FADH2 (an electron carrier like NAD+, which is derived from the B
riboflavin)
1 ATP by substrate level phosphorylation
Many more molecules of ATP are ultimately produced when the NADH + H+ and
the FADH2 are oxidized in the electron transport chain. We will be examining the
process used to make ATP (oxidative phosphorylation) in detail in the next lecture.
Before we close today, we will look briefly at a diagram showing the entire
process of the Krebs cycle. The cycle shown in Fig. 9.11 was elucidated in great part by
Hans Krebs in the 1930s. The cycle has other names as well, after the names of some of
the types of components in the cycle.
Krebs cycle = citric acid cycle = tricarboxylic acid cycle (identify citric acid and
tricarboxylic acids in the pathway)
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Keep in mind that I will not ask you to know the structures of the chemical
intermediates of the Krebs cycle. I am mostly working through those steps important for
producing carbon dioxide, reduced electron carriers and ATP, that is the key products of
the pathway.
Key steps (from # on Fig. 9.11) you should be able to follow are
(1) Acetyl CoA joins the Krebs cycle by transfering acetate to the compound
oxaloacetate, forming citrate.
(3) When isocitrate is converted to -ketoglutarate, one CO2 is released and one
+
NAD is reduced to give NADH + H+.
(4) More CO2 and NADH + H+ are formed in the very next step as ketoglutarate is further oxidized.
(5) Substrate level phosphorylation occur at step 5 in the pathway, the conversion
of succinyl CoA to succinate.
(6) FADH2 is formed in the redox reaction where succinate is oxidized to give
fumarate.
(8) A third NADH + H+ is formed during the oxidation of malate to give
oxaloacetate.
The cycle can then begin again by the addition of two more carbons from acetyl
CoA.
Are there any questions on the Krebs cycle?
Summary: Today we reviewed the process of glycolysis, where glucose is oxidized to
pyruvate. Then we tracked what happens to the carbon atoms of pyruvate as it is further
oxidized to carbon dioxide in the Krebs cycle. The key products of the Krebs cycle as we
observed today are carbon dioxide, ATP, and many molecules of the reduced cofactors
NADH + H+ and FADH2. In the next lecture, we will look at the energy yield from these
electron carriers which is preserved in the processes of electron transport and oxidative
phosphorylation.
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