SCI 260 Stratford University Introduction to Biochemistry Questions

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G LUCO N EO G EN ES IS
I N T R O D U C T I O N TO B I O C H E M I S T RY ( S C I 2 6 0 )
LECTURE 5
DR. OMARI J. BANDELE
OVERVIEW OF GLUCONEOGENESIS
• Brain cells and red blood cells (RBC) need a constant supply of glucose for
energy.
• Gluconeogenesis is a metabolic pathway that produces glucose from
carbohydrate molecules.
non-
– Non-carbohydrate molecules: glycerol, lactic acid, and some amino acids
• Gluconeogenesis is needed to: (1) provide brain cells and RBCs with glucose and
(2) avoid hypoglycemia (i.e., low blood glucose levels).
• Where does gluconeogenesis occur in the body?
– Major site: Liver
– Minor site: Kidney
• Glycolysis: Glucose à Pyruvate
• Gluconeogenesis: Pyruvate à Glucose
Lecture 5
2
GLUCONEOGENESIS IS NOT THE
EXACT REVERSE OF GLYCOLYSIS
WHY NOT?
• Because glycolysis has 3 irreversible reactions
– ΔG of glycolysis = -74 kJ/mol (favorable pathway)
– ΔG of gluconeogenesis = +74 kJ/mol (unfavorable pathway)
• The 3 irreversible glycolysis reactions must be bypassed
for gluconeogenesis to be favorable (-ΔG).
Lecture 5
3
REACTIONS OF GLUCONEOGENESIS
• Gluconeogenesis uses many of the
same enzymes as glycolysis.
• 7 glycolysis rxns are reversible and
also used during gluconeogenesis.
• 3 gluconeogenesis reactions and
enzymes are different from glycolysis.
– These 3 rxns make gluconeogenesis a
favorable (-ΔG) pathway for the
production of glucose from pyruvate.

mations/gluconeogenesis/gluconeogenesis.htm
Lecture 5
4
G LY CO G EN
M ETA BO LIS M
I N T R O D U C T I O N TO B I O C H E M I S T RY ( S C I 2 6 0 )
LECTURE 5
DR. OMARI J. BANDELE
GLYCOGEN
• A large polymer made of glucose
molecules
– Contains up to 120,000 glucose molecules
• Glucose is stored in the body as glycogen.
• In glycogen, glucose molecules are joined
by α(1-4) and α(1-6) glycosidic linkages.
– Remember: α(1-6) glycosidic linkages
create branches
Lecture 5
6
GLYCOGEN
• Glycogen provides an immediate source of glucose
during hypoglycemia.
– Note: This process is separate from
gluconeogenesis.
• What is the advantage of glycogen having
branches?
– Glucose is released from the end of each branch,
one glucose at a time.
– More branches increases the number of ends
available for glucose to be released.
• Liver and muscle are the primary sites where
glycogen is stored in the body.
Lecture 5
7
LIVER GLYCOGEN LEVELS BETWEEN MEALS
Lecture 5
8
PROCESSES OF GLYCOGEN AND GLUCOSE
METABOLISM
• Degradation processes end in “–lysis”
• Synthesis processes end with “–genesis”
Lecture 5
9
GLYCOGEN BREAKDOWN BY GLYCOGEN
PHOSPHORYLASE
• Glycogen phosphorylase breaks the
α(1-4) glycosidic linkage between
glucose molecules in glycogen.
– Glycogen phosphorylase acts only at
the ends of glycogen branches
Lecture 5
10
GLYCOGEN BREAKDOWN TO GLUCOSE 6PHOSPHATE IN THE LIVER
• Glycogen phosphorylase removes glucose
molecules from the ends of glycogen
branches.
– This produces glucose 1-phosphate (G1P)
• G1P is converted to glucose 6-phosphate
(G6P)
– G6P is trapped inside the cell and enters
glycolysis to produce ATP
OR
– G6P is converted to glucose and released
from liver cells
Lecture 5
Released from
the cell
Trapped inside the cell
11
DIFFERENCE BETWEEN GLYCOGEN
METABOLISM IN LIVER AND MUSCLE
• Liver cells have the enzyme glucose 6-phosphatase
(removes phosphate from G6P) which produces glucose.
• Muscle cells don’t have glucose 6-phosphatase
WHY?
• Liver cells release glucose into the blood to be delivered to
the brain and other tissues (“unselfish”).
– Increases blood glucose levels to prevent hypoglycemia.
• Muscle cells retain G6P and uses it for energy only in the
muscle (“selfish”).
– Phosphorylated glucose cannot get outside of muscle cells.
Lecture 5
12
HORMONAL REGULATION OF GLYCOGEN METABOLISM
Glucagon:
• Active during a fast (low glucose) to prevent hypoglycemia
• Stimulates glycogen breakdown
– Releases glucose for energy (ATP) production via glycolysis, TCA cycle, & ETC/OXPHOS
• Inhibits glycogen synthesis
– This occurs because the body does not need to store glucose when it is in need of energy.
Insulin:
• Active after a meal (high glucose) to prevent hyperglycemia
• Stimulates glycogen synthesis
– Stores glucose for later use when the body is in need of energy (ATP)
• Inhibits glycogen breakdown
– This occurs because the body does not need to release glucose for energy production
because ATP is plentiful after meal.
Lecture 5
13
PEN TO S E PHO S PHATE
PATHWAY
I N T R O D U C T I O N TO B I O C H E M I S T RY ( S C I 2 6 0 )
LECTURE 5
DR. OMARI J. BANDELE
OVERVIEW
• The pentose phosphate pathway is another pathway for
the metabolism of glucose.
Phase 1 (Oxidative Phase):
• Production of NADPH
– Used to make fatty acids, cholesterol, steroid hormones,
and amino acids
– Used to detoxify drugs and poisons
• Production of ribulose 5-phosphate
Phase 2 (Non-oxidative Phase):
• Ribulose 5-phosphate is used to make nucleotides, DNA,
and RNA
• Production of glycolysis intermediates
– Fructose 6-phosphate and glyceraldehyde 3-phosphate
• No ATP is produced OR used during the pentose
phosphate pathway.
Lecture 5
15
PENTOSE PHOSPHATE PATHWAY: PHASE 1
• Two (2) NADPHs are produced for each glucose that is metabolized during
the pentose phosphate pathway.
Lecture 5
16
PENTOSE PHOSPHATE PATHWAY: PHASE 2
Lecture 5
Glyceraldehyde 3-phosphate (G3P) and fructose 6-phosphate (F6P)
are products of the PPP that can be used in glycolysis.
17
NADH
NADH VS. NADPH
• NADH and NADPH are both electron (H+) carriers.
• The phosphate group on NADPH allows it to interact with
different types of enzymes compared to NADH.
– NADH and NADPH are nearly identical except for the presence of
a phosphate group on NADPH (see figures to the right).
NADPH
• NADH is used in catabolic reactions that break down
molecules and release energy (e.g., glycolysis)
• NADPH is used in anabolic reactions that consume energy
to synthesize molecules (e.g., fatty acids, cholesterol, etc.)
Phosphate group
Lecture 5
18
SUPPLEMENTAL VIDEOS
Gluconeogenesis

Note: gluconeogenesis uses different enzymes to bypass the 3 irreversible glycolysis steps
Glycogen

Note: We’ll discuss glucagon and epinephrine in more detail later. You don’t have to worry
about some of the other info (i.e., thyroid hormone and cAMP)
Pentose Phosphate Pathway

Note: You can stop watching at the 1 min 56 sec mark (1:56)
Lecture 5
19
SCI260 – Introduction to Biochemistry
Lesson 5
In this week’s class, we discussed gluconeogenesis, glycogen metabolism, and the
pentose phosphate pathway and their roles in glucose metabolism. Based on the topics
discussed in Lecture #5, please answer each of the following questions.
1. Which enzyme-catalyzed reactions are shared between the gluconeogenesis and
glycolysis pathways? (Note: just provide the name of the enzyme that catalyzes
each of the shared reactions).
2. Why are some enzymes needed only in the gluconeogenesis pathway and not in
the glycolysis pathway?
3. Which one of the following reactions only occurs during gluconeogenesis (i.e.,
not during glycolysis)?
a.
b.
c.
d.
1,3-bisphosphglycerate into 3-phosphoglycerate
Glucose 6-phosphate into fructose 6-phosphate
Oxaloacetate into Phosphoenolpyruvate
Phosphoenolpyruvate into Pyruvate
4. An increase in glucagon levels can __________.
a.
b.
c.
d.
Promote the degradation of glycogen (i.e., glycogenolysis)
Occurs during the fasting state (starvation)
Promotes the synthesis of glycogen
A and B
5. A high school girl who is self-conscious about her appearance has been fasting for
several days to fit into a dress she intentionally bought a size too small for a
school dance. Which of her organs/tissues is producing the glucose that is being
synthesized through gluconeogenesis?
a.
b.
c.
d.
Red blood cells
Muscles
Liver
Brain
6. Why do only some tissues (e.g., liver) contain the enzyme glucose-6phosphatase?
7. What are the main products of the pentose phosphate pathway, and how does
the cell use them?
8. How many molecules of ATP are produced during the metabolism of one (1)
molecule of glucose through the pentose phosphate pathway?
9. Which two (2) molecules are produced during the pentose phosphate pathway
that are also produced during glycolysis?
10. Describe two (2) specific things that make NADH and NADPH different from each
other (Note: consider the structural and biological properties of the two
molecules).
You may use your textbook and/or the internet to assist you. You will earn 10 points for
correctly answering each question (maximum points earned = 100 points).

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