16.4 The Glycolytic Pathway Is Tightly Controlled
Overview of Glycolysis Regulation: Glycolysis has two major roles: energy generation (ATP production) and providing building blocks for
biosynthesis (e.g., fatty acids, amino acids). Regulation focuses on the irreversible reactions catalysed by hexokinase, phosphofructokinase
(PFK), and pyruvate kinase, which are key control points. Glycolysis is regulated differently in skeletal muscle and liver, based on their distinct
energy and metabolic roles.
Glycolysis Regulation in Skeletal Muscle:
• Purpose: Provide ATP for contraction.
• Primary Signal: Energy charge of the cell (ATP/AMP ratio). Glycolysis is stimulated when ATP is low, and AMP is high.
• Phosphofructokinase (PFK) (key regulatory enzyme):
o Allosteric inhibition: High ATP binds a regulatory site, reducing enzyme activity.
o Stimulation: AMP competes with ATP at the regulatory site and reverses inhibition.
o pH Regulation: Decreased pH (e.g., from lactic acid during anaerobic activity) further inhibits PFK, protecting muscle from damage
due to acid accumulation.
• Hexokinase (key regulatory enzyme):
o Inhibited by its product, glucose 6-phosphate.
o This ensures glucose is not unnecessarily phosphorylated when PFK is inactive.
• Pyruvate Kinase (key regulatory enzyme):
o Inhibition: High ATP allosterically inhibits the enzyme when energy is sufficient.
o Activation: Fructose 1,6-bisphosphate (product of the previous step) stimulates pyruvate kinase to ensure glycolytic flux matches
upstream activity.
Glycolysis Regulation in the Liver:
• Purpose: Maintain blood glucose, synthesize biosynthetic precursors, and store glucose.
• Regulation: More complex than in muscle due to the liver's diverse roles.
• Phosphofructokinase (PFK) (key regulatory mechanism):
o Inhibited by citrate, indicating sufficient biosynthetic precursors.
o Activated by fructose 2,6-bisphosphate (F-2,6-BP), which increases PFK’s affinity for fructose 6-phosphate and diminishes ATP’s
inhibitory effect.
o Feedforward stimulation: High blood glucose raises F-2,6-BP, accelerating glycolysis.
• Hexokinase & Glucokinase (key regulatory mechanism):
o Hexokinase: Similar regulation as in muscle.
o Glucokinase (hexokinase IV): Active only when glucose is abundant due to a high Km for glucose. It ensures glucose is
phosphorylated for storage as glycogen only when plentiful.
o Glucokinase is regulated by glucokinase regulatory protein (GKRP), which sequesters it in the nucleus during low glucose
conditions.
• Pyruvate Kinase (L Isozyme) (key regulatory mechanism):
o Like muscle but also inhibited by alanine (signal for available building blocks).
o Hormonal Regulation: Phosphorylated and inhibited by glucagon-triggered cAMP signalling during low blood glucose, ensuring
glucose is conserved for essential tissues like the brain.
Glucose Transport:
Glucose transport across membranes is mediated by GLUT transporters, which differ by tissue type and function:
• GLUT1 and GLUT3: Found in most tissues. Low Km (~1 mM) ensures continuous glucose uptake, even at low serum-glucose levels.
• GLUT2: Found in the liver and pancreatic beta cells. High Km (~15–20 mM) ensures glucose uptake only during high blood glucose (e.g.,
after meals). In beta cells, GLUT2 helps regulate insulin secretion.
• GLUT4: Found in muscle and fat cells. Km (~5 mM). Insulin increases GLUT4 expression on membranes to promote glucose uptake.
Exercise training also upregulates GLUT4 expression.
• GLUT5: Found in the small intestine. Specializes in fructose transport.
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,Key Concepts for Exam Preparation:
• Muscle Glycolysis:
o ATP/AMP ratio controls glycolysis rate.
o PFK is the major regulatory enzyme, with ATP as an allosteric inhibitor and AMP as an activator.
o Hexokinase is indirectly regulated by PFK activity.
• Liver Glycolysis:
o Citrate and F-2,6-BP regulate PFK for biosynthetic and energy needs.
o Glucokinase ensures glucose storage only when abundant.
o Pyruvate kinase is regulated by hormones and allosteric signals like alanine.
• Glucose Transport:
o GLUT1/GLUT3 for basal glucose uptake.
o GLUT2 for liver and pancreatic glucose sensing.
o GLUT4 for insulin-dependent glucose uptake in muscle and fat.
o GLUT5 for fructose transport.
Question 13: There are several irreversible reactions that control the rate of glycolysis. By which
enzymes are these catalysed?
a. Aldolase
b. Enolase
c. Glyceraldehyde 3-phosphate dehydrogenase (GAPDH)
d. Hexokinase
e. Phosphoglycerate kinase
f. Phosphofructokinase (PFK)
g. Pyruvate kinase
Answer: These are phosphofructokinase, hexokinase and pyruvate kinase.
Question 13A: On three places the enzymes of the glycolysis (hexokinase, PFK and pyruvate
kinase) are allosterically inhibited. Which allosteric effectors are involved in this inhibition?
Allosteric effectors: citrate, ADP, ATP, ATP, pyruvate, AMP and glucose 6-phosphate.
Answer: At rest little ATP is needed. ATP serves as an allosteric inhibitor for pyruvate kinase and
phosphofructokinase (PFK). Hexokinase is allosterically inhibited by the product of the reaction it
catalyses glucose 6-phosphate. In the liver high levels of citrate indicate that there is no need to
degrade more glucose for biosynthesis.
Question 13B: When the glycolysis needs to be active, two of these enzymes (PFK and pyruvate
kinase) are allosterically stimulated. Which allosteric effectors are involved in this stimulation?
Answer: High levels of fructose 6-phosphate leads to the synthesis of fructose 2,6-biphosphate in the
liver. In the liver this leads to a feedforward stimulation of phosphofructokinase (PFK). Pyruvate kinase
is stimulated by fructose 1,6-biphosphate in a feedforward manner.
Question 14: Place these descriptions in the correct place matching the enzymes (hexokinase,
glucokinase or both). Descriptions: a broad specificity for hexoses, low KM for glucose, specificity for
glucose, found in nonhepatic tissues, requires ATP for reaction, found in liver, and inhibited by glucose
6-phosphate.
Answer: Glucokinase is specific for the liver where it allows uptake of hexose from the blood as soon as the blood glucose reaches its K M. It
therefore helps maintain blood sugar homeostasis. Hexokinase is found in all tissues including the liver. This enzyme traps glucose into the
cells to feed glucose 6-phosphate to the various pathways.
Glucokinase has a low affinity for glucose, because it only must catalyse its phosphorylation when there is a surplus of glucose. Hexokinase has a
high affinity but has a broad specificity for all hexoses. Only hexokinase is inhibited, and this by its final product glucose 6-phosphate. Both are
kinases and require ATP for phosphorylation of the substrate.
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, Question 15A: What are glucose transporters?
a. Intracellular proteins
b. Transmembrane proteins
c. Extracellular proteins
d. No proteins at all
Answer: Transmembrane proteins. These proteins have a 12-transmembrane helix structure.
Question 15B: Over which membranes do GLUT family transporters accomplish the movement of
monosaccharides?
a. Plasma membranes of a cell
b. Internal cell membranes
c. All membranes of a cell
Answer: They all accomplish the movement of glucose and related hexoses across the plasma membrane of cells.
Question 15C: Is the movement of monosaccharides over the membrane unidirectional?
Answer: No. They facilitate diffusion across the membrane, dependant of the diffusion gradient, so this is bidirectional. However, because the
blood glucose levels are generally much higher than the KM of hexokinase in the cytosol of cells, they mediate the uptake of glucose and some
related hexoses into the cell.
Question 15D: On what do the distribution and concentration of glucose transporters in the tissue depend?
a. The tissue type
b. The metabolic state of the organism
Answer: GLUT1 and 3 are present in all mammalian tissues, but GLUT2, 4 and 5 are in specific tissues. GLUT4 is found in muscle cell
membranes, and endurance exercise training increases the amount. So, both are true.
Question 15E: There are five glucose transporters: GLUT1 - 5. How are the transporters of the GLUT-family related?
a. They are isomers
b. They are epimers
c. They are isoforms
d. All the above
Answer: They are protein isoforms. This does not mean that they have the same molecular formula like an isomer; it means they have almost the
same sequence of amino-acids, and differ only in one or a few.
16.5 Metabolism in Context: Glycolysis Helps Pancreatic Beta Cells Sense Glucose
Summary: Glycolysis and Insulin Secretion in Pancreatic β Cells
• Insulin Function:
o A polypeptide hormone secreted by pancreatic β cells in response to
increased blood glucose (e.g., after a meal).
o Stimulates glucose uptake in tissues like muscle and adipose tissue.
• Glucose Uptake by β Cells:
o Glucose enters β cells via the GLUT2 transporter, which functions only
when blood glucose levels are high.
• Metabolism in β Cells:
o Glucose undergoes glycolysis, producing pyruvate, which is processed
through cellular respiration to generate ATP.
o The resulting high ATP/ADP ratio closes ATP-sensitive potassium (K⁺)
channels, preventing K⁺ from exiting the cell.
• Ionic Changes and Insulin Secretion:
o The closure of K⁺ channels alter the cell's ionic balance, leading to the opening of calcium (Ca²⁺) channels.
o Ca²⁺ influx triggers fusion of insulin-containing vesicles with the cell membrane, releasing insulin into the bloodstream.
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Overview of Glycolysis Regulation: Glycolysis has two major roles: energy generation (ATP production) and providing building blocks for
biosynthesis (e.g., fatty acids, amino acids). Regulation focuses on the irreversible reactions catalysed by hexokinase, phosphofructokinase
(PFK), and pyruvate kinase, which are key control points. Glycolysis is regulated differently in skeletal muscle and liver, based on their distinct
energy and metabolic roles.
Glycolysis Regulation in Skeletal Muscle:
• Purpose: Provide ATP for contraction.
• Primary Signal: Energy charge of the cell (ATP/AMP ratio). Glycolysis is stimulated when ATP is low, and AMP is high.
• Phosphofructokinase (PFK) (key regulatory enzyme):
o Allosteric inhibition: High ATP binds a regulatory site, reducing enzyme activity.
o Stimulation: AMP competes with ATP at the regulatory site and reverses inhibition.
o pH Regulation: Decreased pH (e.g., from lactic acid during anaerobic activity) further inhibits PFK, protecting muscle from damage
due to acid accumulation.
• Hexokinase (key regulatory enzyme):
o Inhibited by its product, glucose 6-phosphate.
o This ensures glucose is not unnecessarily phosphorylated when PFK is inactive.
• Pyruvate Kinase (key regulatory enzyme):
o Inhibition: High ATP allosterically inhibits the enzyme when energy is sufficient.
o Activation: Fructose 1,6-bisphosphate (product of the previous step) stimulates pyruvate kinase to ensure glycolytic flux matches
upstream activity.
Glycolysis Regulation in the Liver:
• Purpose: Maintain blood glucose, synthesize biosynthetic precursors, and store glucose.
• Regulation: More complex than in muscle due to the liver's diverse roles.
• Phosphofructokinase (PFK) (key regulatory mechanism):
o Inhibited by citrate, indicating sufficient biosynthetic precursors.
o Activated by fructose 2,6-bisphosphate (F-2,6-BP), which increases PFK’s affinity for fructose 6-phosphate and diminishes ATP’s
inhibitory effect.
o Feedforward stimulation: High blood glucose raises F-2,6-BP, accelerating glycolysis.
• Hexokinase & Glucokinase (key regulatory mechanism):
o Hexokinase: Similar regulation as in muscle.
o Glucokinase (hexokinase IV): Active only when glucose is abundant due to a high Km for glucose. It ensures glucose is
phosphorylated for storage as glycogen only when plentiful.
o Glucokinase is regulated by glucokinase regulatory protein (GKRP), which sequesters it in the nucleus during low glucose
conditions.
• Pyruvate Kinase (L Isozyme) (key regulatory mechanism):
o Like muscle but also inhibited by alanine (signal for available building blocks).
o Hormonal Regulation: Phosphorylated and inhibited by glucagon-triggered cAMP signalling during low blood glucose, ensuring
glucose is conserved for essential tissues like the brain.
Glucose Transport:
Glucose transport across membranes is mediated by GLUT transporters, which differ by tissue type and function:
• GLUT1 and GLUT3: Found in most tissues. Low Km (~1 mM) ensures continuous glucose uptake, even at low serum-glucose levels.
• GLUT2: Found in the liver and pancreatic beta cells. High Km (~15–20 mM) ensures glucose uptake only during high blood glucose (e.g.,
after meals). In beta cells, GLUT2 helps regulate insulin secretion.
• GLUT4: Found in muscle and fat cells. Km (~5 mM). Insulin increases GLUT4 expression on membranes to promote glucose uptake.
Exercise training also upregulates GLUT4 expression.
• GLUT5: Found in the small intestine. Specializes in fructose transport.
1|Page
,Key Concepts for Exam Preparation:
• Muscle Glycolysis:
o ATP/AMP ratio controls glycolysis rate.
o PFK is the major regulatory enzyme, with ATP as an allosteric inhibitor and AMP as an activator.
o Hexokinase is indirectly regulated by PFK activity.
• Liver Glycolysis:
o Citrate and F-2,6-BP regulate PFK for biosynthetic and energy needs.
o Glucokinase ensures glucose storage only when abundant.
o Pyruvate kinase is regulated by hormones and allosteric signals like alanine.
• Glucose Transport:
o GLUT1/GLUT3 for basal glucose uptake.
o GLUT2 for liver and pancreatic glucose sensing.
o GLUT4 for insulin-dependent glucose uptake in muscle and fat.
o GLUT5 for fructose transport.
Question 13: There are several irreversible reactions that control the rate of glycolysis. By which
enzymes are these catalysed?
a. Aldolase
b. Enolase
c. Glyceraldehyde 3-phosphate dehydrogenase (GAPDH)
d. Hexokinase
e. Phosphoglycerate kinase
f. Phosphofructokinase (PFK)
g. Pyruvate kinase
Answer: These are phosphofructokinase, hexokinase and pyruvate kinase.
Question 13A: On three places the enzymes of the glycolysis (hexokinase, PFK and pyruvate
kinase) are allosterically inhibited. Which allosteric effectors are involved in this inhibition?
Allosteric effectors: citrate, ADP, ATP, ATP, pyruvate, AMP and glucose 6-phosphate.
Answer: At rest little ATP is needed. ATP serves as an allosteric inhibitor for pyruvate kinase and
phosphofructokinase (PFK). Hexokinase is allosterically inhibited by the product of the reaction it
catalyses glucose 6-phosphate. In the liver high levels of citrate indicate that there is no need to
degrade more glucose for biosynthesis.
Question 13B: When the glycolysis needs to be active, two of these enzymes (PFK and pyruvate
kinase) are allosterically stimulated. Which allosteric effectors are involved in this stimulation?
Answer: High levels of fructose 6-phosphate leads to the synthesis of fructose 2,6-biphosphate in the
liver. In the liver this leads to a feedforward stimulation of phosphofructokinase (PFK). Pyruvate kinase
is stimulated by fructose 1,6-biphosphate in a feedforward manner.
Question 14: Place these descriptions in the correct place matching the enzymes (hexokinase,
glucokinase or both). Descriptions: a broad specificity for hexoses, low KM for glucose, specificity for
glucose, found in nonhepatic tissues, requires ATP for reaction, found in liver, and inhibited by glucose
6-phosphate.
Answer: Glucokinase is specific for the liver where it allows uptake of hexose from the blood as soon as the blood glucose reaches its K M. It
therefore helps maintain blood sugar homeostasis. Hexokinase is found in all tissues including the liver. This enzyme traps glucose into the
cells to feed glucose 6-phosphate to the various pathways.
Glucokinase has a low affinity for glucose, because it only must catalyse its phosphorylation when there is a surplus of glucose. Hexokinase has a
high affinity but has a broad specificity for all hexoses. Only hexokinase is inhibited, and this by its final product glucose 6-phosphate. Both are
kinases and require ATP for phosphorylation of the substrate.
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, Question 15A: What are glucose transporters?
a. Intracellular proteins
b. Transmembrane proteins
c. Extracellular proteins
d. No proteins at all
Answer: Transmembrane proteins. These proteins have a 12-transmembrane helix structure.
Question 15B: Over which membranes do GLUT family transporters accomplish the movement of
monosaccharides?
a. Plasma membranes of a cell
b. Internal cell membranes
c. All membranes of a cell
Answer: They all accomplish the movement of glucose and related hexoses across the plasma membrane of cells.
Question 15C: Is the movement of monosaccharides over the membrane unidirectional?
Answer: No. They facilitate diffusion across the membrane, dependant of the diffusion gradient, so this is bidirectional. However, because the
blood glucose levels are generally much higher than the KM of hexokinase in the cytosol of cells, they mediate the uptake of glucose and some
related hexoses into the cell.
Question 15D: On what do the distribution and concentration of glucose transporters in the tissue depend?
a. The tissue type
b. The metabolic state of the organism
Answer: GLUT1 and 3 are present in all mammalian tissues, but GLUT2, 4 and 5 are in specific tissues. GLUT4 is found in muscle cell
membranes, and endurance exercise training increases the amount. So, both are true.
Question 15E: There are five glucose transporters: GLUT1 - 5. How are the transporters of the GLUT-family related?
a. They are isomers
b. They are epimers
c. They are isoforms
d. All the above
Answer: They are protein isoforms. This does not mean that they have the same molecular formula like an isomer; it means they have almost the
same sequence of amino-acids, and differ only in one or a few.
16.5 Metabolism in Context: Glycolysis Helps Pancreatic Beta Cells Sense Glucose
Summary: Glycolysis and Insulin Secretion in Pancreatic β Cells
• Insulin Function:
o A polypeptide hormone secreted by pancreatic β cells in response to
increased blood glucose (e.g., after a meal).
o Stimulates glucose uptake in tissues like muscle and adipose tissue.
• Glucose Uptake by β Cells:
o Glucose enters β cells via the GLUT2 transporter, which functions only
when blood glucose levels are high.
• Metabolism in β Cells:
o Glucose undergoes glycolysis, producing pyruvate, which is processed
through cellular respiration to generate ATP.
o The resulting high ATP/ADP ratio closes ATP-sensitive potassium (K⁺)
channels, preventing K⁺ from exiting the cell.
• Ionic Changes and Insulin Secretion:
o The closure of K⁺ channels alter the cell's ionic balance, leading to the opening of calcium (Ca²⁺) channels.
o Ca²⁺ influx triggers fusion of insulin-containing vesicles with the cell membrane, releasing insulin into the bloodstream.
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