Lecture 16- Glucose Fermentation, Fructose & Galactose Metabolism, and Regulation of Glycolysis

Explain why NAD + needs to be regenerated after glycolysis

During glycolysis, glucose is broken down into 2 molecules of pyruvate.

Along the way, 2 NAD⁺ molecules are reduced to 2 NADH when glyceraldehyde-3-phosphate is converted to 1,3-bisphosphoglycerate.

NAD⁺ is the oxidizing agent that accepts electrons — without it, glycolysis cannot continue.

If all available NAD⁺ is converted to NADH and not regenerated, the cell runs out of NAD⁺, and glycolysis stops, halting ATP production.

how does fermentations regenerate NAD+

When oxygen is not available (anaerobic conditions), cells use fermentation to regenerate NAD⁺.

In lactic acid fermentation (like in muscle cells or bacteria):Pyruvate + NADH → Lactate + NAD⁺(Catalyzed by lactate dehydrogenase)

In alcoholic fermentation (like in yeast):Pyruvate → Acetaldehyde + CO₂

Acetaldehyde + NADH → Ethanol + NAD⁺

how does cellular respiration regenerate NAD+

When oxygen is present, cells use aerobic respiration to recycle NADH.

NADH transfers its high-energy electrons to the electron transport chain (ETC) in the mitochondria.

As electrons pass through the chain, energy is used to pump protons and create ATP.

At the end of the chain, oxygen acts as the final electron acceptor, forming water:½ O₂ + 2H⁺ + 2e⁻ → H₂O

List the enzymes involved in alcohol fermentation; identify the oxidants and reductants in both processes

Pyruvate carboxylase catalyzes the decarboxylation of pyruvate.

Alcohol dehydrogenase then reduces acetaldehyde to ethanol, regenerating NAD + for use by glyceraldehyde 3-phosphate dehydrogenase in glycolysis

List the enzymes involved in lactic acid fermentation; identify the oxidants and reductants in both processes

NADH can also be oxidized by converting pyruvate to lactate in a reaction catalyzed by lactate dehydrogenase. The conversion of glucose into two molecules of lactate is called lactic acid fermentation.

Outline how fructose enters the glycolytic pathway by listing enzymes, cofactors/coenzymes, and intermediates involved

in the liver (the main site of fructose metabolism), fructose is metabolized by the fructose 1-phosphate pathway.

In other tissues (e.g., adipose tissue), fructose is directly phosphorylated by hexokinase

Outline how galactose enters the glycolytic pathway by listing enzymes, cofactors/coenzymes, and intermediates involved

Galactose is converted into glucose 6-phosphate by the galactose-glucose interconversion pathway, which begins with the phosphorylation of galactose by galactokinase. Glucose 1-phosphate (G1P) is generated using UDP-glucose, a transferase, and an epimerase. G1P is converted into glucose 6-phosphate by phosphoglucomutase

Compare and contrast deficiencies in galactose metabolism with that of lactose metabolism

Galactose Can Be Highly Toxic with a Defective Metabolic Pathway

Symptoms- vomiting and diarrhea after milk consumption, enlargement of the liver, jaundice, cataracts, lethargy, and delayed neurological development

Causes- inherited deficiency in galactose 1-phosphateuridyl transferase activity

Treatment- remove galactose and lactose from the diet

Lactose intolerance is caused by lactase deficiency in which human lactase is no longer active. Some people can drink milk into adulthood due to lactase persistence

Symptoms- Diarrhea, bloating, and gas after milk consumption

Causes- Lactase deficiency (which is normal; mutations cause lactase persistence)

Treatment- avoiding lactose products or ingesting lactase with milk products

Outline how muscle cells regulate glycolysis and why

Glycolysis in skeletal muscle is regulated to meet the need for ATP

During exercise, glycolysis is activated, as ATP is needed when the skeletal muscle is contracting. Phosphofructokinase is allosterically stimulated by AMP, as energy charge is low and the need for ATP is high. Pyruvate kinase is stimulated by the phosphofructokinase product fructose 1,6-bisphosphate in an example of feedforward stimulation

what does phosphofructokinase do?

Phosphofructokinase catalyzes the rate determining step and is allosterically inhibited by ATP. ATP binding lowers the enzyme's affinity for fructose 6-phosphate. This means that the activity of the enzyme decreases when the ATP/AMP ratio is high(high energy charge)

Explicitly name any positive or negative effectors for each of the three regulated enzymes in glycolysis for the muscle cells

Hexokinase is allosterically inhibited by its product glucose6-phosphate.

Phosphofructokinase is allosterically inhibited by ATP.

Pyruvate kinase is allosterically inhibited by signals ATP

* all are irreversible

Outline how liver cells regulate glycolysis and why

The liver has more diverse biochemical functions than does muscle. The liver maintains blood-glucose concentration, generates reducing power for biosynthesis, and synthesizes biochemicals. As such, regulation is more complex than in the muscle.

Explicitly name any positive or negative effectors for each of the three regulated enzymes in glycolysis for the liver cells

Hexokinase is an allosteric enzyme in the liver, like it is in muscle. However, the hexokinase isozyme in liver that is primarily responsible for phosphorylating glucose is called glucokinase (hexokinase IV).

• Glucokinase is active only after a meal, when blood-glucose levels are high.

• Glucokinase is not inhibited by glucose6-phosphate.

• Glucokinase has a lower affinity for glucose than the other hexokinases, so it operates only when glucose is abundant.

• The role of glucokinase in the liver is to provide glucose 6-phosphate for thesynthesis of glycogen and for theformation of fatty acids.

• Glucokinase is also present in the β cells of the pancreas, where insulin is released in response to high blood glucose levels.

Regulation of Phosphofructokinase in Liver

While liver phosphofructokinase can be inhibited by ATP and citrate, which reports on the status of the citric acid cycle, fructose 2,6-bisphosphate is the most powerful activator of phosphofructokinase in the liver

Regulation of pyruvate kinase in the liver

Pyruvate kinase in the liver is regulated allosterically, as it is in muscle. However, the liver isozyme of pyruvate kinase is also regulated by covalent modification. Low blood glucose leads to the phosphorylation and inhibition of liver pyruvate kinase. Glucagon triggers this phosphorylation, which prevents the liver from consuming glucose when it is more urgently needed by the brain and muscle

Compare and contrast the different isozymes of glycolytic enzymes found in muscle and liver cells

The hexokinase isozyme in liver that is primarily responsible for phosphorylating glucose, glucokinase, is active only after a meal, when blood-glucose levels are high, and it is not inhibited by glucose 6-phosphate

(Maintains blood glucose homeostasis)

in muscle cells, they use glucose for rapid ATP production during contraction

affect of high and low blood sugar for glycolysis in the liver

High blood sugar (insulin) leads to the synthesis of fructose 2,6-bisphosphate and thus the activation of liver phosphofructokinase.

• Pyruvate kinase in the liver is regulated allosterically and by covalent modification.

• Low blood glucose (glucagon) leads to the phosphorylation and inhibition ofliver pyruvate kinase, while high blood sugar (insulin) leads to the dephosphorylation and activation of liver pyruvate kinase

Provide an advantage for why glycolysis enzymes are found physically associated with each other

Enzymes of glycolysis in eukaryotes are organized into complexes in the cytoplasm. This facilitates substrate channeling, a process that facilitates movement of substrates and products between enzymes

Evidence shows that glycolytic complexes increase in hypoxic conditions

In hypoxic (low oxygen) conditions, yeast and other eukaryotes show an increase in glycolytic bodies (g-bodies), also called Core Fermentation granules which are rich in glycolytic producing

Describe how cancer cells influences aerobic glycolysis(Warburg effect), the hypoxia-inducible transcription factor (HIF-1a), and the downstream effectors

Aerobic glycolysis is a property of tumor cells and other rapidly growing cells. Rapidly growing tumor cells will metabolize glucose to lactate even in the presence of oxygen, a process called aerobic glycolysis (aka Warburg effect). Although producing ATP this way is less efficient than using oxidative phosphorylation, nearly all cancer cells use aerobic glycolysis so there must be a selective advantage

what transcription factor is stimulated in cancer cells

Rapidly growing tumor cells experience hypoxia, activating hypoxia-inducible transcription factor (HIF-1a), which increases gene expression of most glycolyticenzymes, the basal GLUT transporters, and even growth factors that stimulate formation of blood vessels.

Describe how endurance training influences aerobic glycolysis(Warburg effect), the hypoxia-inducible transcription factor (HIF-1a), and the downstream effectors

Cancer and endurance training affect glycolysis in a similar fashion. Anaerobic exercise training forces muscles to rely on lactic acid fermentation for ATP production and this also stimulates HIF-1a.

HIF-1a is degraded by ubiquitin-proteosome pathway when O 2 levels rise