Biology Chapter 8: Glycolysis

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Questions and Answers

What is the primary end product of glycolysis in aerobic conditions?

Pyruvate (correct)

ATP

Glucose

Lactate

Which glucose transporter is primarily found in neurons?

GLUT-2

GLUT-4

GLUT-3 (correct)

GLUT-1

How many net molecules of ATP are produced during the glycolysis of one glucose molecule?

2 (correct)

What characterizes the Na+-monosaccharide cotransporter system?

It transports glucose against its concentration gradient. (A) Signup and view all the answers

Which phase of glycolysis involves an investment of ATP?

Energy investment phase (C) Signup and view all the answers

Which GLUT transporter has a significant presence in the liver and kidney?

GLUT-2 (B) Signup and view all the answers

In anaerobic glycolysis, what substance does pyruvate get converted to?

Lactate (D) Signup and view all the answers

Which process is used for glucose transport into cells that do not rely on energy?

Facilitated diffusion (D) Signup and view all the answers

What is the role of phosphorylation in the metabolism of glucose?

It traps glucose as G-6-P within the cell, committing it to metabolism. (C) Signup and view all the answers

Which feature distinguishes glucokinase from other hexokinases?

It functions effectively during hyperglycemic conditions. (D) Signup and view all the answers

How does fructose 6-phosphate (F-6-P) regulate glucokinase activity?

F-6-P inhibits glucokinase by promoting its translocation to the nucleus. (B) Signup and view all the answers

Which statement about hexokinases I-III is accurate?

They efficiently phosphorylate glucose even at low concentrations. (C) Signup and view all the answers

What is a key characteristic of phosphofructokinase-1 (PFK-1) in glycolysis?

It is considered the rate-limiting step of glycolysis. (A) Signup and view all the answers

In which cells does glucokinase primarily function?

Pancreatic β-cells and liver parenchymal cells. (A) Signup and view all the answers

What is the primary function of GA-3P dehydrogenase in glycolysis?

Oxidation of glyceraldehyde 3-phosphate (C) Signup and view all the answers

What happens to glucokinase when blood glucose levels are high?

It is released from the nucleus and becomes active. (B) Signup and view all the answers

What is the consequence of high levels of G-6-P for hexokinases I-III?

It causes inhibition of hexokinases I-III. (D) Signup and view all the answers

Which mechanism allows for the reoxidation of NADH in glycolysis?

Conversion of pyruvate to lactate (D) Signup and view all the answers

How is ATP synthesized from 1,3-BPG during glycolysis?

By substrate level phosphorylation (C) Signup and view all the answers

Which statement accurately describes the role of phosphoglycerate kinase?

It catalyzes the conversion of ADP to ATP (C) Signup and view all the answers

What is the primary difference between substrate-level phosphorylation and oxidative phosphorylation?

Substrate-level phosphorylation does not involve the electron transport chain. (C) Signup and view all the answers

What effect does increased levels of fructose-1,6-bisphosphate have on pyruvate kinase in liver cells?

Activates pyruvate kinase activity (D) Signup and view all the answers

What is the primary effect of glucagon on pyruvate kinase activity in the liver?

Inactivates pyruvate kinase through phosphorylation (C) Signup and view all the answers

What is formed when 2-phosphoglycerate is dehydrated by enolase?

Phosphoenolpyruvate (D) Signup and view all the answers

What effect do high levels of ATP have on regulation by energy levels in a cell?

They promote the conversion of glucose to glycogen. (A), They inhibit allosterically, signaling high energy compounds. (D) Signup and view all the answers

Which of the following statements about fructose 2,6-bisphosphate (F-2,6-bisP) is correct?

It is formed from fructose 6-phosphate by PFK-2. (D) Signup and view all the answers

What happens to F-2,6-bisP levels during fasting?

They decrease, leading to reduced glycolysis. (D) Signup and view all the answers

What is the primary action of aldolase A in glycolysis?

To cleave fructose 1,6-bisphosphate into two three-carbon molecules. (C) Signup and view all the answers

How does fructose 2,6-bisP exert its effect on gluconeogenesis?

It inhibits fructose 1,6-bisphosphatase. (D) Signup and view all the answers

What role does triode phosphate isomerase play in glycolysis?

It interconverts dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. (B) Signup and view all the answers

Which condition leads to activation of PFK-1 through fructose 2,6-bisP?

During elevated levels of insulin and decreased glucagon. (D) Signup and view all the answers

What is the result of the reciprocal regulation of F-2,6-bisP on glycolysis and gluconeogenesis?

It ensures that either glycolysis or gluconeogenesis is fully active. (C) Signup and view all the answers

Study Notes

Unit II: Intermediary Metabolism

Glycolysis is a pathway occurring in all tissues
Glucose is broken down to produce energy in the form of ATP and other intermediates for metabolic pathways
It's central to carbohydrate metabolism, converting various sugars into glucose
Pyruvate is the final product of glycolysis in cells with mitochondria (aerobic conditions)
In anaerobic conditions, pyruvate is converted to lactate

Chapter 8: Glycolysis

Glycolysis occurs in all cells
It breaks down glucose to produce ATP and other metabolic intermediates
The process involves a series of enzymatic reactions
Aerobic glycolysis occurs in cells with mitochondria with adequate oxygen supply. The end product is pyruvate.
In anaerobic conditions, pyruvate is converted to lactate
Glucose transporters (GLUTs) are crucial for glucose uptake
Various GLUTs exist, each with a different tissue distribution, affinity, and regulation

Transport of Glucose into Cells

Glucose cannot diffuse into cells; it requires transporters
Sodium-independent facilitated diffusion transports glucose
Monomeric proteins (GLUTs) form transport channels across the cell membrane.
Extracellular glucose binds to the transporter, changing its conformation to allow glucose transport across the cell membrane.
GLUT-1 is highly concentrated in red blood cells and blood brain barrier, but not in the muscles
GLUT-2 is present in liver, kidney and pancreatic cells
GLUT-3 is present in neurons
GLUT-4 is influenced by insulin, common in muscle and fat tissue
Sodium-dependent glucose cotransporters (SGLTs) move glucose against a concentration gradient, coupling its transport with sodium ions. This is crucial in the intestine and kidneys.

Reactions of Glycolysis

Conversion of glucose to pyruvate occurs in two stages.
Energy investment phase, using ATP to synthesize phosphorylated intermediates
Energy generation phase, producing a net gain of ATP molecules through substrate-level phosphorylation.
The initial five reactions form the energy investment stage of glycolysis. The following reactions represent the energy generation phase of glycolysis.
The conversion of glucose to pyruvate involves a net gain of 2ATP per glucose molecule.

Phosphorylation of Glucose

Phosphorylated glucose molecules cannot diffuse out of cells due to their negative charge.
This 'traps' glucose within cells.
This reaction involves an enzyme called hexokinase.
Several hexokinase isozymes (I-IV) exist with differing affinities for glucose and varied regulatory mechanisms.
Reaction controlled by Km and Vmax.

Hexokinase IV (Glucokinase)

Specifically found in liver and pancreatic cells
Acts as a glucose sensor for pancreatic beta cells, influencing insulin secretion and responding to hypoglycemia in neurons.
Participates in glucose metabolism during high blood glucose levels.
High Km, allowing liver to utilize glucose even at low concentrations within blood and prevents excessive blood glucose levels after a meal

Regulation by Fructose 6-Phosphate and Glucose

Glucokinase activity is not allosterically inhibited by G-6-P as in other hexokinases.
Glucokinase controlled by regulatory protein (GKRP), which keeps glucokinase inactive in the nucleus under low glucose conditions.
High glucose levels cause GK to detach from GKRP, allowing it enter the cytoplasm, where it can phosphorylate glucose to produce glucose-6-phosphate.
Fructose-1-phosphate inhibits the formation of the GK-GKRP complex

Regulation by Fructose 2,6-Bisphosphate

Fructose 2,6-bisphosphate is a potent activator of phosphofructokinase-1 (PFK-1) and enhances glycolysis at high ATP levels, playing a critical signaling role.
Formed from fructose 6-phosphate by phosphofructokinase-2 (PFK-2)
Dephosphorylation of PFK-2 reverses the process and enhances gluconeogenesis in low glucose states.
It works in opposition to fructose 1,6-bisphosphatase, ensuring that both glycolysis and gluconeogenesis do not operate simultaneously.

Cleavage of Fructose 1,6-Bisphosphate

Aldolase A cleaves fructose 1,6-bisphosphate into two 3-carbon molecules: dihydroxyacetone phosphate and glyceraldehyde-3-phosphate,
Reversible reaction, not a major regulatory step
In liver and kidney, Aldolase B is involved in metabolizing dietary fructose

Isomerization of Dihydroxyacetone Phosphate

Triose phosphate isomerase interconverts dihydroxyacetone phosphate into glyceraldehyde-3-phosphate
Reversible and readily occurs

Oxidation of Glyceraldehyde 3-Phosphate

Catalyzed by dehydrogenase in Glycolysis which requires NAD+
Produces the high-energy intermediate 1,3-bisphosphoglycerate via an oxidation reaction coupled to the attachment of inorganic phosphate
NADH are reoxidized either by the production of lactate or by the respiratory chain

Synthesis of 3-Phosphoglycerate

Phosphoglycerate kinase catalyzes the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate, producing ATP through substrate-level phosphorylation.
Important step in the glycolysis, producing energy in the form of ATP

Shift of Phosphate Group & Dehydration

3-Phosphoglycerate is rearranged to 2-phosphoglycerate by phosphoglycerate mutase.
2-phosphoglycerate is dehydrated creating high-energy intermediate, phosphoenolpyruvate by enolase
Reversible steps of glycolysis

Formation of Pyruvate

Conversion of phosphoenolpyruvate to pyruvate is catalyzed by pyruvate kinase, this reaction produces ATP through substrate level phosphorylation
Pyruvate kinase reaction, irreversible, favored to proceed in the direction towards pyruvate formation in anaerobic conditions

Regulation of Pyruvate Kinase

Pyruvate kinase activity is regulated by a feedforward mechanism influenced by high levels of fructose-1,6-bisphosphate
Covalent modulation involves phosphorylation and dephosphorylation of the enzyme. Inactivation occurs via phosphorylation by cAMP-activated protein kinase. Dephosphorylation re-activates the enzyme.
Covalent modulation is mediated through signaling pathways responsive to glucagon and blood glucose concentrations, crucial for regulating glycolysis and gluconeogenesis, ensuring they do not operate simultaneously.

Genetic Defects of Glycolytic Enzymes

Defects in genes responsible for glycolysis can result in various symptoms
Pyruvate kinase deficiency is a common cause of chronic hemolytic anemia (a condition in which red blood cells are destroyed faster than they can be produced)

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Description

This quiz focuses on Chapter 8 which covers glycolysis, the critical pathway for glucose metabolism. It explores the enzymatic breakdown of glucose into pyruvate, energy production through ATP, and differences in aerobic and anaerobic processes. Additionally, the role of glucose transporters (GLUTs) in cellular glucose uptake is highlighted.