Section E - Carbohydrate Structure and Metabolism

Dr. Olson

What are the four major pathways of Glucose utilization?

1. Synthesis of Structural Polymers (yields extracellular matrix and cell wall polysaccharides)

2. Oxidation via PPP (yields Ribose 5-Phosphate)

3. Storage (yields Glycogen, Starch, and Sucrose)

4. Oxidation via Glycolysis (yields Pyruvate)

What is Glycolysis?

What are the major inputs? Outputs?

Glycolysis is the central pathway of glucose intermediary metabolism

Inputs — 6 carbon carbohydrates, i.e. Glucose

Outputs — 2 Pyruvates or Lactates (3 carbons)

What does the term ‘Intermediary Metabolism’ mean?

Term used to describe pathways that are used for (1) synthesis of molecules needed by our cells or (2) degradation of molecules for ATP synthesis or elimination from the cell (anabolic and catabolic pathways, respectively)

These pathways go through a series of Intermediates (hence the name)

What is the committed step in a metabolic pathway?

First reaction in which the product has no other fate except to proceed through the pathway towards a certain product outcome

What are the two phases of Glycolysis?

1. Preparatory Phase

2. Payoff Phase

What happens during the Preparatory Phase of Glycolysis?

1. ATP is consumed

2. delta G of the intermediates increases (more +)

3. Hexose (5) carbon chains are turned into Glyceraldehyde 3-Phosphate

What happens during the Payoff Phase of Glycolysis?

1. Energy is conserved as 2 ATP and 2 NADH

2. Production of 2 Pyruvates (or Lactates)

What are the three main noteworthy chemical transformations that occur during Glycolysis?

1. Degradation of Glucose carbon skeleton (glucose oxid.) —> Pyruvate

2. Phosphorylation of ADP by compounds with high phosphoryl group transfer potential —> ATP

3. Transfer of a Hydride ion (H-) to NAD+ (red. via e- from Glucose oxid.) —> NADH

What is the overall reaction equation for Glycolysis?

Glucose + 2 NAD⁺ + 2 ADP + Pi —> 2 Pyruvate + 2 NADH + 2 H⁺ + 2 ATP + 2 H₂O

Which two thermodynamic processes can Glycolysis be divided into?

1. Conversion of Glucose —> Pyruvate (EXERGONIC)

Glucose + 2 NAD⁺ —> 2 Pyruvate + 2 NADH + 2H⁺

delta G’˚₁ = -146 kJ/mol

2. Formation of ATP from ADP and Pi (ENDERGONIC)

2 ADP + 2 Pi —> 2 ATP + 2 H₂O

delta G’˚₂ = +62.0 kJ/mol

What gives us the Overall Standard Free-Energy Change (∆G′°Sum) of Glycolysis?

The sum of the two thermodynamic processes that Glycolysis is divided into (Conversion of Glucose to Pyruvate and Formation of ATP from ADP and Pi)

∆G′°Sum = -85kJ/mol

Is Glycolysis reversible or irreversible under standard, cellular conditions?

Irreversible, because of an overall negative free energy change (∆G′°Sum < 0)

What two ways are used to extract energy from Pyruvate?

1. Aerobic Processes that lead to formation of CO2

• Oxid. rxns in the Citric Acid Cycle

• Oxidative Phosphorylation

2. Anaerobic Processes

• Reduction to Lactate

What other way can energy be used from Pyruvate, excluding extracting the energy through Aerobic or Anaerobic processes?

Pyruvate can provide the carbon skeleton for Alanine Synthesis or Fatty Acid Synthesis

What is the first step in Glucose metabolism?

Hexokinase phosphorylates at Glucose C6, yielding Glucose 6-Phosphate

What is the phosphate donor in the first step of Glucose Metabolism in the cell?

What is required for Hexokinase activity during this step?

Is this first step reversible or irreversible under intracellular conditions?

Phosphate donor for the first step of Glucose Metabolism is ATP

Hexokinase requires Mg2+ for activity

This first step is irreversible under intracellular conditions

What is Glucose 6-phosphate?

When is it produced?

High energy compound that cannot be transported out of the cell

Produced during the first step of Glucose Metabolism

What is the second step of Glucose Metabolism?

Phosphofructokinase-1 (PFK-1) catalyzes the transfer of a Phosphoryl group from ATP to Fructose 6-Phosphate, yielding Fructose 1,6-Bisphosphate

Why is the second step of Glucose Metabolism so important?

Is this second step reversible or irreversible under cellular conditions?

It is the first “committed” step in Glycolysis, as well as the Rate-Limiting Step (slowest step of the pathway)

Irreversible

What causes Phosphofructokinase-1 (PFK-1) activity to increase?

Activity increases when either (1) ATP supply is depleted or (2) ADP and AMP accumulate

What is Fructose 2,6-Bisphosphate?

How is it made? Activated?

Potent allosteric activator compound

It is made by the bifunctional enzyme PFK-2

It is activated by hormone signals indicating a need for energy

What is Citrate?

Key intermediate in the Aerobic Oxid. of Pyruvate, FAs, and AAs

What effect does Citrate have on PFK-1?

Citrate allosterically regulates PFK-1

High [Citrate] increases the inhibitory effect of ATP

It serves as an intracellular signal that the cell has its needs met for energy-yielding metabolism (through oxid. of fats and proteins)

What happens during the Payoff Phase of Glycolysis?

Each of the two molecules of Glyceraldehyde 3-phosphate undergo oxidation at C1

Energy from this oxidation rxn is conserved as 1 NADH and 2 ATP per triose phosphate oxidized

What is Substrate-Level Phosphorylation?

Formation of ATP by phosphoryl group transfer from a substrate; it is DIFFERENT than Respiration-Linked Phosphorylation

How is NAD+ regenerated?

Why is it important this molecule is regenerated?

Pyruvate is reduced into Lactate, regenerating 2 NAD+

We need NAD+ in order for Glycolysis to occur as Glycolysis converts 2 NAD+ to 2 NADH

What is Pyruvate Kinase?

What does it require to function?

Enzyme that catalyzes the transfer of the phosphoryl group from Phosphoenolpyruvate to ADP, yielding Pyruvate

Requires K+ and either Mg2+ or Mn2+

Where do we get other monosaccharides from in our diet?

Lactose in dairy —> Galactose and Glucose

Sucrose —> Fructose and Glucose

Fruits and Vegetables —> D-Mannose

What is Galactose?

What enzyme phosphorylates Galactose?

Galactose is a product of Lactose hydrolysis and is an important component in the infant diet

Galactose is phosphorylated at C1 by Galactokinase

What is Galactokinase?

What is the reaction equation for Galactokinase function?

Galactokinase is the enzyme responsible for phosphorylating Galactose at the C1 by using ATP

Galactose + ATP —(Mg2+)—> Galactose 1-Phosphate + ADP

How does Galactose convert into Glucose 1-Phosphate?

Via conversion through UDP-galactose and UDP-glucose intermediates

What causes Galactosemia Diseases?

How are they treated?

Galactosemia diseases are caused by a genetic defect in the enzymes of the pathway where Galactose is converted to Glucose 1-Phosphate

They are typically treated by carefully controlling dietary Galactose

How can Fructose and Mannose be used for Glycolysis?

They can be phosphorylated and funneled into Glycolysis

How is Fructose phosphorylated in the Small Intestine and Liver?

Small Intestine: via Hexokinase

Fructose + ATP —(Mg2+)—> Fructose 6-Phosphate + ADP

Liver: via Fructose 1-Phosphate Aldose

Fructose + ATP —(Mg2+)—> Fructose 1-Phosphate + ADP

How does Fructose 1-Phosphate Aldolase work?

Cleaves Fructose 1-Phosphate into Dihydroxyacetone and Glyceraldehyde

What happens to the products of Fructose 1-Phosphate hydrolysis?

They are entered into Glycolysis as Glyceraldehyde 3-Phosphate

What is the main function of Triose Phosphate Isomerase?

Triose Kinase?

Triose Phosphate Isomerase — converts Dihydroxyacetone Phosphate into Glyceraldehyde 3-Phosphate

Triose Kinase — phosphorylates Glyceraldehyde into Glyceraldehyde 3-Phosphate using ATP

How is Mannose used for Glycolysis?

Mannose is converted into Fructose for Glycolysis

What does Hexokinase do to Mannose?

Phosphohexose Isomerase?

Hexokinase — phosphorylates Mannose at C6

Mannose + ATP —(Mg2+)—> Mannose 6-Phosphate + ADP

Phosphohexose Isomerase — converts Mannose 6-Phosphate into Fructose 6-Phosphate

What are the three catabolic fates of Pyruvate?

1. NADH must be recycled to regenerate NAD+

2. Under aerobic conditions, Pyruvate is oxid. to Acetyl-CoA

3. Under anaerobic conditions (hypoxia), Pyruvate is red. to Lactate or Ethanol

Which pathway does Glucose 6-Phosphate feed into?

What determines this?

Glucose 6-Phosphate feeds into both Glycolysis and the Pentose Phosphate Pathway (PPP)

It is partitioned between both pathways, based on relative conc. of NADP+ and NADPH

What is the main function of the Pentose Phosphate Pathway?

Oxidizes Glucose 6-Phosphate, producing pentose phosphates and NADPH

What is responsible for beginning the oxidative portion of the Pentose Phosphate Pathway again?

The conversion of Pentose Phosphates into Glucose 6-Phosphate

What are some examples of cells and tissues that use the Pentose Phosphate Pathway?

Rapidly Dividing Cells — use Ribose 5-Phosphate to make RNA, DNA, and coenzymes

Liver, Adipose, and Lactating Mammary Glands — required NADPH from the PPP to carry out extensive FA synthesis

Liver, Adrenal Glands, and Gonads — require NADPH from the PPP for active synthesis of Cholesterol and Steroid hormones

What is Glycogen?

Polymeric storage form of Glucose in animals, found primarily in the muscle and liver

What does Glycogen breakdown in the muscle provide?

In the liver?

In the muscle — delivers glucose needed for muscle contraction within seconds

In the liver — reservoir for maintaining homeostasis of blood glucose

What is the primer for Glycogen Synthesis?

Glycogenin Dimer

How many glucose residues are located in a Glycogen Beta-Granule?

What linkages exist in the glucose residues?

Why is this important?

12-14 residues in each chain

(alpha1 —> 4) linkage chains and (alpha1 —> 6) linked branches

These linkages are important as they provide many free nonreducing ends

What is the starting molecule for Glycogenesis?

Where is this molecule sourced from?

What is the name for the first reaction this molecule is involved in for Glycogenesis?

Starting molecule is Glucose 6-phosphate

Sourced from Glucose (converted via Hexokinase) or from Lactate (converted by the Liver through Gluconeogenesis)

First step reaction is Phosphoglucomutase Reaction

What happens during the Phosphoglucomutase reaction?

Glucose 6-Phosphate is converted into Glucose 1-Phosphate

What are the steps of Glycogenesis?

COME BACK TO THIS ONE

What is the second step of Glycogenesis?

Formation of UDP-glucose

Glucose 1-Phosphate is converted into UDP-glucose via UDP-Glucose Pyrophosphorylase

Glucose 1-Phosphate + UTP —> UDP-glucose + PPi

What is the main function of UDP-glucose Pyrophosphorylase?

Enzyme that converts Glucose 1-Phosphate into UDP-glucose

What is the main function of UDP-glucose?

Sugar nucleotide that donates the Glucose necessary for Glycogenesis

What are Sugar Nucleotides?

Compounds in which the anomeric carbon of a sugar is activated via attachment to a nucleotide through a phosphate ester linkage

What is the Anomeric Carbon?

Carbon that opens and closes to form the monosaccharide ring structure

What is the main function of Glycogenin?

Protein that serves as a primer to start Glycogenesis