Lecture 25 - Catabolic Pathways/Glycolysis

How is glucose pulled into the cell

By transporter

• active transport

• requires ATP

5 principles of metabolic pathways

1. series of separate reactions

2. each reaction is catalyzed by a specific enzyme

3. many metabolic pathways are similar in all organisms (conserved function)

4. in eukaryotes, metabolic pathways are compartmentalized in specific organelles

5. key enzymes can be inhibited or activates to alter the rate of the pathway (allosteric)

What is glycolysis

Oxidation of glucose

• catabolic

• very highly exergonic

• free energy releases is harvested + used to drive anabolic processes (makes ATP)

What three processes are used to harvest energy from glucose

• glycolysis - anaerobic

• cellular respiration - aerobic (O₂ available)

• fermentation - anaerobic (No O₂ available)

Glucose is partially oxidized by the end of ________

glycolysis

Which process is not favored to produce ATP

Fermentation, because net production of ATP is 2

Redox reactions

transfer of electrons

OIL RIG

Oxidation is loss

Reduction is gain

Couple reactions, allows occur together

• when something is oxidized in a reaction, the partner has to be reduced

When a molecule loses H atoms it becomes oxidized

Move from long, weaker, nonpolar bonds to more stable, shorter, polar bonds

• no energy left to extract

• all hydrogens have been removed, electrons taken

Coenzme NAD⁺/NADH electron carrier

NAD⁺ is oxidized form

• overall charge on nitrogen is positive

• nicotinamide ring

H⁺ and 2e^- are transferred

WhNADH is reduced form

• loses overall positive charge

  • because 1e^- neutralizes positive charge of nitrogen

Where does NAD⁺ come from

Vitamin B3 (niacin)

• mobile carrier

• cofactor

What does terminal electron acceptor O₂ accept electrons from

NADH

NADH becomes oxidized to NAD⁺, it is recycled and available to take more electrons in glycolysis

Where does glycolysis occur

cytosol

Where does pyruvate oxidation occur

Mitochondria

Where does Citric Acid Cyle (CAC) occur

Mitochondria

Where does ETC/ATP synthesis occur

Mitochondria

Where does Fermentation occur

Cytosol

Where does complete oxidation of glucose occur

CAC

Glycolysis

• 10 steps

  • steps 1-5 are energy investing

  • steps 6-10 are energy harvesting

• occurs in cytosol

• converts glucose into 2 molecule of pyruvate

• produces 2 ATP and 2 NADH

Glycolysis Step 1

Glucose is a 6 carbon sugar

Phosphorylation occurs on C-6

• 1 ATP invested

Cannot occur on C-1 because it is a carbonyl group and cannot be phosphorylated (due to double bond)

Forms Glucose 6-phosphate

Glycolysis Step 2

Now, the glucose-6 phosphate needs to be symmetrical on the carbon-1, but can’t due to carbonyl oxygen

So the doubled bonded O and OH switch positions

Forms Fructose 6-phosphate

• ketose

Glycolysis Step 3

Now, carbon-1 can be phosphorylated

1 ATP invested

Forms Fructose 1-6 biphosphate

Glycolysis Step 4

Carbonyl group at C-2 (double bond) facilitates C-C bond cleavage

Forms 2 3-carbon products

1. Dihydroxyacetone phosphate (DHAP)

2. Glyceraldehyde 3-phosphate

Glycolysis Step 5

Dihydroxyacetone phosphate is interconverted into glyceraldehyde 3-phosphate

Now, there are 2, identical Glyceraldehyde 3-phosphate products

Glycolysis Step 6

oxidative phosphorylation of glyceraldehyde 3-phosphate occurs

1 NADH produced —> reduced form, goes to ETC

Forms 1-3- biposphoglycerate

Glycolysis Step 7

ATP produced, 1 phosphate removed

Glycolysis Step 8

Remaining phosphoryl group moved from C-3 to C-2

Forms 2-phosphoclycerate

Glycolysis Step 9

Dehydration activates phosphoryl for transfer in step 10

Forms phosphoenolpyruvate

Glycolysis Step 10

Phosphate transferred to another ATP - 1 ATP produced

Final produce is Pyruvate

Steps 6-10 occurs 2x

So Net NADH and ATP production is 2 of each (1 during each round)