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Glycolysis

Why cells need energy

Energy supports:

  • transport
  • mitosis
  • detoxification
  • growth
  • movement
  • synthesis

Digestion removes electrons from atoms, then feeds them into the electron transport chain (ETC).

Where glycolysis occurs

  • in the cytoplasm

ATP basics

  • ATP stores energy in phosphoanhydride bonds (bonds between phosphate groups)

Metabolism vocabulary

  • catabolism: break down
  • anabolism: fuse together

Cellular respiration (big picture)

Energy-yielding nutrients (carbohydrates, fats, proteins), with or without O\(_2\), produce ATP.

It is multi-step to increase energy-harvest efficiency.

Substrate-level phosphorylation

  • creates a high-energy intermediate
  • breaks the high-energy intermediate bond to form the ATP bond

Oxidative phosphorylation

  • uses an energy gradient from electron transfer to make ATP

Where each macronutrient enters

Carbohydrates

  • monosaccharides enter glycolysis (stage 1)

Proteins

  • amino acids enter at end of stage 2 (pyruvate oxidation → acetyl-CoA) or stage 3 (citric acid cycle)

Fat

  • glycerol backbone enters glycolysis
  • fatty acids enter at end of stage 2 (acetyl-CoA)

Reaction coupling

Couples an endergonic reaction to an exergonic reaction.

Example:

  1. glucose + P\(_i\) → glucose-6-phosphate + H\(_2\)O, \(\Delta G = 3.3\,\text{kcal/mol}\)
  2. ATP + H\(_2\)O → ADP + P\(_i\), \(\Delta G = -7.3\,\text{kcal/mol}\)

Sum:

  • \(\Delta G = -4.0\,\text{kcal/mol}\)

Coenzymes (often vitamin-derived)

NAD\(^+\)

  • nicotinamide adenine dinucleotide (from B3)
  • reduced to NADH by accepting a hydride ion (H\(^+\) + 2e\(^-\))

FAD

  • flavin adenine dinucleotide (from B2)
  • reduced to FADH\(_2\) by accepting 2 protons and 2 electrons

Glycolysis overview

  • cytoplasmic, carbohydrate-specific pathway
  • overlaps with other pathways
  • converts glucose into high-energy phosphorylated intermediates that can phosphorylate ADP to ATP

Steps of glycolysis

1) Glucose entry

  • enters the cell via GLUTs (insulin-dependent for muscle and fat cells)

2) Hexokinase / glucokinase

  • glucose + ATP → glucose-6-phosphate (G6P) + ADP
  • enzyme: hexokinase
  • adds phosphate to the C\(6'\) position of glucose
  • \(\Delta G\) is negative; traps glucose in the cell
  • produces proton

Hexokinase isoforms:

  • hexokinase I, II, III: found widely; regulated by product inhibition
  • glucokinase (hexokinase IV): liver (storage) and pancreas (sensor); high \(K_m\), high \(V_{max}\)

3) Phosphoglucose isomerase

  • glucose-6-phosphate → fructose-6-phosphate (aldo- to keto-)

4) PFK-1 (committed step)

  • fructose-6-phosphate + ATP → fructose-1,6-bisphosphate (F-1,6-BP) + ADP
  • enzyme: phosphofructokinase-1 (PFK-1)
  • removes \(\gamma\) phosphate
  • irreversible; committed to glycolysis; speed control
  • produces proton

Regulation:

  • 2 substrate sites (ATP, F-6-P)

  • 4 allosteric sites

  • inhibitors: ATP, citrate (TCA cycle)

  • activators: F-2,6-BP, AMP
  • F-2,6-BP made by PFK-2; accelerates PFK-1 (overwrites ATP inhibition)

5) Aldolase

  • F-1,6-BP → glyceraldehyde-3-phosphate (GAP) + dihydroxyacetone phosphate (DHAP)

6) Triose phosphate isomerase

  • DHAP ↔ GAP

7) GAP dehydrogenase

  • GAP + P\(_i\) + NAD\(^+\) → 1,3-bisphosphoglycerate (1,3-BPG) + NADH
  • enzyme: glyceraldehyde-3-phosphate dehydrogenase
  • produces proton
  • 1,3-BPG is high-energy

8) Phosphoglycerate kinase (substrate-level phosphorylation)

  • 1,3-BPG + ADP → 3-phosphoglycerate + ATP
  • enzyme: phosphoglycerate kinase

Note:

  • steps 7 and 8 are coupled

9) Phosphoglycerate mutase

  • 3-phosphoglycerate → 2-phosphoglycerate

10) Enolase

  • 2-phosphoglycerate → phosphoenolpyruvate (PEP) + H\(_2\)O

11) Pyruvate kinase

  • PEP + ADP → pyruvate + ATP
  • enzyme: pyruvate kinase
  • regulated

Regulation notes:

  • in liver, phosphorylated pyruvate kinase is less active than dephosphorylated pyruvate kinase

  • glucagon promotes phosphorylation

  • insulin promotes dephosphorylation

  • allosteric regulation:

  • F-1,6-BP activates (feed-forward)

  • ATP inhibits

Fates of NADH and pyruvate

With O\(_2\)

  • NADH enters ETC
  • pyruvate enters citric acid cycle

Without O\(_2\)

  • NADH and pyruvate are converted back to NAD\(^+\) and lactate via lactate dehydrogenase

  • lactate can be transported to other parts of the body

  • liver: gluconeogenesis converts it back to glucose

  • muscle: converts it to pyruvate

Alcohol dehydrogenase note

  • uses NAD\(^+\) to convert ethanol → acetaldehyde
  • increases NADH, which pushes pyruvate → lactate

Other sugar entry points

Galactose

  • enters via glucose-6-phosphate

Fructose

  • enters via:

  • fructose-1-phosphate (via fructokinase), or

  • fructose-6-phosphate (via hexokinase, less)

Fructose-1-phosphate pathway:

  • fructose-1-phosphate cleaved by fructose-1-phosphate aldolase into:

  • DHAP (then triose phosphate isomerase → GAP)

  • glyceraldehyde (then triose kinase → glyceraldehyde-3-phosphate)

Inhibitors

Fluoride

  • blocks enolase by forming a complex with Mg\(^{2+}\)
  • disrupts proper positioning of 2-phosphoglycerate

Arsenate

  • structural analog of phosphate
  • causes cancers
  • dysregulates glycolysis by acting as an analog

  • replaces phosphate and prevents formation of 1,3-bisphosphoglycerate

  • then ADP → ATP conversion in the next step is lost (substrate-level phosphorylation)