Glycolysis

Glycolysis

Glycolysis is the process by which glucose is oxidized to pyruvate (to enter TCA cycle), producing ATP

tissues without mitochondria (RBC’s, renal medulla) are entirely dependent upon glycolysis for ATP

Glycolysis takes place in two steps:

(1) Glucose Þ Fructose-1,6-diphosphate – six carbon species, requires energy

(2) Glyceraldehyde-3-Phosphate Þ Pyruvate – three carbon species (two for each glucose), provides energy

The Glycolytic Pathway

Enzymes in Glycolysis

Nine main enzymes (Note: an asterix (*) indicates regulated steps)

(1) Hexokinase – in every cell – K_m = 0.1, so the enzyme is usually saturated

Glucokinase is liver hexokinase – K_m = 10 mM, so liver absorbs much more sugar than other tissues

regulated differently – glucose-6-phosphate inhibits hexokinase but not glucokinase

glucose enters cell through GLUT receptors – GLUT2 (liver B cells), GLUT4 (requires insulin – muscle and fat cells)
hexokinase/glucokinase does not work in reverse, so phosphorylation effectively traps glucose inside the cell

(2) Phosphoglucose Isomerase – reversable and unregulated – driven by mass action

*(3) Phosphofructokinase-1 (PFK-1) – irreversable, D G << 0 – first committed step to glycolysis, so very tightly controlled

regulation: ATP (-), citrate (-) (both indicate that enough energy already) – AMP (+), Fructose-2,6-diphos.(+)

ATP and AMP both elevated in energy depleted states – 2ADP Þ ATP + AMP (adenylate kinase)

AMP rescues PFK function at high ATP levels

PFK-2 – reversably makes fructose-2,6-dP from fructose-6-P when glucose levels are high

liver isomer of PFK-2 is inhibited by cAMP (stop liver glycolysis when fasting); muscle isomer is not

(4) Aldolase – reversable – produces two three-carbon molecules (G3P and DHAP)

(5) G3P Dehydrogenase – reversable, but driven forward by low NADH/NAD⁺ ratio – NAD⁺ must be quickly regenerated

(6) 3-Phosphoglycerate Kinase – produces ATP (two per glucose, since two three-carbon species were formed)

alternate pathway: diphosphoglycerate mutase – 1,3-diP-glycerate to 2,3-dP-glycerate (BPG) in RBC’s

(7) Phosphoglyceromutase – reversable activated by BPG in RBC’s

(8) Enolase – reversable

*(9) Pyruvate Kinase – irreversable, D G << 0 – different isoforms in muscle and liver are regulated differently

muscle: ATP(-) – already enough energy; liver: cAMP(-), alanine(-), ATP(-) – stop glycolysis during fasting
both isoforms are activated if earlier steps in glycolysis are occuring – Fructose-1,6-bisphosphate(+)

Energetics of Glycolysis

Each molecule of glucose yields:

(1) 2 ATP – two used during the six-carbon leg, but four made during the three-carbon leg

(2) 2 NADH – since glycolysis occurs in the cytosol, it must enter the mitochondria to be used in oxidative phosphorylation

phosphoglycerate shuttle – in muscle – NADH reduces DHAP to 3-phosphoglycerol (3P-glycerol dehydr.)

3P-glycerol enters mito and reduces FAD Þ FADH₂ on the membrane (via flavoprotein dehydrogenase)

thus each NADH produces only 2 ATP in muscle (FADH₂ is less energetic)

malate-aspartate shuttle – in liver – NADH reduces oxaloacetate (OAA) to malate (malate dehydrogenase)

malate enters mitochondria, where it is made back into oxaloacetate, producing NADH

malate Þ OAA Þ aspartate (via aspartate transaminase); aspartate is transpoted to cytosol, then aspartate Þ OAA

thus each NADH prodcues 3 ATP in liver (NADH retained)

(3) pyruvate – two principal fates:

1 to lactate (via lactate dehydrogenase) – does not require O₂, but uses 2 NADH (from above) – so 0 ATP

2 to acetyl CoA (via pyruvate dehydrogenase complex) – enter TCA cycle (12 ATP per turn) – so net 24 ATP

Therefore:

glucose Þ lactate – 2 ATP

glucose + O₂ Þ CO₂+ H₂O – 36 ATP in muscle, 38 ATP in liver difference due to different NADH shuttles