Glycolysis

1
Glycolysis
2
Cellular glucose transport

• Glucose gradient from extracellular to
  intracellular environment
• Glucose - polar molecule
• Membrane transporter required
• GLUT transporter family
• GLUT transporters in RBC, brain, liver and kidney
  cells unregulated
• GLUT transporters in skeletal muscle, fat and
  heart regulated
• Predominantly GLUT-4

3
Regulation of GLUT-4 transporter activity

• Insulin
• Binding to receptor signals translocation of
  GLUT-4 transporters from intracellular storage
  sites to membrane
• Muscle contraction
• Elevated calcium signals translocation of GLUT-4
  to membrane
• Glucose uptake displays Michaelis-Menten (ie.
  saturation) kinetics
• Insulin and contraction effects additive

From Houston ME (2001) Biochemistry Primer for
Exercise Science (2nd Ed.) Champaign Human
Kinetics p 85.
4
Glycolysis

• Glycolysis
• Occurs in cytosol
• Converts 6-C glucose to two 3-C pyruvate
• Utilises 2 ATP and yields 4 ATP
• Net gain 2 ATP

From Summerlin LR (1981) Chemistry for the Life
Sciences. New York Random House p 543.
5
Glycolysis - Step 1

• Glucose ? G-6-P
• Catalysed by hexokinase in skeletal muscle
  (glucokinase in liver)
• Non-equilibrium reaction
• Energy from ATP used to phosphorylate C6
• Traps glucose in cell

From Summerlin LR (1981) Chemistry for the Life
Sciences. New York Random House p 543.
6
Glycolysis - Step 1

• Glucokinase has higher Km for glucose than
  Hexokinase
• Ensures hexokinase has first call on circulating
  glucose
• Glucokinase
• provides G-6-P for glycogen synthesis
• Not allosterically inhibited by G-6-P
• Hexokinase
• Provides G-6-P for energy production
• Allosterically inhibited by G-6-P

From Mathews CK van Holde KE (1990)
Biochemistry. Redwood City Benjamin Cummings p
439.
7
Glycolysis - Step 2

• G-6-P ? Fructose-6-P
• Catalysed by Phosphoglucose isomerase
• Equilibrium reaction
• Generated hydroxyl group (-OH) at C1
• Alters energy distribution within molecule
• Allows phosphorylation of C1 in next step

From Summerlin LR (1981) Chemistry for the Life
Sciences. New York Random House p 543.
8
Glycolysis - Step 3

• F-6-P ? Fructose 1,6-Bisphosphate
• Catalysed by Phosphofructokinase
• Non-equilibrium reaction
• First non-reversible reaction unique to
  glycolytic pathway
• Committed step
• Primary control site
• Inhibited by ATP, citrate and H
• Stimulated by AMP, ADP, Epinephrine, Pi, NH4
• Adds phosphate group to C1
• prepares for cleavage into two phosphorylated
  molecules in next step

From Summerlin LR (1981) Chemistry for the Life
Sciences. New York Random House p 543.
9
Glycolysis - Step 4

• Cleavage of F 1,6-Bisphosphate
• Catalysed by Aldolase
• Equilibrium reaction
• Cleaves 6C molecule of F 1,6- Bisphosphatase to
  form two 3C molecules of DHAP and G-3-P
• G-3-P on direct glycolytic pathway
• DHAP and G-3-P in equilibrium
• 96 held as DHAP
• All reactions in pathway doubled from this point
  on

From Summerlin LR (1981) Chemistry for the Life
Sciences. New York Random House p 543.
10
Glycolysis - Step 5

• G-3-P ? 1,3-BPG
• Catalysed by G-3-P dehydrogenase
• Equilibrium reaction
• First reaction where energy produced (indirectly)
• G-3-P oxidised by removal of H
• NAD reduced to NADH
• NADH to ETC to give 3 ATP
• Oxidation provides energy to incorporate Pi into
  structure as high energy phosphate group
• Doubling of reaction gives 2 x NADH 6 ATP

From Summerlin LR (1981) Chemistry for the Life
Sciences. New York Random House p 543.
11
Glycolysis - Step 6

• 1,3-BPG ? 3-PG
• Catalysed by phosphoglycerate kinase
• Equilibrium reaction
• High energy phosphate group removed
• Energy release used to phosphorylate ADP to ATP
  (substrate level phosphorylation)
• Doubling of reaction gives 2 x ATP

From Summerlin LR (1981) Chemistry for the Life
Sciences. New York Random House p 543.
12
Glycolysis - Step 7

• 3-PG ? 2-PG
• Catalysed by phosphoglyceromutase
• Equilibrium reaction
• Phosphate group moved from C3 to C2

From Summerlin LR (1981) Chemistry for the Life
Sciences. New York Random House p 543.
13
Glycolysis - Step 8

• 2-PG ? PEP
• Catalysed by enolase
• Equilibrium reaction
• Dehydration causes redistribution of energy
  within molecule creating a high energy phosphate
  group

From Summerlin LR (1981) Chemistry for the Life
Sciences. New York Random House p 543.
14
Glycolysis - Step 9

• PEP ? Pyruvate
• Catalysed by Pyruvate kinase
• Non-equilibrium reaction
• High energy phosphate group removed
• Energy release used to phosphorylate ADP to ATP
  (substrate level phosphorylation)
• Intermediate step in reaction forms enolpyruvate
  which spontaneously converts to pyruvate
• Doubling of reaction yields 2 ATP

From Summerlin LR (1981) Chemistry for the Life
Sciences. New York Random House p 543.
15
Lactic acid

• NADH produced at Step 5 reoxidised to NAD in ETC
• If unable to oxidise NADH glycolysis would stop
• Energy only put in prior to step 5
• Would be energy consuming pathway
• NADH usually oxidised in ETC
• NADH also oxidised by LDH reducing pyruvate to
  lactate

From Summerlin LR (1981) Chemistry for the Life
Sciences. New York Random House p 543.
16
Lactic acid

• Lactate has numerous fates
• Remain within cell until can be reoxidised to
  pyruvate
• Released from cell
• Taken up by other less active fibres in same
  muscle
• Enter circulation to be taken up by
• other less active fibres in other skeletal muscle
• Heart for oxidation
• Liver for conversion to glucose via Cori cycle
• Release of lactate from skeletal muscle
  represents means of redistributing CHO stores
  throughout body
• Lactate shuttle hypothesis