Glycolysis

Glycolysis Overview • The Glycolytic pathway describes the oxidation of

glucose to pyruvate with the generation of ATP and NADH

• Glycolysis is a universal pathway; present in all organisms: from yeast to mammals.

• In eukaryotes, glycolysis takes place in the cytosol• Glycolysis is anaerobic; it does not require oxygen.

• In the presence of O2, pyruvate is further oxidized to CO2.

• In the absence of O2, pyruvate can be fermented to lactate or ethanol.

Glycolysis

Eric NiederhofferSIU-SOM

Glucose

Pyruvate Lactate

10 steps

No O2

Energy (ATP) and metabolites

3 regulated steps

Net Reaction: Glucose + 2NAD+ + 2 Pi + 2 ADP = 2 pyruvate + 2 ATP + 2 NADH + 2 H2O

Glycolysis summary•Inputs:

•Glucose•2 NAD+•2 ATP

•4 ADP + 4P

•Outputs:•2 pyruvate•2 NADH•2 ADP

•2 ATP (net gain)

Regulation of Glycolysis

• Steps 1, 3 and 10 are regulatory

1. First regulatory enzyme – Hexokinase (Step 1)

• Hexokinase catalyses phosphorylation of Glucose > G-6-P

• First reversible step of glycolysis

• Glucose >>>>>>> G-6-P (lots) (rate of glycolysis low) • so Hexokinase inhibited (Feed back inhibition)

• Glucose enters cell but since the hexokinase inactive it can readily diffuse back to the blood through the glucose transporter in the plasma membrane.

• This step uses ATP so prevent wastage.

Step 2 Phosphofructokinase

• Is the rate limiting enzyme

• Is inhibited by [H+]• Production of lactate leads to an increase in [H+] so

Phosphofructokinase is inhibited. During vigorous exercise all the lactate can not be removed by the blood leading to a increase in [H+] causing muscle pain.

• Is inhibited by ATP but stimulated by AMP and Pi• This shows that this enzyme is stimulated when the cell

needs energy.

• Is inhibited by citrate• Is an early intermediate in the TCA cycle. When the TCA

cycle rises citrate is transported out of the mitochondria into the cytosol and inhibits Phosphofructokinase and so slows glycolysis.

Step 10

• Pyruvate Kinase

Summary of 10 steps

Step Importance Enzyme Type

1 Traps glucose inside cellATP investmentRegulated 1

Hexokinase Phosphoryl transfer

2 Rearrangement Phosphoglucose isomerase

Isomerization

3 Rate limiting step ie committed stepATP investment (second)Major regulation 2

Phosphofructokinase Phosphoryl transfer

4 Get two, 3 carbon compounds Aldolase Aldol cleavage

5 Freely - interconversion Triose phosphate isomerase

Isomerization

6 NADH formed i.e. first energy trapping reaction.

Glyceraldehyde 3-phosphate dehydrogenase

Phosphorylation coupled to oxidation

7 ATP generation (2)Substrate level Phosphorylation

Phosphoglycerate kinase

Phosphoryl transfer

8 Phosphoryl shift Phospoglyceromutase

Phosphoryl shift

9 Energy trapping reaction – in the form of PEP. (used to generate ATP in the next rxn)

Enolase Dehydration

10 ATP generation (2)Regulated 3

Pyruvate kinase Phosphoryl transfer

Regulation

Step Enzyme Inhibitors Stimulators

1 Hexokinase Epinephrine + Glucagon (low blood glucose)

Insulin

3 Phosphofructokinase ATP[H+]

citrate

AMPF-2,6-BP(learn in detail)

10 Pyruvate kinase ATPGlucagon

F1,6-BP (remember is product of Phosphofructokinase)