1. Metabolism: Glycolysis

3. Glycolysis
1897: Hans and Eduard Buchner (Sucrose cell-free experiments; fermentation can take place outside of living cells) METABOLISM became simple chemistry
Glycolysis: “Embden-Meyerhof pathway”

4. The all-important Glucose
The only fuel the brain uses in non-starvation conditions
The only fuel red blood cells can use
WHY?
Evolutionary: probably available for primitive systems (from formaldehyde)
Low tendency to glycosylate proteins, strong tendency to exist in ring form (recall: all equatorial!)

5. The products and their fates

6. Sidebar: Fermentation
WHY? (lower energy yield)
Oxygen is not required!
OBLIGATE ANAEROBES (Cannot live with oxygen)
FACULTATIVE ANAEROBES (Can live with or without oxygen)

7. Glycolysis: a 10-step pathway
Three major stages
Glucose (G) to Fructose-1,6-bisphosphate (F-1,6-BP)
Three steps, INVESTMENT of 2 ATP
F-1,6-BP to two three-carbon fragments
Two component steps to glyceraldehde-3-phosphate (G3P)
G3P to Pyruvate
Five steps, YIELD of 4 ATP

11. RXN 1: Phosphorylation of G
The first reaction - phosphorylation of glucose
Hexokinase or glucokinase
This is a priming reaction - ATP is consumed here in order to get more later
ATP makes the phosphorylation of glucose spontaneous
Hexokinase (and glucokinase) act to phosphorylate glucose and keep it in the cell
ATP is used, thus FIRST PRIMING REACTION G large, negative

13. RXN 2: G6P to F6P
Isomerization of glucose-6-phosphate to fructose-6-phosphate
By phosphoglucose isomerase
Why does this reaction occur??
next step (phosphorylation at C-1) would be tough for hemiacetal -OH, but easy for primary -OH
isomerization activates C-3 for cleavage in aldolase reaction

15. RXN 3: F6P to F-1,6-BP
Fructose-6-P to Fructose-1,6-bisphosphate = the “COMMITTED STEP”
By phosphofructokinase (PFK)
ATP is used, thus SECOND PRIMING REACTION G large, negative

16. On PFK
Committed step and large, neg delta G - means PFK is highly regulated
ATP inhibits, AMP reverses inhibition
Citrate is also an allosteric inhibitor
Fructose-2,6-bisphosphate is allosteric activator
PFK increases activity when energy status is low
PFK decreases activity when energy status is high

17. Recall Stage 1

18. RXN 4: C6 cleaves to 2 C3s (DHAP, Gly-3-P)
Done by Aldolase

19. RXN 5: DHAP converted to Gly-3-P
Triose phosphate isomerase is used
Important: Glu and His in active site; mechanism through ene-diol intermediate

21. Recall stage 2

22. So far… We’ve USED UP 2 ATP molecules to process 1 glucose molecule
We are left with 2 G3P now
Time for energy payback, thus STAGE 3!
Recall that stage 3 happens in parallel to the two G3P molecules

23. RXN 6: G3P is oxidized to 1,3-BPG
Glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate
By glyceraldehyde 3-phosphate dehydrogenase

24. RXN 7: 1,3-BPG to 3-PG 1,3-bisphosphogycerate to 3-phosphoglycerate
By phosphoglycerate kinase (ATP yield!)

25. RXN 8: 3PG to 2PG Simply Phosphoryl group from C-3 to C-2
Done by phosphoglycerate mutase
Rationale for this enzyme - repositions the phosphate to make PEP

26. RXN 9: 2PG to PEP 2-phosphoglycerate to phosphoenolpyruvate By Enolase
"Energy content" of 2-PG and PEP are similar
Enolase just rearranges to a form from which more energy can be released in hydrolysis

27. RXN 10: PEP to Pyruvate
By pyruvate kinase
These two ATP (from one glucose) can be viewed as the "payoff" of glycolysis
Large, negative G - regulation!
Allosterically activated by AMP, F-1,6-bisP
Allosterically inhibited by ATP and acetyl-CoA

28. Review stage 3

30. Energetics of Glycolysis
The elegant evidence of regulation!
Standard state G values are scattered: + and -
G in cells is revealing:
Most values near zero
3 of 10 Rxns have large, negative  G
Large negative  G Rxns are sites of regulation!

31. GLYCOLYSIS NET Glucose + 2Pi + 2 ADP + 2 NAD+ ->
2 pyruvate + 2 ATP + 2 NADH + 2H+ + 2H2O

32. What Now?: The Fate of NADH and Pyruvate
Aerobic or anaerobic, that is the question.
NADH is energy - two possible fates:
If O2 is available, NADH is re-oxidized in the electron transport pathway, making ATP in oxidative phosphorylation
In anaerobic conditions, NADH is re-oxidized by lactate dehydrogenase (LDH), providing additional NAD+ for more glycolysis

33. The Fate of NADH and Pyruvate
Pyruvate is also energy - two general possible fates (AEROBIC/ANAEROBIC):
aerobic: citric acid cycle
anaerobic: to lactate (lactic acid fermentation) or to ethanol (alcoholic fermentation)