12-13 glycolysis

Why is glucose a central energy source for many cells?

Its oxidation releases usable energy, it can be stored as glycogen, and glycolysis works without requiring oxygen.

What are four major fates of glucose shown in the lecture?

(1) Glycolysis → pyruvate; (2) pentose phosphate pathway → ribose-5-phosphate; (3) glycogen/starch/sucrose storage; (4) synthesis of structural polysaccharides.

Why is glucose also considered a biochemical precursor?

Its carbon skeletons feed biosynthesis of molecules such as amino acids, membrane lipids, nucleotides, and cofactors.

What is glycolysis, and where does it occur?

A 10-reaction pathway in the cytosol that converts one 6-carbon glucose into two 3-carbon pyruvates; it does not directly require O₂.

Which steps are the investment phase, and which are the payoff phase?

Steps 1–5 are the investment/splitting phase; steps 6–10 are the payoff phase. After step 5, two GAP molecules run through the remaining reactions.

Step 1 — Draw the transformation and name the enzyme, substrate, product, and ATP use.

Hexokinase: glucose + ATP → glucose-6-phosphate (G6P) + ADP. Phosphate is added to C6, trapping glucose in the cell. Mg²⁺ assists; essentially irreversible.

Step 2 — How does glucose-6-phosphate become fructose-6-phosphate?

Phosphohexose isomerase converts G6P ⇌ F6P. It rearranges an aldose into a ketose (carbonyl at C1 → C2). Reversible; enediol intermediate.

Step 3 — Draw the phosphorylation catalyzed by PFK-1.

Phosphofructokinase-1 (PFK-1): F6P + ATP → fructose-1,6-bisphosphate (F1,6BP) + ADP. Adds phosphate to C1. Essentially irreversible; first committed step.

Step 4 — Which carbon skeleton is split by aldolase, and into what products?

Aldolase cleaves F1,6BP (6 carbons) into glyceraldehyde-3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP), both 3-carbon sugars.

Step 5 — What does triose phosphate isomerase do?

Triose phosphate isomerase converts DHAP ⇌ GAP (ketose → aldose). Now one starting glucose yields 2 GAP, so steps 6–10 occur twice.

Step 6 — What changes chemically during GAP dehydrogenase? Identify the coenzymes.

GAP dehydrogenase: GAP + Pi + NAD⁺ ⇌ 1,3-bisphosphoglycerate + NADH + H⁺. The aldehyde is oxidized; inorganic phosphate is attached. Occurs twice per glucose.

Step 7 — Which enzyme makes the first ATP in glycolysis?

Phosphoglycerate kinase transfers phosphate from 1,3-bisphosphoglycerate to ADP → ATP + 3-phosphoglycerate. This is substrate-level phosphorylation, twice per glucose.

Step 8 — What chemical rearrangement does phosphoglycerate mutase catalyze?

3-phosphoglycerate ⇌ 2-phosphoglycerate. The phosphate shifts from carbon 3 to carbon 2; the carbon skeleton is otherwise unchanged.

Step 9 — How does enolase convert 2-phosphoglycerate into PEP?

Enolase removes H₂O (dehydration), producing phosphoenolpyruvate (PEP) with a high-energy enol phosphate. This occurs twice per glucose.

Step 10 — Which molecule donates phosphate to ADP, and what is the final product?

Pyruvate kinase transfers phosphate from PEP to ADP → ATP + pyruvate. Substrate-level phosphorylation; essentially irreversible; happens twice per glucose.

Which glycolytic steps involve an ATP/NAD⁺ cofactor, and what are their roles?

ATP is consumed at steps 1 and 3; NAD⁺ is reduced to NADH at step 6; ADP is phosphorylated to ATP at steps 7 and 10. Mg²⁺ supports several phosphate-transfer enzymes.

Draw the three distinct chemical features of G6P, F6P, and F1,6BP.

G6P: 6-carbon aldose with a phosphate on C6. F6P: 6-carbon ketose with phosphate on C6. F1,6BP: ketose with phosphates on C1 and C6. The carbonyl shifts C1 → C2 in step 2.

Why can glycolysis proceed in the absence of oxygen?

None of its ten reactions directly consumes O₂. However, glycolysis needs a continuing supply of NAD⁺ to sustain the GAP dehydrogenase step.

Why must NADH produced by glycolysis be reoxidized to NAD⁺?

GAP dehydrogenase requires NAD⁺. If NAD⁺ runs out, step 6 stops and glycolysis cannot keep producing ATP.

Compare the fate of pyruvate under aerobic conditions versus low-oxygen muscle conditions.

With sufficient O₂, pyruvate can be oxidized further through acetyl-CoA and the TCA cycle; when oxygen delivery is limiting, pyruvate can be reduced to lactate to regenerate NAD⁺. [Source note: The lactate dehydrogenase mechanism is needed for the practice-final application but is not detailed in this PDF.]

Practice final Q9: Your legs burn during a sprint. What produces lactate, which enzyme is responsible, and WHY?

Lactic acid fermentation: lactate dehydrogenase converts pyruvate + NADH + H⁺ → lactate + NAD⁺. The purpose is to regenerate NAD⁺ so glycolysis continues making ATP rapidly. [Source note: Practice-final application; lactate dehydrogenase details are beyond the text of these lecture slides.]

Practice final Q9 follow-up: Where can circulating lactate go, and what happens to its carbon?

The liver can take up lactate and convert it into glucose by gluconeogenesis; glucose can return to tissues. This is the Cori cycle. [Source note: Cori-cycle detail comes from the Fall 2024 practice-final question rather than these glycolysis slides.]

Write the net products of glycolysis per one glucose.

2 pyruvate + 2 ATP net + 2 NADH + 2 H₂O + 2 H⁺ (with 2 NAD⁺, 2 ADP, and 2 Pi consumed overall).

Where are the 2 ATP invested, and why are they invested?

Step 1 (hexokinase) and step 3 (PFK-1), each use one ATP. These phosphorylations trap/prime the six-carbon sugar before splitting.

Where are the 4 ATP generated, and why does that mean only 2 net ATP?

Steps 7 and 10 make ATP. Each occurs twice per glucose (2 GAP), producing 4 total ATP. Subtract 2 ATP invested → 2 ATP net.

At which glycolytic step are NADH molecules generated? How many per glucose?

Step 6, glyceraldehyde-3-phosphate dehydrogenase. Each GAP makes one NADH; two GAP per glucose → 2 NADH. No FADH₂ is made in glycolysis.

At the end of step 7, how many ATP have been made and what is the net ATP balance so far?

Two ATP have been made (one from each 1,3-BPG). Because two ATP were used in steps 1 and 3, the running net balance is 0 ATP.

Practice final Q3: If one glycerol enters as one GAP after costing 1 ATP, what does the GAP-to-pyruvate segment yield?

From one GAP: 1 pyruvate, 1 NADH, 2 ATP gross, 0 FADH₂. Subtract the stated 1-ATP entry cost → net 1 ATP through pyruvate. [Source note: This answer counts only the glycolysis segment; the practice-final asks for complete oxidation, addressed in the next card.]

Practice final Q3: Fully oxidize one glycerol entering as one GAP (1 ATP entry cost), but do NOT count ETC ATP. Totals?

5 NADH (1 from GAP→pyruvate, 1 from pyruvate dehydrogenase, 3 from TCA); 1 FADH₂ (TCA); net 2 ATP/GTP (2 glycolytic ATP + 1 TCA GTP − 1 entry ATP). [Source note: Requires pyruvate dehydrogenase and TCA-cycle material not taught in this particular PDF. Assumes standard complete oxidation of one 3-carbon pyruvate.]

For an enzyme-coupled step, how do you obtain the overall ΔG°′? Use hexokinase as the example.

Add component free-energy changes: glucose phosphorylation +13.8 kJ/mol plus ATP hydrolysis −30.5 kJ/mol = net −16.7 kJ/mol. A favorable coupled reaction can drive an unfavorable step.

Which three glycolysis steps are essentially irreversible and tightly regulated?

Step 1 hexokinase, step 3 PFK-1, and step 10 pyruvate kinase. Their large negative ΔG values make them regulatory control points.

How does accumulation of glucose-6-phosphate regulate hexokinase?

Product feedback inhibition: high G6P inhibits hexokinase, slowing additional glucose phosphorylation and uptake into glycolysis.

Why is PFK-1 the first committed step rather than hexokinase?

PFK-1 makes F1,6BP, committing carbon to glycolysis. G6P formed by hexokinase still has other fates (e.g., glycogen synthesis and pentose phosphate pathway).

What activates or inhibits PFK-1, and what does this reveal about cell energy status?

High ATP inhibits (high energy); AMP/ADP and fructose-2,6-bisphosphate activate (energy demand/glucose availability). The slide also shows citrate as an inhibitor.

ATP is both PFK-1’s substrate and inhibitor. How can the same molecule do both?

ATP binds the catalytic active site as phosphate donor, but at high levels ATP also binds an allosteric regulatory site and lowers PFK-1 activity.

How is pyruvate kinase regulated in the lecture?

It is inhibited by ATP, acetyl-CoA, and alanine, which signal that additional pyruvate production is less needed.

At rest versus during exercise, how does muscle glycolysis respond and why?

At rest, high energy charge favors inhibitory signals. During exercise, ATP is consumed and AMP/ADP rise, stimulating glycolysis so ATP production increases.

Why can PFK-1 show a sigmoidal response to F6P instead of a standard hyperbolic Michaelis–Menten curve?

PFK-1 is allosterically regulated. Its activity depends on effector-dependent conformational changes as well as substrate concentration; the lecture shows ATP-dependent shifts.

What hormone is released when blood glucose is high, and what is its effect on liver glycolysis?

Insulin signals the fed/high-glucose state and promotes hepatic glycolysis and glucose use or storage.

What hormone signals low blood glucose, and how does liver glycolysis respond?

Glucagon signals low blood glucose, reducing hepatic glycolysis and favoring glucose export to restore blood glucose.

How should you describe epinephrine’s effect on glucose metabolism without assuming it acts identically everywhere?

Epinephrine mobilizes glucose fuel during stress/exercise. In skeletal muscle it can support glycolysis by increasing fuel availability; in liver it favors glucose mobilization/release. [Source note: Epinephrine is named in the learning objectives but its tissue-specific mechanism is not explained in the PDF; this is supporting physiology context.]

Application: Compare liver metabolism after a carbohydrate-rich meal versus a prolonged fast.

Fed: insulin dominates → liver uses/stores glucose; glycolysis tends to rise. Fasting: glucagon dominates → liver lowers glycolytic flux and conserves/releases glucose.

What exact reaction does PFK-2 catalyze?

PFK-2 converts fructose-6-phosphate (F6P) into fructose-2,6-bisphosphate (F2,6BP), a regulatory signaling metabolite.

What does fructose-2,6-bisphosphate do to PFK-1?

It allosterically activates PFK-1, encouraging F6P → F1,6BP and increasing glycolysis.

Is F2,6BP an intermediate of the 10-step glycolysis pathway? Why does that distinction matter?

No. F2,6BP is a regulator, not the step-3 product. The glycolytic intermediate F1,6BP is made by PFK-1, while F2,6BP is made by PFK-2.

Predict: If liver F2,6BP falls, what happens to PFK-1 activity and glycolysis?

PFK-1 loses an allosteric activating signal, so hepatic glycolysis decreases. This supports the low-glucose/glucagon response.

Compare PFK-1 and PFK-2: what do they make and why?

PFK-1: F6P → F1,6BP (committed glycolytic reaction, consumes ATP). PFK-2: F6P → F2,6BP (regulatory metabolite, promotes PFK-1 activity).