Glycolysis: Key Steps, Enzymes, and Regulation in Cellular Respiration

What is the primary definition of Glycolysis?

The conversion of 1 glucose to 2 pyruvates, yielding 2 ATP molecules.

How many enzymatic steps are in the Glycolysis pathway?

10 steps.

Glycolysis converts one C6 hexose into how many C3 molecules?

Two C3 triose molecules (pyruvates).

What are the two main stages of Glycolysis?

Stage I: Energy Investment and Stage II: Energy Recovery.

How many ATP molecules are consumed during the energy investment stage?

2 ATP.

How many ATP molecules are produced during the energy recovery stage?

4 ATP.

What is the net ATP 'profit' of glycolysis?

2 ATP.

What is the net chemical equation for glycolysis?

Glucose + 2NAD+ + 2ADP + 2Pi → 2Pyruvate + 2NADH + 2ATP + 2H2O + 4H+.

Which steps in glycolysis are irreversible?

Steps 1 (Hexokinase), 3 (PFK), and 10 (Pyruvate Kinase).

Where in the cell does glycolysis occur?

In the cytosol.

What enzyme catalyzes the phosphorylation of glucose?

Hexokinase.

What is the product of the Hexokinase reaction?

Glucose-6-phosphate (G6P).

Why is G6P trapped in the cell?

The negative charge of the phosphate prevents it from crossing the plasma membrane.

What divalent cation is required by Hexokinase?

Mg2+ (to shield negative charges on ATP).

What enzyme converts G6P to Fructose-6-phosphate (F6P)?

Phosphoglucose Isomerase (PGI).

What type of reaction is the PGI step?

Isomerization (Aldose to Ketose).

What enzyme catalyzes the phosphorylation of F6P?

Phosphofructokinase-1 (PFK-1).

What is the product of the PFK reaction?

Fructose-1,6-bisphosphate (FBP).

Why is PFK considered the 'rate-limiting' step?

It is the primary control point and the first committed step of glycolysis.

What enzyme cleaves FBP into two trioses?

Aldolase.

What are the two products of the Aldolase reaction?

Glyceraldehyde-3-phosphate (GAP) and Dihydroxyacetone phosphate (DHAP).

What enzyme interconverts DHAP and GAP?

Triose Phosphate Isomerase (TIM).

Why is TIM considered a 'perfect' enzyme?

Its rate is limited only by diffusion.

At the end of Stage I, how many GAP molecules exist per glucose?

Two GAP molecules.

What enzyme oxidizes GAP?

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH).

What high-energy intermediate is formed by GAPDH?

1,3-Bisphosphoglycerate (1,3-BPG).

What electron carrier is reduced in this step?

NAD+ is reduced to NADH.

What toxic element can replace phosphate in this reaction?

Arsenate (AsO4^3-).

What enzyme produces the first 2 ATPs of glycolysis?

Phosphoglycerate Kinase (PGK).

What is the product of the PGK reaction?

3-Phosphoglycerate (3PG).

Define Substrate-Level Phosphorylation.

Synthesis of ATP from ADP by direct transfer of a phosphate group from a high-energy substrate.

What enzyme moves the phosphate from C3 to C2?

Phosphoglycerate Mutase (PGM).

What is the product of the PGM reaction?

2-Phosphoglycerate (2PG).

What enzyme dehydrates 2PG?

Enolase.

What is the high-energy product of the Enolase reaction?

Phosphoenolpyruvate (PEP).

What inhibits Enolase?

Fluoride (F-).

What enzyme catalyzes the final step of glycolysis?

Pyruvate Kinase (PK).

What are the final products of Step 10?

Pyruvate and ATP.

How many ATP are produced here per glucose?

2 ATP.

Which three enzymes are the major flux control points?

Hexokinase, Phosphofructokinase, and Pyruvate Kinase.

How does ATP affect PFK-1?

It is an allosteric inhibitor (high energy signals to slow down).

How does AMP affect PFK-1?

It is an allosteric activator (low energy signals to speed up).

What is the most potent allosteric activator of PFK-1 in the liver?

Fructose-2,6-bisphosphate (F2,6P).

How does Citrate affect PFK-1?

It inhibits PFK-1 (signals TCA cycle is saturated).

What is the difference between Hexokinase I and Hexokinase IV (Glucokinase)?

Glucokinase (liver) has a higher Km and is not inhibited by G6P.

What activates Pyruvate Kinase via 'feed-forward' regulation?

Fructose-1,6-bisphosphate (FBP).

What inhibits Pyruvate Kinase?

ATP and Alanine.

In the liver, how is Pyruvate Kinase inactivated?

Phosphorylation (triggered by glucagon/low blood sugar).

Why must NAD+ be regenerated under anaerobic conditions?

To allow the GAPDH step of glycolysis to continue.

What enzyme reduces pyruvate to lactate in muscles?

Lactate Dehydrogenase (LDH).

What is the product of homolactic fermentation?

Lactate.

What are the two steps of alcoholic fermentation?

1. Decarboxylation by Pyruvate Decarboxylase; 2. Reduction by Alcohol Dehydrogenase.

What cofactor is required by Pyruvate Decarboxylase?

Thiamine pyrophosphate (TPP).

What are the products of alcoholic fermentation?

CO2 and Ethanol.

Is there a net change in oxidation state during fermentation?

No, it is a redox-neutral process.

Where does Galactose enter glycolysis?

It is converted to Glucose-1-phosphate, then G6P.

What is Galactosemia?

A genetic deficiency in enzymes needed to metabolize galactose, causing cataracts and liver damage.

How does Fructose enter glycolysis in the muscle?

Hexokinase phosphorylates it to Fructose-6-phosphate.

How does Fructose enter glycolysis in the liver?

Via the Fructose-1-phosphate pathway (bypassing PFK).

Why might high fructose consumption lead to obesity?

It bypasses the PFK regulatory step, providing unregulated flux into lipid synthesis.

What enzyme converts Mannose to Mannose-6-phosphate?

Hexokinase.

What enzyme converts Mannose-6-P to Fructose-6-P?

Phosphomannose Isomerase.

What are the two primary products of the Pentose Phosphate Pathway (PPP)?

NADPH and Ribose-5-phosphate.

What is the main use of NADPH?

Reducing power for anabolic biosynthesis (e.g., fatty acids).

What is the main use of Ribose-5-phosphate?

Synthesis of nucleic acids (DNA/RNA).

Where does the PPP occur?

Cytosol.

What enzyme is the rate-limiting step of the PPP?

Glucose-6-phosphate dehydrogenase (G6PD).

What inhibits G6PD?

High levels of NADPH.

What are the two phases of the PPP?

Oxidative phase (irreversible) and Non-oxidative phase (reversible).

Which phase of the PPP produces NADPH?

The oxidative phase.

What is the 'Warburg Effect'?

The observation that cancer cells carry out glycolysis at a much higher rate than normal cells, even in oxygen.

How is glycolysis used in medical imaging?

PET scans use a glucose analog (FDG) to identify high-glucose-consuming tumors.

What is the net yield of NADH per glucose?

2 NADH.

Which step involves a thioester intermediate?

The GAPDH reaction (Step 6).

What is the role of 2,3-Bisphosphoglycerate (2,3-BPG) in red blood cells?

It regulates oxygen binding to hemoglobin (it is a byproduct of glycolysis).

What enzyme converts 3PG to 2,3-BPG?

Bisphosphoglycerate mutase.

How many ATP are invested per glucose in the liver if starting from glycogen?

Only 1 ATP (because glycogen breakdown yields G1P, bypassing hexokinase).

What is the Km of Hexokinase I for glucose?

~0.1 mM (very high affinity).

What is the Km of Glucokinase for glucose?

~10 mM (low affinity, active only when glucose is high).

Which enzyme in glycolysis uses a 'Schiff base' mechanism?

Class I Aldolase.

Which enzyme requires TPP for its function in yeast?

Pyruvate Decarboxylase.

What happens to the pH in muscles during intense exercise?

It drops (becomes more acidic) due to lactic acid production.

What is the standard free energy change (ΔG°') of the total glycolysis process?

Approximately -73 kJ/mol.

Which step is the only oxidation in glycolysis?

GAPDH (Step 6).

What is the function of the enzyme Phosphoglucomutase?

Converts Glucose-1-phosphate to Glucose-6-phosphate.

Name a non-carbohydrate precursor for glycolysis.

Glycerol (from fats) can enter at the DHAP step.

What is the structure of Pyruvate?

A 3-carbon alpha-keto acid.

How many CO2 molecules are produced in glycolysis?

Zero (decarboxylation happens later in the mitochondria or in yeast).

Which step is inhibited by high levels of Phosphoenolpyruvate (PEP) in some organisms?

PFK-1.

What is the 'Recovery Phase'?

Steps 6 through 10, where energy is captured as NADH and ATP.

What is the role of NAD+ in GAPDH?

It acts as an oxidizing agent (electron acceptor).

Why is the PGI reaction (Step 2) necessary?

It moves the carbonyl to C2, allowing for a symmetric cleavage later by aldolase.

Which enzyme is used as a clinical marker for liver damage?

LDH (Lactate Dehydrogenase).

What is the role of the 'swinging arm' in some dehydrogenase complexes?

It moves intermediates between active sites (though not used in GAPDH, used in PDH later).

How many high-energy phosphate bonds are in 1,3-BPG?

Two (but only one is high-energy enough to make ATP).

Which enzyme is most sensitive to the ATP/AMP ratio?

PFK-1.

What is the symmetry of Fructose-1,6-bisphosphate?

It is nearly symmetric, which allows it to be split into two 3-carbon sugars.

Is glycolysis aerobic or anaerobic?

It is anaerobic (does not require oxygen).

What is the first step that produces H2O as a byproduct?

Enolase (Step 9).

What is the 'First Committed Step' of glycolysis?

The PFK-1 reaction (Step 3).