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Questions and Answers
Which of the following correctly describes a function of ATP?
Which of the following correctly describes a function of ATP?
What occurs during an oxidation reaction?
What occurs during an oxidation reaction?
Which of the following is not a carrier molecule in metabolic reactions?
Which of the following is not a carrier molecule in metabolic reactions?
What is the primary function of phosphoryl-transfer potential?
What is the primary function of phosphoryl-transfer potential?
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What is the primary role of alpha-amylase in carbohydrate digestion?
What is the primary role of alpha-amylase in carbohydrate digestion?
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Which enzyme is responsible for transferring the phosphate group from 1,3-BPG to ADP during glycolysis?
Which enzyme is responsible for transferring the phosphate group from 1,3-BPG to ADP during glycolysis?
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Which enzyme is responsible for converting maltose into glucose?
Which enzyme is responsible for converting maltose into glucose?
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What structural bond is responsible for linking glucose subunits in glycogen?
What structural bond is responsible for linking glucose subunits in glycogen?
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Which of the following statements about substrate-level phosphorylation is false?
Which of the following statements about substrate-level phosphorylation is false?
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In which pathway does fructokinase act, and what does it produce?
In which pathway does fructokinase act, and what does it produce?
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What type of reaction does the enzyme pyruvate kinase catalyze?
What type of reaction does the enzyme pyruvate kinase catalyze?
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Which intermediate in the TCA cycle has a high phosphate-transfer potential?
Which intermediate in the TCA cycle has a high phosphate-transfer potential?
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Which polysaccharide serves as the primary storage form of glucose in animals?
Which polysaccharide serves as the primary storage form of glucose in animals?
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Which of the following statements describes citrate synthase?
Which of the following statements describes citrate synthase?
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Which of the following substances is produced from the overall reaction of the TCA cycle?
Which of the following substances is produced from the overall reaction of the TCA cycle?
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What is the primary function of succinyl-CoA synthetase in the TCA cycle?
What is the primary function of succinyl-CoA synthetase in the TCA cycle?
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What is the primary function of hexokinase in glycolysis?
What is the primary function of hexokinase in glycolysis?
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What inhibits phosphofructokinase (PFK) activity?
What inhibits phosphofructokinase (PFK) activity?
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Which enzyme is considered the rate-limiting step in glycolysis?
Which enzyme is considered the rate-limiting step in glycolysis?
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How does pyruvate kinase regulate the glycolytic pathway?
How does pyruvate kinase regulate the glycolytic pathway?
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Which statement is true about the reaction catalyzed by phosphoglycerate kinase?
Which statement is true about the reaction catalyzed by phosphoglycerate kinase?
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What is a major product of the glycolytic breakdown of glucose?
What is a major product of the glycolytic breakdown of glucose?
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What is the effect of ATP on phosphofructokinase?
What is the effect of ATP on phosphofructokinase?
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Which enzyme catalyzes the conversion of G3P and DHAP?
Which enzyme catalyzes the conversion of G3P and DHAP?
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What is the role of the β subunits in the structure described?
What is the role of the β subunits in the structure described?
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What occurs during the 'T' conformation of the β subunits?
What occurs during the 'T' conformation of the β subunits?
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In terms of the regulation of ATP synthesis, what effect does a lower mass-action ratio have?
In terms of the regulation of ATP synthesis, what effect does a lower mass-action ratio have?
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What is the primary function of the electron transport chain (ETC)?
What is the primary function of the electron transport chain (ETC)?
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What does the concept of proton-motive force (PMF) refer to?
What does the concept of proton-motive force (PMF) refer to?
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What does a negative reduction potential (E0) indicate about a molecule?
What does a negative reduction potential (E0) indicate about a molecule?
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During ATP synthesis, where does the proton enter after binding to a glutamate residue on the c-ring subunit?
During ATP synthesis, where does the proton enter after binding to a glutamate residue on the c-ring subunit?
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Which statement accurately describes the effect of inhibitors on the electron transport chain?
Which statement accurately describes the effect of inhibitors on the electron transport chain?
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What is the main function of uncouplers in the electron transport chain?
What is the main function of uncouplers in the electron transport chain?
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Which complex in the electron transport chain directly pumps the most protons per molecule of substrate?
Which complex in the electron transport chain directly pumps the most protons per molecule of substrate?
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How do mobile electron carriers contribute to the electron transport chain?
How do mobile electron carriers contribute to the electron transport chain?
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Which of the following is a characteristic of hydrogen peroxide?
Which of the following is a characteristic of hydrogen peroxide?
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What is the role of superoxide dismutase within the context of reactive oxygen species?
What is the role of superoxide dismutase within the context of reactive oxygen species?
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Which statement best describes the proton-motive force (PMF)?
Which statement best describes the proton-motive force (PMF)?
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Which of the following is true regarding cytochrome c?
Which of the following is true regarding cytochrome c?
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What distinguishes complex II in the electron transport chain from the other complexes?
What distinguishes complex II in the electron transport chain from the other complexes?
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Study Notes
ATP and its functions
- ATP is the cellular currency, derived from fuel or light.
- ATP hydrolysis is highly exergonic due to unstable bonds.
- ATP is used for mechanical work, active transport, macromolecule synthesis and unfavorable reaction coupling.
- ATP consists of adenine, ribose, and three phosphate groups.
Oxidation-reduction reactions
- Oxidation reactions involve losing electrons, hydrogen atoms, or gaining oxygen atoms.
- Reduction reactions involve gaining electrons, hydrogen atoms, or losing oxygen atoms.
Carrier Molecules
- Carrier molecules are used to carry molecular groups or electrons in metabolic reactions.
- ATP carries phosphoryl groups.
- NAD+ and FAD carry electrons.
- Coenzyme A carries acyl groups, like the acetyl group.
Entry of saccharides into the glycolytic pathway
- Alpha-amylase cleaves α1→4 glycosidic bonds in starch.
- Maltase breaks down maltose into glucose.
- Glycogen phosphorylase adds a phosphate group to glycogen, producing glucose 1-phosphate.
- Phosphoglucomutase moves the phosphate group from C-1 to C-6 in glucose 1-phosphate to form glucose 6-phosphate.
- Galactokinase phosphorylates galactose at C-1 using ATP.
- G1P uridyltransferase converts galactose 1-phosphate to glucose 1-phosphate using UDP-glucose.
- Fructokinase phosphorylates fructose at C-1 using ATP.
- Aldolase cleaves fructose 1-phosphate into dihydroxyacetone phosphate (DHAP) and glyceraldehyde.
- Triose kinase phosphorylates glyceraldehyde at C-3, producing glyceraldehyde 3-phosphate.
Polysaccharides
- Starch is a storage form of glucose in plants.
- It is made up of two polymers: amylose and amylopectin.
- Glycogen is the storage form of glucose in animals.
- It is a polymer of α1→4 linked glucose subunits with α1→6 linked branch points every 8-12 residues.
- Cellulose is a linear, unbranched polysaccharide found only in plants.
- It is linked by β1→4 glycosidic bonds.
- Chitin is a polysaccharide composed of N-acetyl glucosamine residues in β1→4 linkage.
- It is the second most abundant polysaccharide.
Overall Reaction Equation for Glycolysis
- Glucose + 2 NAD+ + 2 ADP + 2Pi → 2 pyruvate + 2NADH + 2H+ + 2 ATP + 2H2O
- Free energy change: -85 kJ/mol (irreversible)
Enzymes of Glycolysis
-
Hexokinase:
- Phosphorylates glucose at C-6, forming glucose 6-phosphate (G6P).
- Inhibited by G6P.
- Traps glucose in the cell.
- Largely irreversible.
-
Phosphoglucose isomerase:
- Converts G6P to fructose 6-phosphate (F6P).
- Reversible reaction.
-
Phosphofructokinase (PFK):
- Phosphorylates F6P at C1, yielding fructose 1,6-bisphosphate (F16P).
- Traps sugar as fructose.
- Irreversible.
- Allosterically regulated.
- Inhibited by ATP and citrate
- Activated by AMP
- Rate-limiting step of glycolysis.
-
Aldolase:
- Cleaves F16P into G3P and DHAP.
- Reversible reaction.
-
Triose phosphate isomerase:
- Interconverts G3P and DHAP.
- Reversible.
-
Glyceraldehyde 3-phosphate dehydrogenase (G3PDH):
- Oxidizes G3P and adds phosphate, forming 1,3-bisphosphoglycerate (1,3-BPG).
- Reversible.
- Occurs in two steps:
- Exergonic oxidation and reduction of NAD+ to NADH.
- Endergonic addition of phosphate.
-
Phosphoglycerate kinase:
- Transfers phosphate group from 1,3-BPG to ADP, forming ATP and 3-phosphoglycerate (3-PG).
- Reversible.
-
Phosphoglycerate mutase:
- Shifts the phosphate group from C-2 to C-3 in glycerate, forming 2-phosphoglycerate (2-PG).
- Reversible.
-
Enolase:
- Removes H2O from 2-PG, producing phosphoenolpyruvate (PEP).
- Reversible.
- PEP has a high phosphate-transfer potential.
-
Pyruvate kinase:
- Transfers the phosphate group from PEP to ADP, yielding ATP and pyruvate.
- Irreversible.
- Highly exergonic.
Regulation of Glycolysis
- Hexokinase is allosterically inhibited by G6P.
-
Phosphofructokinase (PFK) is allosterically inhibited by ATP and allosterically activated by AMP.
- ATP lowers PFK's affinity for F6P.
- AMP competes with ATP for the allosteric site.
- Pyruvate kinase is allosterically inhibited by ATP and alanine, but stimulated by F16P.
Phosphoryl-Transfer Potential
- The phosphoryl-transfer potential is the standard free energy change involved in the hydrolysis of a phosphoryl-containing compound.
- It is used to compare the tendency of organic molecules to transfer a phosphate group to an acceptor molecule.
- ATP has an intermediate phosphate-transfer potential among important phosphate molecules.
Substrate-Level Phosphorylation
- Substrate-level phosphorylation is the formation of ATP via the transfer of a phosphate group from a substrate to ADP.
- It does not require oxygen.
- Substrate-level phosphorylation is the only source of ATP production in anaerobic conditions or organisms.
Substrate-Level Phosphorylation Enzymes in Glycolysis
- Phosphoglycerate kinase transfers the phosphate group from 1,3-BPG to ADP, forming ATP and 3- phosphoglycerate (3-PG).
- Pyruvate kinase transfers the phosphate group from PEP to ADP, yielding ATP and pyruvate.
Substrate-Level Phosphorylation Enzymes in the TCA Cycle
-
Succinyl-CoA synthetase (ligase) cleaves the thioester bond of succinyl-CoA, forming succinate.
- It regenerates CoA.
- The intermediate, succinyl phosphate, has a high phosphate-transfer potential and donates the phosphate group to ADP forming ATP.
TCA Cycle
- The TCA cycle harvests electrons from carbon fuels.
- It occurs in the mitochondrial matrix.
- The overall reaction equation is: 2 acetyl CoA + 6 NAD+ + 2 FAD + 2ADP + 2Pi → 4 CO2 + 6 NADH + 6 H+ + 2 FADH2 + 2 ATP
TCA Cycle Enzymes
-
Citrate synthase (ligase):
- Joins acetyl CoA with oxaloacetate, forming citrate.
- Irreversible.
- CoA is regenerated.
- Exhibits induced fit.
-
Aconitase (isomerase):
- Rearranges citrate, forming isocitrate.
- Reversible.
- Endergonic.
-
Isocitrate dehydrogenase:
- Converts isocitrate to α-ketoglutarate via oxidative decarboxylation, releasing CO2.
- Irreversible.
- Reduces NAD+ to NADH.
-
α-ketoglutarate dehydrogenase complex:
- Converts α-ketoglutarate to succinyl-CoA via oxidative decarboxylation, releasing CO2.
- Composed of 3 alpha subunits and 3 beta subunits.
-
Succinyl-CoA synthetase:
- Cleaves the thioester bond of succinyl-CoA, forming succinate and regenerating CoA.
- The intermediate, succinyl phosphate, has high phosphate-transfer potential and donates the phosphate group to ADP forming ATP.
-
Succinate dehydrogenase:
- Oxidizes succinate to fumarate.
- Reduces FAD to FADH2.
-
Fumarase:
- Adds H2O to fumarate, forming malate.
- Reversible.
-
Malate dehydrogenase:
- Oxidizes malate to oxaloacetate.
- Reduces NAD+ to NADH.
Mechanism of ATP Synthesis
- Proton enters the intermembrane space.
- Proton enters the half-channel facing the intermembrane space.
- Proton binds to glutamate residue on a c-ring subunit.
- After a full rotation of the c-ring, the proton enters the half-channel facing the matrix.
- Proton moves into the matrix, down the proton gradient.
Regulation of ATP Synthesis
- The [ATP]/([ADP][P]1) ratio regulates respiration:
- A low energy charge ratio increases the rate of respiration.
- ADP regulates oxidative phosphorylation:
- Low ADP levels decrease the rate of oxidative phosphorylation.
Uncouplers and Inhibitors of ETC and ATP Synthesis
-
Uncouplers:
- Increase IMM permeability to protons, dissipating the proton gradient.
- Decrease ATP synthesis.
-
Inhibitors:
- Block electron flow through the ETC.
- Interfere with ATP synthesis.
Chemiosmotic Theory
- The ETC and ATP synthesis are coupled by a proton gradient across the inner mitochondrial membrane (IMM).
- The energy stored in the electrochemical gradient created by the ETC is used to produce ATP.
- Protons are pumped from the matrix into the intermembrane space, creating higher [H+] in the intermembrane space and lower [H+] in the matrix.
- The potential difference between the two compartments is called the proton-motive force (PMF).
Free Energy Change, Reduction Potential, and Redox Reactions
- The free energy change (ΔG´°) indicates the likelihood of a redox reaction occurring.
- The reduction potential (E0) measures a molecule's electron-transfer potential.
- Negative E0: Strong reducing agent.
- Positive E0: Strong oxidizing agent.
- Electrons transfer from positive to negative reduction potentials.
Electron Transport Chain (ETC)
- The ETC is a series of oxidation-reduction reactions that generate a proton gradient used to power ATP synthesis.
- It consists of four large complexes within the IMM:
- Complex I (NADH- Q oxidoreductase): pumps 4 protons per NADH.
- Complex II (Succinate-Q reductase): does not pump protons.
- Complex III (Q-cytochrome c oxidoreductase): pumps 4 protons per QH2.
- Complex IV (cytochrome c oxidase): pumps 2 protons per 2 cyt c.
-
Mobile Electron Carriers:
- Ubiquinone (Coenzyme Q10): transfers two electrons at a time and moves between the four complexes.
- Cytochrome c: carries one electron at a time.
Reactive Oxygen Species (ROS)
- ROS are oxygen-containing compounds that damage cellular biomolecules.
- Common ROS include:
- Superoxide (O2−)
- Hydrogen peroxide (H2O2)
- Hydroxyl radical (OH·)
-
Protective Enzymes:
- Superoxide dismutase: converts superoxide into H2O2.
- Catalase: cleaves H2O2.
ATP Synthesis
- The energy stored in the electrochemical gradient across the IMM drives ATP synthesis.
- The PMF is created by the ETC as protons are pumped from the matrix into the intermembrane space.
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