sign in sign up
<<
Home , Adenosine triphosphate, Anaerobic respiration, Beta oxidation, Catabolism, Cellular respiration, Chemiosmosis

The Structure and of ATP groups • ATP drives endergonic reactions (a) The structure of ATP by , transferring a phosphate group to some other , triphosphate (ATP) such as a reactant • The recipient molecule is now called a

Inorganic phosphorylated (ADP) phosphate intermediate (b) The hydrolysis of ATP How the Hydrolysis of ATP Performs Work

• The bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis • Energy is released from ATP when the terminal phosphate bond is broken • This release of energy comes from the to a state of lower free energy, not from the phosphate bonds themselves • The three types of cellular work (mechanical, transport, and chemical) are powered by the hydrolysis of ATP • In the , the energy from the exergonic reaction of ATP hydrolysis can be used to drive an • Overall, the coupled reactions are exergonic The Regeneration of ATP • ATP is a that is regenerated by addition of a phosphate group to adenosine diphosphate (ADP) • The energy to phosphorylate ADP comes from catabolic reactions in the cell • The ATP cycle is a revolving door through which energy passes during its transfer from catabolic to anabolic pathways

ATP H2O

Energy from Energy for cellular (exergonic, work (endergonic, energy-releasing ADP P i energy-consuming processes) processes) speed up metabolic reactions by lowering energy barriers • A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction • An is a catalytic • Hydrolysis of by the enzyme is an example of an enzyme-catalyzed reaction

Sucrase

Sucrose

(C12H22O11) (C6H12O6) (C6H12O6) Figure 8.13

Course of

reaction EA without without enzyme enzyme EA with enzyme is lower Reactants

Course of ΔG is unaffected reaction by enzyme

Free energy with enzyme

Products

Progress of the reaction Oxidation of Organic During • During cellular respiration, the fuel (such as glucose)

is oxidized, and O2 is reduced

becomes oxidized

becomes reduced Stepwise Energy Harvest via NAD+ and the Transport Chain • In cellular respiration, glucose and other organic molecules are broken down in a series of steps • from organic compounds are usually first transferred to NAD+, a coenzyme • As an , NAD+ functions as an oxidizing agent during cellular respiration • Each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP

Dehydrogenase NAD+ NADH Reduction of NAD+

(from food) Oxidation of NADH Nicotinamide (oxidized form) (reduced form)

• NADH passes the electrons to the • Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reaction

• O2 pulls electrons down the chain in an energy-yielding tumble • The energy yielded is used to regenerate ATP 1 1 H2 + /2 O2 2 H + /2 O2 (from food via NADH) Controlled release of 2 H+ + 2 e- energy for synthesis of Electron transport ATP

ATP G G

Explosive chain ATP release of heat and light ATP energy Free energy, Free energy, Free energy, Free energy, 2 e- 1/ O 2 H+ 2 2

H2O H2O

(a) Uncontrolled reaction (b) Cellular respiration The Stages of Cellular Respiration: A Preview

• Harvesting of energy from glucose has three stages – (breaks down glucose into two molecules of pyruvate) – The citric cycle (completes the breakdown of glucose) – Oxidative phosphorylation (accounts for most of the ATP synthesis) Figure 9.6-3

Electrons Electrons carried carried via NADH and

via NADH FADH2

Oxidative Pyruvate Glycolysis oxidation Citric phosphorylation: acid electron transport Glucose Pyruvate Acetyl CoA cycle and

CYTOSOL

ATP ATP ATP

Substrate-level -level Oxidative phosphorylation phosphorylation phosphorylation Oxidative Phosphorylation • The process that generates most of the ATP is called oxidative phosphorylation because it is powered by reactions • Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration • A smaller amount of ATP is formed in glycolysis and the cycle by substrate-level phosphorylation

• For each molecule of glucose degraded to CO2 and by respiration, the cell makes up to 32 molecules of ATP Glycolysis harvests by oxidizing glucose to pyruvate • Glycolysis (“splitting of ”) breaks down glucose into two molecules of pyruvate • Glycolysis occurs in the and has two major phases – Energy investment phase – Energy payoff phase

• Glycolysis occurs whether or not O2 is present Figure 9.8 Energy Investment Phase Glucose

2 ADP + 2 P 2 ATP used

Energy Payoff Phase

4 ADP + 4 P 4 ATP formed

2 NAD+ + 4 e- + 4 H+ 2 NADH + 2 H+

2 Pyruvate + 2 H2O

Net Glucose 2 Pyruvate + 2 H2O 4 ATP formed - 2 ATP used 2 ATP 2 NAD+ + 4 e- + 4 H+ 2 NADH + 2 H+ Figure 9.9a

Glycolysis: Energy Investment Phase

ATP Glucose Glucose 6-phosphate Fructose 6-phosphate ADP

Hexokinase Phosphogluco- 1 2 Figure 9.9b

Glycolysis: Energy Investment Phase

ATP Fructose 6-phosphate Fructose 1,6-bisphosphate ADP

Phospho-

3 Aldolase 4

Dihydroxyacetone phosphate 3-phosphate

To Isomerase 5 step 6 Figure 9.9c

Glycolysis: Energy Payoff Phase

2 ATP 2 NADH 2 NAD+ + 2 H+ 2 ADP 2 2

Triose Phospho- phosphate glycerokinase 2 P i dehydrogenase 1,3-Bisphospho- 7 3-Phospho- 6 glycerate glycerate Figure 9.9d

Glycolysis: Energy Payoff Phase

2 ATP 2 H O 2 2 ADP 2 2 2 2

Phospho- Pyruvate glyceromutase 9 3-Phospho- 8 2-Phospho- Phosphoenol- 10 Pyruvate glycerate glycerate pyruvate (PEP) After pyruvate is oxidized, the completes the energy-yielding oxidation of organic molecules

• In the presence of O2, pyruvate enters the mitochondrion (in eukaryotic cells) where the oxidation of glucose is completed • Before the citric acid cycle can begin, pyruvate must be MITOCHONDRION converted to acetyl (acetyl CoA), which links glycolysisCYTOSOL to the citric acid cycle CO2 Coenzyme A

1 3

2

NAD+ NADH + H+ Acetyl CoA Pyruvate

Transport protein Pyruvate

The Citric Acid CO2 NAD+ CoA Cycle NADH + • The citric acid cycle, + H Acetyl CoA also called the Krebs CoA

cycle, completes the CoA break down of pyrvate

to CO2 • The cycle oxidizes organic fuel derived Citric acid 2 CO from pyruvate, cycle 2

+ generating 1 ATP, 3 FADH2 3 NAD NADH, and 1 FADH 2 FAD 3 NADH per turn + 3 H+

ADP + P i

ATP • The citric acid cycle has eight steps, each Acetyl CoA catalyzed by a specific CoA-SH enzyme NADH + H+ 1 H2O • The of NAD+ 8 Oxaloacetate acetyl CoA joins the 2

cycle by combining with Malate Citrate Isocitrate oxaloacetate, forming NAD+ Citric NADH citrate 3 7 acid + H+ H O 2 CO • The next seven steps cycle 2 Fumarate decompose the citrate CoA-SH back to oxaloacetate, α-Ketoglutarate 6 4 making the process a CoA-SH FADH 5 cycle 2 CO NAD+ 2 FAD • The NADH and FADH NADH 2 Succinate P i produced by the cycle GTP GDP Succinyl + H+ CoA relay electrons extracted ADP

from food to the electron ATP transport chain Figure 9.12a

Acetyl CoA

CoA-SH

1 H2O

Oxaloacetate 2

Citrate Isocitrate Figure 9.12b

Isocitrate NAD+ NADH 3 + H+

CO2

CoA-SH α-Ketoglutarate 4

CO2 NAD+

NADH Succinyl + H+ CoA Figure 9.12c

Fumarate

6 CoA-SH 5 FADH2 FAD

Succinate P i GTP GDP Succinyl CoA ADP

ATP Figure 9.12d

NADH + H+ NAD+ 8 Oxaloacetate

Malate

7 H2O

Fumarate During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis • Following glycolysis and the citric acid cycle, NADH and

FADH2 account for most of the energy extracted from food • These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation

• Electrons are transferred from NADH or FADH2 to the electron transport chain • Electrons are passed through a number of including

(each with an ) to O2 • The electron transport chain generates no ATP directly

• It breaks the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts NADH The Pathway of 50 2 e- NAD+ FADH Electron Transport 2 2 e- FAD Multiprotein 40 I complexes • The electron transport chain is FMN II Fe S Fe•S in the inner membrane • Q III (kcal/mol) (cristae) of the mitochondrion 2 Cyt b 30 Fe•S Cyt c • Most of the chain’s 1 IV components are proteins, Cyt c Cyt a

which exist in multiprotein relativeO ) to Cyt a3

G 20 complexes • The carriers alternate reduced

and oxidized states as they 10 2 e- accept and donate electrons Free energy ( (originally from NADH or FADH2) • Electrons drop in free energy

+ 1 as they go down the chain and 0 2 H + /2 O2

are finally passed to O2, forming H2O H2O Energy-Coupling Mechanism Electron transfer in the H+ • Stator electron transport chain Rotor causes proteins to pump H+ from the to the intermembrane space • H+ then moves back across the membrane, passing through the , ATP Internal rod Catalytic • ATP synthase uses the knob exergonic flow of H+ to drive phosphorylation of ATP ADP • This is an example of +

chemiosmosis, the use of P i ATP + energy in a H gradient to MITOCHONDRIAL MATRIX drive cellular work • The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis • The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work

H+ H+

+ Protein H H+ complex Cyt c of electron carriers IV Q III I ATP II synth- + 1 2 H + /2O2 H2O ase FADH2 FAD

NADH NAD+ ADP + P i ATP (carrying electrons from food) H+

1 Electron transport chain 2 Chemiosmosis Oxidative phosphorylation • During cellular respiration, most energy flows in this sequence: glucose → NADH → electron transport chain → proton-motive force → ATP • About 34% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 32 ATP • There are several reasons why the number of ATP is not known exactly

Electron shuttles MITOCHONDRION span membrane 2 NADH or

2 FADH2

2 NADH 2 NADH 6 NADH 2 FADH2

Glycolysis Pyruvate oxidation Oxidative Citric phosphorylation: Glucose 2 Pyruvate 2 Acetyl CoA acid electron transport cycle and chemiosmosis

+ 2 ATP + 2 ATP + about 26 or 28 ATP

About Maximum per glucose: 30 or 32 ATP

CYTOSOL and enable cells to produce ATP without the use of

• Most cellular respiration requires O2 to produce ATP

• Without O2, the electron transport chain will cease to operate • In that case, glycolysis couples with fermentation or anaerobic respiration to produce ATP • Anaerobic respiration uses an electron transport chain with a final electron acceptor other than O2, for example • Fermentation uses substrate-level phosphorylation instead of an electron transport chain to generate ATP Types of Fermentation • Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis • Two common types are fermentation and fermentation • In alcohol fermentation, pyruvate is converted to in two

steps, with the first releasing CO2 • Alcohol fermentation by is used in , , and baking • In , pyruvate is reduced to NADH,

forming lactate as an end product, with no release of CO2 • Lactic acid fermentation by some fungi and is used to make and • Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce Figure 9.17

2 ADP + 2 P i 2 ATP 2 ADP + 2 P i 2 ATP

Glucose Glycolysis Glucose Glycolysis

2 Pyruvate

2 NAD + 2 NADH 2 NAD + 2 NADH 2 CO2 + 2 H+ + 2 H+ 2 Pyruvate

2 Ethanol 2 2 Lactate (a) Alcohol fermentation (b) Lactic acid fermentation Comparing Fermentation with Anaerobic and Aerobic Respiration • All use glycolysis (net ATP =2) to oxidize glucose and harvest chemical energy of food • In all three, NAD+ is the oxidizing agent that accepts electrons during glycolysis • The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration • Cellular respiration produces 32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule • Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2 • Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration • In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes Figure 9.18 Glucose

Glycolysis CYTOSOL

Pyruvate

No O2 present: O2 present: Fermentation Aerobic cellular respiration

MITOCHONDRION Ethanol, Acetyl CoA lactate, or other products Citric acid cycle Glycolysis and the citric acid cycle connect to many other metabolic pathways • Gycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways • Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration • Glycolysis accepts a wide range of • Proteins must be digested to amino ; amino groups can feed glycolysis or the citric acid cycle • are digested to (used in glycolysis) and fatty acids (used in generating acetyl CoA) • Fatty acids are broken down by and yield acetyl CoA • An oxidized gram of produces more than twice as much ATP as an oxidized gram of Proteins Carbohydrates Fats

Amino Glycerol Fatty acids acids

Glycolysis • The body uses Glucose small molecules to Glyceraldehyde 3- P build other substances NH3 Pyruvate • These small molecules Acetyl CoA may come directly from

Citric food, from acid glycolysis, or cycle from the citric acid cycle

Oxidative phosphorylation Glucose

AMP Regulation of Cellular Glycolysis Fructose 6-phosphate Stimulates Respiration via + - - Mechanisms Fructose 1,6-bisphosphate Inhibits Inhibits • Feedback inhibition is the most common mechanism for control • If ATP begins Pyruvate to drop, respiration speeds ATP Citrate up; when there is plenty of Acetyl CoA ATP, respiration slows down Citric • Control of catabolism is acid cycle based mainly on regulating the activity of enzymes at Oxidative strategic points in the phosphorylation catabolic pathway