The Generation of Biochemical Energydfard.weebly.com/uploads/1/0/5/3/10533150/cas_21.pdf · 32...

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Chapter Twenty one

The Generation of Biochemical Energy

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Fig 21.1 The flow of energy through biosphere

21-1 Energy and Life

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Energy Requirements , living organisms:

Must be released gradually.Must be available when needed.Must Maintain constant body temperature.Must be available for unfavorable reactions.

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Fig 21.1 (a) Energy diagram of an (a) exergonic and (b) endergonic reaction

21.2 Energy and Biochemical Reactions

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Photosynthesis

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21.3 Cells and Their Structures

- prokaryotic cells, usually found in single-celled organisms including bacteria and blue-green algae.- eukaryotic cells, found in some single-celled organisms and all plants and animals.

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http://www.cellsalive.com/cells/cell_model.htm

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Cell Components and Function

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A Mitochondrion :Cell’s power plants

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Metabolism involves:Catabolic reactions that break down large, complex molecules to provide energy and smaller molecules.Anabolic reactions that use ATP energy to build larger molecules.

Metabolism21.4 An Overview of Metabolism and Energy Production

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Food molecules undergo catabolism to provide energy in Food molecules undergo catabolism to provide energy in four stages as shown in the following Fig 21.5four stages as shown in the following Fig 21.5

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Stages of Metabolism

Catabolic reactions are organized as stages:In Stage 1, digestion breaks down large molecules into smaller ones that enter the bloodstream.In Stage 2, molecules in the cells are broken down to two- and three-carbon compounds.In Stage 3, compounds are oxidized in the citric acid cycle to provide CO2 and reduced enzymes.In Stage 4, Reduced enzymes cause production of ATP

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Stages of Metabolism

& Stage 4

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ATP and Energy

In cells, energy is stored in adenosine triphosphate(ATP).

21.5 Strategies of Metabolism: ATP and Energy Transfer

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Biochemical energy production, transport, and use all depends on the ATP ADP interconversions.

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Hydrolysis of ATP to ADP and ADP to AMP

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Hydrolysis of ATP

The hydrolysis of ATP to ADP releases 7.3 kcal (31 kJ/mole). ATP ADP + Pi + 7.3 kcal (31 kJ/mole)

The hydrolysis of ADP to AMP releases 7.3 kcal (31 kJ/mole). ADP AMP + Pi + 7.3 kcal (31 kJ/mole)

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Learning Check

Match the following:1) ATP 2) ADP + Pi

A. Used in anabolic reactions.B. The energy-storage molecule.C. Coupled with energy-requiring

reactions.D. Hydrolysis products.

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Solution

Match the following:1) ATP 2) ADP + Pi

A. 1 Used in anabolic reactions.B. 1 The energy-storage molecule.C. 1 Coupled with energy-requiring

reactions.D. 2 Hydrolysis products.

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21.6 Strategies of Metabolism: Metabolic Pathways and Coupled Reactions

Metabolic pathways of catabolism release energy bit by bit in a series of reactions.The overall reaction and the overall free-energy change for any series of reactions can be found by summing up the equations and the free-energy changes for the individual steps.The reactions of all metabolic pathways add up to favorable processes with negative free-energy changes.

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Not every individual step in every metabolic pathway is a favorable reaction. Metabolic strategy is to couple an energetically unfavorable step (endergonic) with an energetically favorable (exergonic)step so that the overall energy change for the two reactions is favorable.

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21.7 Strategies of Metabolism: Oxidized and Reduced Coenzymes

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NAD+/NADH2NicotinamideNicotinamide adenine adenine dinucleotidedinucleotide (NAD(NAD++))is a biological oxidizing agent

HHH

O

HO OH

N

CNH2

-O-P-O-CH2

O

O

AMP H

O

a β-N-glycosidic bond

+

The plus sign on NAD +

represents the positivecharge on this n itrogen

Nicotinamide;derivedfrom niacin

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NAD+/NADHNAD+ is a two-electron oxidizing agent, and is reduced to NADHNADH is a two-electron reducing agent, and is oxidized to NAD+

NAd

C NH2

OH

H+ 2e-

NAd

C NH2

OH H

+ +

NAD +

(oxidized form)NADH

(reduced form)

:

+

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FAD/FADH2

FlavinFlavin adenine adenine dinucleotidedinucleotide (FAD)(FAD) is also a biological oxidizing agent

O=P-O-AMPO-

CH2

C

O

CCCH2

N

H OH

OHHH

N

N

NH3C

H3C O

HO

OH Ribitol

Flavin

Riboflavin

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FAD/FADH2FAD is a two-electron oxidizing agent, and is reduced to FADH2

FADH2 is a two-electron reducing agent, and is oxidized to FAD

AdN

N

N

NHH3 C

H3 C O

O

+ 2H++ 2e-

H3 C

H3 C O

OH

HAdN

N

N

NHFAD

FADH2

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Coenzyme ACoA activates acyl groups such as the two-carbon acetyl group for transfer.

O O|| ||

CH3—C— + HS—CoA CH3—C—S—CoA

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overview: the two carbon acetyl group of acetyl CoA is fed into the cycle and oxidized to 2 CO2

there are four oxidation steps in the cycle

FAD

FADH2

NAD+

NADH

NAD+

NADHCO2

NAD+

NADHCO2

Acetyl-CoA

GDPGTP

Citric acidcycle

(8 steps)

Coenzyme A

21.8 The Citric Acid Cycle

http://www.wiley.com/legacy/college/boyer/0470003790/animations/tca/tca.htm

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Citric acid cycle also known as Krebs cycle: The series of biochemical reactions that breaks down acetyl groups to produce energy carried by reduced coenzymes and carbon dioxide.The eight steps of the cycle produce two molecules of carbon dioxide, four molecules of reduced coenzymes, and one energy rich phosphate. The final step regenerates the reactant for step 1 of the next turn of the cycle.

21.8 The Citric Acid Cycle

http://www.wiley.com/college/pratt/0471393878/student/animations/citric_acid_cycle/index.html

http://www.science.smith.edu/departments/Biology/Bio231/krebs.html

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CH3 C-SCoAO

+C-COO-

CH2 -COO-O

C-COO-HOCH2 -COO-

CH2 -COO-

+ CoA-SHAcetyl-CoA

Oxaloacetate

Coenzyme A

citratesynthase

Citrate

Citric Acid Cycle: Step 1

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Condensation of acetyl CoA with oxaloacetatethe high-energy thioester of acetyl CoA is hydrolyzedthis hydrolysis provides the energy to drive Step 1citrate synthase is an allosteric enzyme; it is inhibited by NADH, ATP, and succinyl-CoA


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C-COO-HO

CH2 -COO-

CH2 -COO-

Citrate

C-COO-

CH2 -COO-

C-COO-H

CH- COO-

CH2 -COO-

AconitateHO

IsocitrateCH- COO-

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Citric Acid Cycle:Step 2: dehydration and rehydration, catalyzed by aconitase, gives isocitrate

citrate is achiral; it has no stereocenteraconitate is also achiralisocitrate is chiral; it has 2 stereocenters and 4 stereoisomers are possibleonly one of the 4 possible stereoisomers is formed in the cycle

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C-COO-HCH- COO-

CH2 -COO-

HOIsocitrate

C-COO-HC-COO-

CH2 -COO-

C-HHC-COO-

CH2 -COO-

NADHNAD+

α-Ketoglutarate

CO2

isocitratedehydrogenase

O OOxalosuccinate

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Oxidation of isocitrate followed by decarboxylation gives α-ketoglutarate

isocitrate dehydrogenase is an allostericenzyme; it is inhibited by ATP and NADH, and activated by ADP and NAD+


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Review 2: Stages of Metabolism

Catabolic reactions are organized as stages:In Stage 1, digestion breaks down large molecules into smaller ones that enter the bloodstream.In Stage 2, molecules in the cells are broken down to two- and three-carbon compounds.In Stage 3, compounds are oxidized in the citric acid cycle to provide CO2 and reduced enzymes.In Stage 4, Reduced enzymes cause production of ATP

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CH3 C-SCoAO

+C-COO-

CH2 -COO-O

C-COO-HOCH2 -COO-

CH2 -COO-

+ CoA-SHAcetyl-CoA

Oxaloacetate

Coenzyme A

citratesynthase

Citrate


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C-COO-HO

CH2 -COO-

CH2 -COO-

Citrate

C-COO-

CH2 -COO-

C-COO-H

CH- COO-

CH2 -COO-

AconitateHO

IsocitrateCH- COO-

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C-COO-HCH- COO-

CH2 -COO-

HOIsocitrate

C-COO-HC-COO-

CH2 -COO-

C-HHC-COO-

CH2 -COO-

NADHNAD+

α-Ketoglutarate

CO2

isocitratedehydrogenase

O OOxalosuccinate

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CH2

C-COO-

CH2 -COO-

α-KetoglutarateO

CoA-SH

NADHNAD+

α-ketoglutaratedehydrogenase

complex

CH2

C

CH2 -COO-

SCoAOSuccinyl-CoA

+ CO2

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Step 4: oxidative decarboxylation of α-ketoglutarate to succinyl-CoA

the two carbons of the acetyl group of acetyl CoA are still present in succinyl CoA and in succinatethis multienzyme complex is inhibited by ATP, NADH, and succinyl CoA; it is activated by ADP and NAD+


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CH2

C

CH2 -COO-

SCoAO+ GDP + Pi

CH2 -COO-CH2 -COO-

+ GTP + CoA-SH

Su cciny l-CoA Su ccinate

succinyl -CoAsynth etase

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Step 5: formation of succinatethe two CH2-COO- groups of succinate are now equivalentthis is the first energy-yielding step of the cycle; a molecule of GTP is produced


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FAD FADH2

CH2 -COO-CH2 -COO-

Succinate

succinatedehydrogenase

CC

H

H

COO-

-OOC

Fumarate

Step 6: oxidation of succinate to fumarate

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Step 7: hydration of fumarate to L-malate

CC

H

H

COO-

-OOCFumarate

H2 O CH- COO-HOCH2 -COO-

L-Malate

fumarase


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C-COO-

CH2 -COO-

Oxaloacetate

NAD+ NADH

malatedehydrogenase

CH- COO-HO

CH2 -COO-

L-Malate

O

Step 8: oxidation of malate

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Citric Acid Cycle

oxaloacetate now can react with acetyl CoAto start another round of the cycle by repeating Step 1The overall reaction of the cycle is

CH3 C-SCoAO

+ GDP + Pi + 3NAD+ + FAD + 3H2 O

2CO2 + GTPCoA + 3NADH + FADH2+ + 3H+

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Citric Acid CycleControl of the cycle

controlled by three feedback mechanismscitrate citrate synthasesynthase:: inhibited by ATP, NADH, and succinyl CoA; also product inhibition by citrateisocitrateisocitrate dehydrogenasedehydrogenase:: activated by ADP and NAD+, inhibited by ATP and NADHαα--ketoglutarateketoglutarate dehydrogenasedehydrogenase complexcomplex::inhibited by ATP, NADH, and succinyl CoA; activated by ADP and NAD+

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Fig 21.9 The citric acid cycleFig 21.9 The citric acid cycle

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21.10 Harmful Oxygen By-Products and Antioxidant Vitamins

About 90% of the oxygen we breathe are utilized in the electron transport-ATP synthesis reactions. These and other oxygen consuming reactions produces some harmful oxygen containing highly reactive products such as hydroxyl free radical, HO., superoxide ion, O2

-.,

and hydrogen peroxide, H2O2. These reactive species can cause damage by breaking covalent bonds in enzymes and other proteins, DNA, and the lipids in the cell membranes.

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The outcomes of such damages are cancer, liver The outcomes of such damages are cancer, liver damage, heart disease, immune system damage damage, heart disease, immune system damage etc.etc.

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Superoxide dismutase and catalase, some very fast acting enzymes in our body, provides protection against these harmful free radicals and hydrogen peroxide by destroying them as they are produced. Protection is also provided by the vitamins E, C, and A. These vitamins make the free radicals harmless by bonding with them.

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Chapter SummaryWe derive energy by oxidation of food molecules that contain energy.The energy is released slowly in exergonic reactions and is available to do work and also drive endogonic reactions.Food molecules breakdown (catabolism) to provide energy in four stages:

- digestion

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Chapter Summary Contd.- decomposition into two-carbon acetyl

group that are bonded to coenzyme A in acetyl coenzyme

- reaction of the acetyl groups via the citric acid cycle generate energy rich reduced coenzymes and liberate carbon dioxide.

- electron transport and transfer of the energy of the reduced coenzymes of the citric acid cycle to the principle energy transporter ATP.

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Chapter Summary Contd.Using energy from exergonic reactions,

ADP is phosphorylated to ATP.When and where energy is needed, ATP is hydrolyzed back to ADP with the release of energy.

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Chapter Summary Contd.Citric acid cycle is a series of eight reactions in which an acetyl group is oxidized and four reduced coenzymes and one ATP are produced. The reduced coenzymes carry energy for the subsequent production of additional ATP.Citric acid cycle is activated when energy is in short supply and inhibited when energy is in good supply.

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Chapter Summary Contd.Harmful by-products of oxygen consuming reactions are hydroxyl free radical, superoxide ion (also a free radical), and hydrogen peroxide. They cause damage by breaking bonds. Enzymes such as superoxide dismutase and catalase, and vitamins E, C, and A protect our body against such harmful species.

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TermsReperfusion injury refers to damage to tissue caused when restriction in blood supply, generally due to factors in the blood vessels,results inflammation and oxidativedamage to the tissue

http://en.wikipedia.org/wiki/Tissue_%28biology%29

http://en.wikipedia.org/wiki/Blood_vessel




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Animations:Cell structure:http://www.wisc-online.com/objects/index_tj.asp?objid=AP11604http://www.cellsalive.com/cells/cell_model.htm

Citric AcidCyclehttp://www.wiley.com/legacy/college/boyer/0470003790/animations/tca/tca.ht

mhttp://www.wiley.com/college/pratt/0471393878/student/animations/citric_aci

d_cycle/index.html

http://www.science.smith.edu/departments/Biology/Bio231/krebs.html

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End of Chapter Twenty One

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