9. Bioenergetics

217

218

Flow of organic Matter, Electrons, and Energy

in Metabolism

O2

Cellular

Components

Chemical Energy

C-Compounds

Electrons

e-

Respiration

(aerobic)Photosynthesis

Organic

Nurtients

CO2 + H2O

O2

Light energy

ATP

KATABOLISM

(Degradation)

ANABOLISM

(Biosynthesis)

Fermen-

tations

(anaerobic)

Central questions in understanding metabolic pathways:

▪ Which C-C bods are broken and which are formed?

▪ Which C-atoms become oxidized and which reduced?

▪ Why are some reactions able to generate ATP while others consume ATP?

General thermodynamic “rules” in metabolism:

▪ the following conditions apply for all metabolic reactions (p = constant)

ΔG = ΔH - TΔS

▪ only metabolic reactions with DG < 0 erfüllt sein

▪ catabolic reactions oxidize C-H bonds to C-O, C=O and O-H bonds (the opposite

applies for anabolic reactions!); the latter are energetically at a lower level than

C-H bonds, and thus for catabolic reactions DH < 0, whereas for anabolic

reactions DH < 0

▪ in catabolic reactions larger organic molecules are degraded to smaller ones, which

results in an increase in enrtropy, hence DS > 0. The opposite applies for

anaboic reactions, which are therefore lead to a decrease in entropy DS < 0.

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▪ as a result:

DH - TDS = DG < 0 for C-compounds in catacolic reactions

and DH - TDS = DG > 0 for C-compounds in in anabolic reactions

→ anabolic reactions can only occur by coupling with reactions for which

applies DG < 0 (exergonic reactions)

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The Phosphorylation Potential of ATP/ADP

has an intermediate Value

Free energy of hydrolysis

(„phosphorylation potential“)

223

Vermittlerolle des ATP bei der

Übertragung metabolischer Energie (II)

224

Example for ATP Generation

in a catabolic Reaction

226

Example for ATP consumption

In an anabolic Reaction

225

229

NAD(P)H and FADH2 as

“Cellular Electron Currency”

NADH, NADPH und FADH2 are nucleotides with intermediate redox potentials

(= electron affinites) compared to other metabolites

NADH, NADPH and FADH2 transfer electrons onto metabolites (“reducing

agents”)

NAD+, NADP+ und FAD withdraw electrons from biomolecules (“oxidation agents”)

NAD(P)H + X NAD(P)+ + X-H-

FADH2 + Z FAD + XH2

Transfer of H- (Hydride anion = 2 electrons + 1 proton)

Transfer of 2 H

(2 Hydrogen atoms = 2 electronen + 2 protons)

NAD(P)H + Y + H+ NAD(P)+ + H-X-H

Transfer of H- and H+

oder_

Redox Co-Faktoren von Enzymen (I)

NAD+ bzw. NADP+ NADH bzw. NADPH

230

[ H-

]

Redox Co-Faktoren von Enzymen (II)

FAD

Isoalloxazin

231

Presence of O2 (aerobic)

Absence of O2 (anaerobic)

Pentosephosphate

Pathway (PPP)Glycolysis

Glucose Catabolism

Alcoholic

Fermentation

NADP+

NADPH

Homolactic

Fermentation

Glucose

237

Citric Acid

CycleCitric Acid

Cycle

PDH

Complex

ADP + Pi

ATP

Oxidative

Phosphorylation

ADP + Pi

ATP

The Citric Acic Cycle as Hub

for Cellular Metabolites

301

Red arrows:

anaplerotic reactions

304

▪ Generation of ATP though phosphorylation of ADP

Oxidative Phosphorylation

▪ Regeneration of NAD+ and FAD

Sum Reactions:

NADH + 1/2 O2 + H+

3 ADP

+ 3 Pi3 ATP

NAD+ + H-O-H

FADH2 + 1/2 O2

2 ADP

+ 2 Pi2 ATP

FAD + H-O-H

Mechanism:

▪ At the inner mitochondrial membrane (IMM) the above redox reaction are coupled

with the translocation of protons from the mitochondrial matrix into the

intermembrane space → Build up of a H+ gradient across the IMM

= respiratory chain / electron transport chain

▪ The flux of protons from the intermembrane space into the mitochondrial matrix in

with the phosphorylation of ADP = chemiosmosis

306

Oxidative Phosphorylation

(schematic overview)

äußere Mito.

-membran

(ÄMM)

innere Mito.

-membran (IMM)

Porin Porin

Intermembranraum

(IMR)

e-

NADH

O2

+ 4 H+NAD+ FADH2 FAD 2 H2O

H+

H+

H+

H+

Mito. Matrix

(MM)

Cytoplasma

(CP)

ADP

+ Pi

ATP

+ H2O

Electron transport chain

ATP

Synthase

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Photosynthesis

6 CO2 + 6 H2O 6 O2 + C6H12O6

Light energy

Photophosphorylation

Carbon assimmilation reactions

▪ Photosynthesis is the ultimate energy and carbon source for (almost) all life on

earth (except for chemolithothrophs in the deep sea).

▪ “Light reactions” = Photophosphorylation

“Dark reactions“ = Carbon assimilation reactions

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Photophosphorylation

▪ Photosystem II uses light energy to oxidize H2O to O2 and donate the electrons to

an electron transport chain that consists of plastoquinone (PQ→PQH2), the

cytochrome b6f complex (Cytb6fox→Cytb6f

red) and plastocyanin (PCox→PCred).

As the redox potentials increase along the electron transport chain it is a strongly

exergonic process, which enables the transport of H+ against the electrochemical

gradient (stroma = pH 8, thylkakoid lumen = pH 5).

▪ Takes place in the thylakoid membrane = the interface between the chloroplast

stroma and the thylakoid lumen

▪ Photosystem I uses light energy to catalyze the transfer electrons from PCred to

ferredoxin (Fdox→Fdred). Fdred transfers the electrons to NADP+ generating

NADPH.

▪ A H+-driven ATP synthase (CFoCF1-ATP Synthase) in the thylakoid membrane

uses the H+ gradient to phosphorylate ADP (1 ATP produced per 3H+ transported).

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Photophosphorylation

8 H2O + 8 photons + 2 NADP+ + 3 ADP + 3 Pi O2 + 3 ATP + 2 NADPH

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Carbon Assimilation Reactions▪ In the Calvin Cycle, NADPH and ATP are consumed to attach CO2 to a

monosaccharide backbone and reduce it to an aldehyde group thus producing

Glyceraldehyde-3-phosphate.

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Photosynthesis▪ In photosynthesis-independent metabolic pathways Glyceraldehyde-3-Phosphate

can be converted inside the chloroplast to the glucose polymer starch (energy

and organic-C storage), used for ATP production in the cytoplasm (glycolysis)

and mitochondria (PDH complex, citric acid cycle, oxidative (phosphorylation), or

converted to sucrose (O-α-D-glucopyranosyl-(1→2)-β-D-fructofuranoside) to be

delivered to other tissues of the plant (as energy and organic-C source).