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BIOENERGETICS AND OXYGEN TOXICITY

Rondang R. Soegianto2009

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Terminology: Bioenergetics Energy trransduction Biochemical thermodynamics Central theme: Understanding the mechanism of ATP synthesis through the oxidation of substrates (Nicholls) Specificity: Describes the transfer and utilization

of energy in biologic systems (Lippincott)

I. Bioenergetics

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G = H - TS

T = absolute temperatureH = enthalpy, heatS = degree of organization of atoms involved in reaction

G = available useful energy

Gibbs free energy (G) and Gibbs change of free energy (G)

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In the human body, T is constantThus:

H (enthalpy) changes are negligible and principally associated with

chemical bonds known as Internal Energy (Chemical Energy).Meaning: H = G Hence: G = E - TS (Lange, Exam. & Board Review)

Note: Nonbiologic systems utilize heat energy to

perform work. Biologic systems are isothermic and utilize chemical energy for living processes.

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Sign of G predicts the direction of a reaction

G = negative: Reaction is exergonic Proceeds with net loss of free energy Reaction goes spontaneously G = positive: Reaction is endergonic Proceeds only with net gain of energy

G = zero: Sistem is at equilibrium No net change takes place

Exothermic and endothermic reactions involve H as variable.

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Coupling of Endergonic to Exergonic ProcessesHarper 21st, Fig. 11.1

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Transfer of Energy via a High-energy Intermediate Compound Harper 21st, Fig 11-3

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Transduction of energy through ........Harper 21st, Fig 11-4

Transduction of energy thru common high-E comp.

Harper 21st, Fig 11-4

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Devlin 5th, p 538, Fig 13.1

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High-energy phosphates are involved in coupling processes

Harper 26th, p 82

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ATP = energy currency of the cell

Other nucleoside triphosphates: UTP, GTP, CTPMay take part in phosphorylations in the cell transferring free energy.

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II. Biologic Oxidation- Oxidation processes in living systems - Catalyzed by class I enzymes: Oxidoreductases

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Definition: Oxidation = Loss of electrons

Reduction = Gain of electrons Oxidation-reduction (redox) reactions are reversible

A ox + B red A red + B ox

Fe2+ Fe3+ + e-

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Oxidoreductases (Harper 26th)

1. Oxidases: A Containing Cu B As flavoproteins

2. Dehydrogenases: A. NAD+ or NADP+ as coenzyme B Flavin as coenzyme C Cytochromes (Fe-porphyrin as coenzyme)

3. Hydroperoxidases: A Peroxidase B Catalase 4. Oxygenases: A Dioxigenase B Monooxigenase

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1. Oxidases: - Remove 2 protons (H+) from substrate and pass to oksigen - Generate H2O or H2O2 Two groups of oxidases: A Containing Cu Example: Cytochrome a3 (cyt a3) also known as cyt aa3 Is a cytochrome oxidase Terminal compound of the respiratory chain in

mitochondria B. Flavoproteins, contain FMN or FAD Ex. : L-aminoacid oxidase Xanthine oxidase Aldehyde dehydrogenase

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2. Dehydrogenases cannot use O2 as H or e- acceptor

A. NAD+ or NADP+ as coenzyme Generally: NAD+-linked dehydrogenase in

energy transduction reaction

NADP+-linked (as NADPH) dehydrogenase in reductive synthesis

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B. Flavin as coenzyme Tightly bound to apoenzyme (prosthetic group) Linked to e- transport of the respiratory chain

C. Cytochromes Fe-containing hemoproteins

In the resp. chain: cyt b, c1, c, a (and cyt a3 which is an oxidase)

Cyt also in endoplasmic reticulum (P450 and P5), in plant cells, bacteria and yeast.

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3. Hydroperoxidases use H2O2 as substrate

A. Peroxidase reduces peroxides using various e- acceptors

H2O2 + AH2 2 H2O + A

In erythrocytes and other tissues:

H2O2 + 2 GSH GSSG + H2O GSH = Reduced gluthatione Glutamyl-cysteinyl-glycine (a tripeptide) -SH = Reducing group of cysteine residue

Peroxidase

Gluthatione peroxidase

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B. CatalaseHemoprotein with 4 heme groups

2 H2O2 2 H2O + O2

Found in: Blood, bone marrow, mucous membranes Kidney, liver Catalase destroys peroxides formed by oxidases

Catalase

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4. OxygenasesCatalyze direct transfer & incorporation of oxygen into asubstrate molecule. A. Dioxygenases Incorporate both atoms of molecular oxygen into the

substrate.

A + O2 AO2

Example: Homogentisate dioxygenase (oxidase)

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B. Monooxygenases (Mixed-Function Oxidases, Hydroxylases) Incorporate only one O atom into substrate. The other O atom is reduced to water.

AH + O2 ZH2 AOH + H2O + Z

Examples: Detoxication of many drugs Hydroxylation of steroids

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Free radicals

Transfer of a single e to O O (superoxide anion)Can damage membranes, DNA, etc.

Destructive effects Amplified by: Free radical chain reaction Removed by: Superoxide dismutase (SOD) in the reactions

O + O 2H H O + O

H O 2H O + O Catalase22 2

-

2

-2

+2 2

SOD

2 2

2

-2 2

-

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Mitochondria Make > 90% of cellular ATP

“Powerplant” of the cell Four Compartments

Matrix has numerous enzymes that reduce NAD+ to NADH during catabolism of “foodstuffs”

Inner Membrane has: Proteins that transfer electrons (the ETS) ATP synthase

Intermembrane SpaceOuter Membrane

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Faces of mitochondrial membrane (V & V Fig. 20-3)

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Role of RC of mitochondria in the conversion of food energy to ATPHarper 26 Fig. 12-2

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Harper 26 Fig. 12-4

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Cardiac muscle has high ATP demand.

Higher content of mitochondria than mosttissues.

High content of Electron Transport Chainsproteins: ATP synthase, ATP-ADP translocase, TCA cycle.

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Also:

High content of creatine kinase as energybuffer and energy shuttle (as well as brain) Heart (and brain) sensitive to ischemia andanoxia decreased ATP production

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Consequence of decreased ATP:

- Ion influx (Na+, Ca2+)- Swelling of tissue

Cardiac mitochondria can sequester Ca2+

Effect:Low amt stimulates TCAHigh amt activates phopholipase degrades membrane lipids

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