(Rondang) Bioxygencgetyitoitxics - 18 September 2012

BIOENERGETICS AND OXYGEN TOXICITYRondang R. Soegianto2009

I. BioenergeticsTerminology: 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)

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)

In the human body, T is constantThus: H (enthalpy) changes are negligible and principally associated withchemical 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.

Sign of G predicts the direction of a reactionG = 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.

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

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

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

Transduction of energy thru common high-E comp.

Harper 21st, Fig 11-4

Devlin 5th, p 538, Fig 13.1

High-energy phosphates are involved in coupling processes

Harper 26th, p 82

ATP = energy currency of the cell

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

II. Biologic Oxidation

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

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-

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

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

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

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.

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

PeroxidaseGluthatione peroxidase

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

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)

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

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

Catalase222 -2 -2+22SOD222- 22 -

Mitochondria Make > 90% of cellular ATPPowerplant of the cell Four CompartmentsMatrix has numerous enzymes that reduce NAD+ to NADH during catabolism of foodstuffsInner Membrane has:Proteins that transfer electrons (the ETS)ATP synthaseIntermembrane SpaceOuter Membrane

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

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

Harper 26 Fig. 12-4

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.

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

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