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Definition of Bioenergetics It is a study of the energy changes that accompany biochemical reactions Fuel essential metabolic processes in the bodyBioenergetics

Energy in biological systems is described in terms of free energy

Biologic systems conform to the general laws of thermodynamics

Gibbs free energy is the most important thermodynamic function in biochemistry

Gibbs Free Energy

Gibbs change in free energy ( G) is the amount of energy capable of doing work during a chemical reaction , at constant temperature and pressure

A. First Law of Thermodynamics

The total energy of a system, including its surroundings, remains constant

energy is never destroyed, merely conserved, transforming from one form to another

B. Second Law of Thermodynamics

The total entropy of a system including its surroundings must increase if a process is to occur spontaneously

Concepts:

Enthalpy (H)

Heat content of the reacting system

In exothermic reactions:

Heat content of products > reactants

H is negative

In endothermic reactions:

Heat is taken from surroundings

H is positive

Entropy (S)

Extent of disorder or randomness of a system

Becomes maximum as equilibrium is approached

Becomes maximum as equilibrium is approached

When products of a reaction are disordered than the reactants, the reaction is said to proceed with a gain in entropy

When entropy(((, S is positive

Formula:

G = H - T(S)

G

free energy change

H

change in enthalpy (heat)

T

absolute temperature

S

change in entropy

G is always negative for a spontaneously reacting system

Favorable conditions:

S is (+) change in entropy

H is (-) heat content

If G < 0

Process is feasible

(exergonic)

If G = 0 Equilibrium prevails

(isoergonic)

If G > 0

Process not feasible

(endergonic)

Equilibrium constant

aA + bB cC + dD

K eq = _[C] c [D] d __

[A] a [B] bEquilibrium

When equilibrium is reached, no net change in the concentration of the reactants and products occur

When a reacting system is not at equilibrium, the tendency to move towards equilibrium represents a driving force (tendency to move towards equilibrium)

The force driving the system toward equilibrium is defined as the standard free energy change, G0

The biochemical standard state :

25(C

Concn of reactants and products = 1M

pH = 7

Free energy change is directly related to the equilibrium constant

The standard free energy change of a chemical reaction is an alternative mathematical way of expressing its equilibirum constant

G0 = -RTlnKeq

Where:

R = 8.315 J/mol (gas constant)

T = 298 K (25(C) (absolute temp.)

ln = natural logarithm

G and G0

G0 - concerned with the direction a reaction will take for proceeding toward equilibrium

The rate at which this reaction will occur could not be predicted

G - depends on reactant and product concentrations, and the prevailing temperature

G0

Standard free energy values are additive

Example: Phosphorylation of Glucose to Glucose 6-P

G0 (kJ(mol-1)

Endergonic Pi + glucose Glucose-6-P + H2O +13.8

Exergonic ATP + H2O ADP + Pi - 30.5

Overall ATP + glucose ADP + Glucose-6-P - 16.7

Coupled

Reaction

Keq

In contrast, equilibrium constants are multiplicative

Example:

G0 (kJ(mol-1) Keq

Pi + glucose Glucose-6-P + H2O +13.8 3.9 x 10-3 M

ATP + H2O ADP + Pi

- 30.5 2 x 105 M

ATP + glucose ADP + Glucose-6-P - 16.7 7.8 x 102

The relationship between G0 and Keq is logarithmic

Common intermediates

Pi and H2O

Used by all living cells in the synthesis of metabolic intermediates & cellular components

This works only if compounds such as ATP are continuously available

ATP

ADP + PiG0 of hydrolysis of some organophosphates of biochemical significance

Compound

G0 (kcal/mol)Phosphoenolpyruvate

- 14.8

Carbamoyl phosphate

- 12.3

1,3 Biphosphoglycerate

- 11.8

( to 3-phosphoglycerate)

Creatine phosphate

- 10.3

ATP ( AMP + PPi

- 7.7

ATP ( ADP + Pi

- 7.3

Glucose 1-phosphate

- 5.0

Pyrophosphate

- 4.6

Fructose 6-phosphate

- 3.8

Glucose 6-phosphate

- 3.3

Glycerol 3-phosphate

- 2.2

Low energy and high-energy phosphates

Low-energy phosphates - include ester phosphates found in the intermediates of glycolysis pathway (carb metab)

High-energy phosphates( - include ATP, anhydrides, enolphosphates (phosphoenolpyruvate) and phosphoguanidines (creatine phosphate and arginine phosphates)

Sources of high-energy phosphates

Oxidative phosphorylation

Glycolysis

The citric acid cycleRate of ATP Turnover

ATP is continuously being hydrolyzed & regenerated

Brain cells have only a few seconds supply

Muscle & nerve cells have a high ATP turnover but have a free energy reservoir that functions to regenerate ATP rapidly

ATP Regeneration

G0 (kJ/mol-1)

Endergonic PEP + H2O Pyruvate + Pi - 61.9

Exergonic ADP + Pi ATP + H2O + 30.5

Overall PEP + ADP Pyruvate + ATP - 31.4

Coupled

Reaction

ATP is regenerated by coupling its synthesis from ADP & Pi to the even more exergonic hydrolysis of phosphoenolpyruvate (PEP)

Phosphagens

Act as storage forms of high-energy phosphate

Include:

Creatine phosphate (found in skeletal muscle, heart, spermatozoa, brain)

Arginine phosphate (invertebrate muscle

Formation of ATP

Substrate level phosphorylation

Oxidative phosphorylation

Adenylate kinase reaction

The process by which cells derive energy in the form of ATP is called cellular respirationBiologic Oxidation

Oxidation = removal of electrons

Reduction = gain of electrons

Free energy change may also be expressed in terms of the redox potential

Enzymes involved are called oxidoreductases

Oxidoreductases

Oxidases

Dehydrogenases

Hydroperoxidases

Oxygenases

Oxidases

Cytochrome oxidase

Terminal component of the chain of respiratory carriers found in mitochondria

Transfers electrons to final oxygen acceptor

Flavoprotein enzymes

L-amino acid oxidase

Xanthine oxidase

Dehydrogenases

Transfer of hydrogen from one substrate to another in a coupled redox reaction

Function as components in the respiratory chain of electron transportDehydrogenases & Nicotinamide Coenzymes

NAD-linked dehydrogenases catalyze redox reactions in:

Glycolysis

Citric acid cycle

Respiratory chain of mitochondria

NADP-linked dehydrogenases catalyze reactions in reductive pathways:

NADP-linked dehydrogenases catalyze reactions in reductive pathways:

Fatty acid synthesis

Steroid synthesis

Pentose phosphate pathway

Hydroperoxidases

Use H2O2 or an organic peroxide as a substrate

Protect the body against harmful peroxides which could lead to free radical formation

Include peroxidase and catalase

Oxygenases

Catalyze direct transfer and incorporation of oxygen into a substrate molecule

Dioxygenases incorporate both atoms of molecular O2 into the substrate

Monooxygenases incorporate only one atom of molecular O2 into the substrate

Monooxygenases Also called mixed function oxidases or hydroxylases

Notable example is the superfamily of heme-containing monooxygenases cytochrome P450 (metabolism of many drugs)

Both NADH & NADPH donate reducing equivalents for the reduction of these cytochromes

Substrate Level Phosphorylation ATP may be formed from phosphoenolpyruvate by direct transfer of a phosphoryl group from a high energy compound to ADP

These reactions most commonly occur in the early stages of carbohydrate metabolism

Oxidative phosphorylation

Act to generate a proton (H) concentration gradient across a membrane

Discharge of this gradient is enzymatically coupled to formation of ATP from ADP & Pi (the reverse of ATP hydrolysis)

Mitochondria and enzyme compartments

Inner membrane:

Electron carriers (complexes I - IV)

ATP synthase

Membrane transporters

Mitochondrial matrix

Outer membrane

Metabolic pathways oxidizing glucose & fatty acids

Electron transport chain

Respiratory Chain components The flow of electrons through complexes I, III & IV results in the pumping of protons from the matrix across the inner mitochondrial membrane into the intermembrane space

ATP synthase spans the membrane and acts like a rotary motor using the potential energy of the proton gradient to synthesize ATP

Impermeability of the inner mitochondrial membrane to protons and other ions necessitates exchange transporters

Substrate shuttles: glycerophosphate, malate & creatine phosphate

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- geLowfish -(ANABOLISM)

Energy utilization

Synthesis of macromolecules

Muscle contraction

Active ion transport

Thermogenesis

(CATABOLISM)

Energy production

Carbohydrate

Lipid

Protein

Catalyze removal of H from a substrate using oxygen as a hydrogen acceptor

AH2 1/2 O2 AH2 O2

Oxidase Oxidase

A H2O A H2O2

Glucose Fatty acids

Acetyl CoA

Citric acid

cycle 2H ATP

2CO2

Oxidation of NADH and FADH2

Electron transport chain

Complex I Complex IIIComplex IV

NADH FMN-(Fe-S)7 b562 b566 a a3

Q c O2

Fe-S c1 CuA CuB

Succinate FAD-(Fe-S)3