ch 6 an introductin to metabolism cells & chemistry

CH 6 An Introductin to CH 6 An Introductin to Metabolism Metabolism

CELLS & CHEMISTRY

I. Metabolism, Energy and I. Metabolism, Energy and LifeLife

metabole=change

The totality of an organism’s chemical reactions is called metabolism.

Metabolic pathways alter molecules in a series of controlled steps, selectively regulated by enzymes, and assisted by ATP energy.

1. The chemistry of life is organized into 1. The chemistry of life is organized into metabolic pathways.metabolic pathways.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

2 Types of metabolic 2 Types of metabolic PathwaysPathways

1. Catabolic pathways release energy by breaking down complex molecules to simpler compounds.– Example: Cellular Respiration

2. Anabolic pathways consume energy to build complicated molecules from simpler ones.– Example: Photosynthesis

Energy Coupling: The energy from catabolic reactions can be used to drive anabolism.


2. Organisms transform energy2. Organisms transform energy

Energy is the capacity to do work - to move matter against opposing forces.

…..The ability to rearrange matter

Kinetic energy is the energy of motion.– Objects in motion, photons, and heat are

examples.

Potential energy : stored energyExample: an object on a hill; water behind a dam

– Chemical energy is a form of potential energy in molecules .

Kinds of Energy energyKinds of Energy energy

Energy can be converted from one form to another.

– As the boy climbs a ladder to the top of the slide he is converting his kinetic energy to potential energy.

– As he slides down, the potential energy is converted back to kinetic energy.

– The potential energy in the food – he had eaten earlier provided – the energy that permitted him

to climb up initially.


Fig. 6.2

Organisms Are Energy TransformersOrganisms Are Energy Transformers

Thermodynamics:the study of energy transformations.

A closed system, like liquid in a thermos, is isolated from its surroundings.

In an open system energy (and often matter) can be transferred between the system and surroundings.

Energy transformations of life are Energy transformations of life are subject to two laws of thermodynamicssubject to two laws of thermodynamics


Organisms are Organisms are openopen systems. systems.

They absorb energy –– light or chemical energy in organic

molecules –

– and release heat and metabolic waste products

ThermodynamicsThermodynamics

1st Law: conservation of energy; E transferred/transformed, not created/destroyed

2nd Law: transformations increase entropy (disorder, randomness)

In most energy transformations, ordered forms of energy are converted at least partly to heat.

Heat is energy in its most random state.

Combining the two laws, the quantity of energy is constant, but the quality is not..


Not all energy is available to do work.

Free energy (G) is the amount of a system’s energy that is available to do work.


Fig. 6.5

The free energy (G) in a system:

(G= Total Energy - Energy NOT available to do work)

G = H – TS

H = Total Energy

S = entropy

T = temperature in Kelvin units ( K = C + 273 )


Gibbs Free Energy (G)Gibbs Free Energy (G)

Reactions can be classified based upon their free energy changes :final - Initial (Δ G)

1. Exergonic Reactions proceed with a net release of free energy; they are spontaneous ; the products have less free energy.

net loss of free energy. (cellular respiration) - Δ G

2. Endergonic reactions absorb free energy-- net gain in free energy, not spontaneous: (photosynthesis)

+ Δ G

– For the overall reaction of cellular respiration:C6H12O6 + 6O2 6CO2 + 6H2O

– ΔG = -686 kcal/mol

– Through this reaction 686 kcal have been made available to do work in the cell.

– The products have 686 kcal less energy than the reactants.


Photosynthesis is steeply endergonic, powered by the absorption of light energy.

Δ G = + 686 kcal / mol.


Endergonic: Building molecules/tissues/proteins C + D CD

Exergonic: Ex: Glucose catabolism in

mitochondriaAB A + B

Energy is Absorbed-Energy is Absorbed-

Endergonic Endergonic ReactionsReactions

product withmore energy

(plus by-products602 and 6H2O)

ENERGY IN

6 12

An exergonic reaction proceeds with a net

release of free energy and Δ G is negative.


Fig. 6.6a

A system at equilibrium is at maximum stability.– In a chemical reaction at equilibrium, the rate of

forward and backward reactions are equal and there is no change in the concentration of products or reactants.


Metabolic DisequilibriumMetabolic Disequilibrium

Reactions in closed systems eventually reach equilibrium and can do no work.

At equilibrium Δ G = 0 and the system can do no work

A cell that has reached metabolic equilibrium has a Δ G = 0 is dead! Metabolic disequilibrium is one of the defining features of

life.

An Open System


Fig. 6.7b

Cells maintain disequilibrium because they are open with a constant flow of material in and out of the cell.

A cell continues to do work throughout its life

BUT---Catabolic pathways in cells releases free energy in a series of reactions, not in a single step.

GOOD THING, esp. since some of this energy released is heat energy!!!


Fig. 6.7c

Let’s Recap..Let’s Recap.. EXERGONIC REACTIONS

Chemical products have less free energy than the reactant molecules.

Reaction is energetically downhill

Spontaneous reaction

Δ G is negative - Δ G is the maximum amount

of work the reaction can perform.

ENDERGONICREACTIONS

Products store more free energy than reactants

Reaction is energetically uphill

Non-spontaneous reaction (requires energy input)

Δ G is positive + Δ G is the minimum amount of

work required to drive the reaction.

A cell does three main kinds of work:– Mechanical work, beating of cilia, contraction of

muscle cells, and movement of chromosomes.– Transport work, pumping substances across

membranes against the direction of spontaneous movement.

– Chemical work, driving endergonic reactions such as the synthesis of polymers from monomers.

In most cases, the immediate source of energy that powers cellular work is ATP.

5. ATP powers cellular work by 5. ATP powers cellular work by coupling exergonic rxs. to endergonic coupling exergonic rxs. to endergonic

rxs.rxs.


ATP (adenosine triphosphate) .– A nucleotide.

Consists of the nitrogen base adenine, the sugar ribose, and a chain of three phosphate groups.


Fig. 6.8a

P P P

ribose

adenine

**ATPATP--**Body can store it—like batteriesBody can store it—like batteries*Energy is released upon hydrolysis of ATP.*Energy is released upon hydrolysis of ATP.* This energy powers NONSPONTANEOUS * This energy powers NONSPONTANEOUS

metabolic reactionsmetabolic reactions..

How ATP performs work How ATP performs work 1. ATP Hydrolysis1. ATP Hydrolysis

ATP tail: negative charge—makes it unstable

ATP hydrolysis: release

of free E.

– ATP + H2O ADP + Pi + energy

– and releases 7.3 kcal of energy per mole of ATP under standard conditions.

– In the cell Δ G = -13 kcal/mol.

How ATP performs workHow ATP performs work

2.2. Phosphorylation of anPhosphorylation of an Intermediate Intermediate

A phosphate group (Pi )is

transferred to another molecule (Intermediate).

– This molecule is now phosphorylated.

– This molecule is now more reactive b/c has more energy—so ATP is used to supply free energy to nonspontaneous reactions.

ATP is a renewable resoure.

Energy released by breakdown reactions in the cell is used to phosphorylate ADP, regenerating ATP


Fig. 6.10

The ATP CycleThe ATP Cycle


Fig. 6.9 The energy released by the hydrolysis of ATP is harnessed to the endergonic reaction that synthesizes glutamine from glutamic acid through the transfer of a phosphate group from ATP.

Chemical Reactions either Chemical Reactions either involve..involve..

Bond Breaking

ORBond Forming

In either case, some energy is needed.

Enzymes & Chemical ReactionsEnzymes & Chemical Reactions

A chem. reaction will occur spontaneously if it releases free energy.– But this may be too slow!!– Enzymes—speed up reactions

EnzymesEnzymes Catalyst: chemical that accelerated a reaction Enzyme: Biological Catalyst: accelerate

reactions w/o being consumed; most are proteins.

Reduce the Free Energy of Activation (activation E): the E required start a chemical reaction

Transition State:

Substrate, enzyme ,reactant

Active site: pocket or groove on enzyme that binds to substrate

Induced fit model

Enzyme Enzyme PropertiesProperties

1. Biological catalysts 2. Speed up rate of chem rx. 3 Not used up or altered (reusable) in chem rxs. 4.Lower Ea, so the transition state can be reached at

cellular temperatures. 5. They are picky (substrate specific). 6. Made of proteins (mostly) 7. Sensitive to environmental conditions (temp, ph, ions

HOW DO ENZYMES HOW DO ENZYMES WORK?WORK?1.Strains bonds-they break

2.Encourage bond formation

–Joins 2 molecules together

Induced FitInduced FitSubstrate enters ACTIVE SITE

(groove), enzyme changes shape–Bonds broken–Or–New bond forms–Product(s) then released

two substrate

molecules

active sight

substratescontactingactive siteof enzyme

endproduct

enzymeunchangedby thereaction

Join Join molecules molecules togethertogether

.

Split moleculesSplit molecules

Factors Factors Influencing Influencing

Enzyme ActivityEnzyme Activity1. Temperature

2. PH

3. Concentration of Enzyme

4. Concentration of Substrate

Effect of TemperatureEffect of Temperature

Effect of Effect of TemperatureTemperature

Effect of pHEffect of pH(Pepsin, and Trypsin)(Pepsin, and Trypsin)

Substrate and Substrate and enzyme enzyme

concentrationconcentrationThe higher, the faster (positively

correlated)

Materials that may assist Materials that may assist enzymes.enzymes.

Cofactors: Non-organic helpers–Zinc, iron, copper

Coenzymes: organic molecules–Most vitamins

Enzyme InhibitorsEnzyme Inhibitors

Irreversible (covalent); Reversible (weak bonds)

Competitive: competes for active site (reversible); mimics the substrate

Noncompetitive: bind to another part of enzyme (allosteric site) altering its conformation (shape); poisons, antibiotics

Example of Competitive Inhibition (Many medications work by competitive inhibition)

This type of enzyme may OR may not also have an ALLOSTERIC SITE

Here, the BROWN dots are substrate. The BLUE dots are competitive inhibitors. What has happened?

They will bind to another part of an enzyme molecule, causing it to change shape-the active site changes shape.

III. The Control of MetabolismIII. The Control of Metabolism

1. Allosteric Regulation– Regulatory molecules may inhibit or activate enzyme activity.

Bind to a second site allosteric site.—a specific receptor site remote from the active site. (allo= different).

– Result may be inhibition or stimulation.

2. Feedback Inhibition– The switching off of a metabolic pathway by its end product.

3. Cooperativity-amplifies response of enzymes to substrates. Shape may change so increase enzyme activity.

One of the common methods of metabolic control is feedback inhibition in which a metabolic pathway is turned off by its end product.

The end product acts as an inhibitor of an enzyme in the pathway.

When the product is abundant the pathway is turned off, when rare the pathway is active.


Fig. 6.19

In enzymes with multiple subunits, binding by a substrate to one active site stabilizes favorable conformational changes at all other subunits, a process called cooperativity.

This mechanism amplifies the response of enzymes to substrates, priming the enzyme to accept additional substrates


Fig. 6.20

Energy inputrequired topush Roveruphill

Potential energy released by thedownhill run (but not captured to do useful work)

Fig. 5.5a, p. 78

ATP-ATP-Coupled ReactionsCoupled Reactions

When a reaction that absorbs energy is DRIVEN by the conversion of :

ATP ADP +P + energy

Energy ABSORBED & energy RELEASED

Energy RelationshipsEnergy Relationships

ATP

BIOSYNTHETIC PATHWAYS(ANABOLIC)

ENERGY INPUT

DEGRADATIVE PATHWAYS(CATABOLIC)

energy-poor products(such as carbon dioxide, water)

large energy-rich molecules(fats, complex carbohydrates,

proteins, nucleic acids)

simple organic compounds(simple sugars, amino acids,

fatty acids, nucleotides)

ADP + Pi

ch 6 an introductin to metabolism cells & chemistry

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