Chapters 2.3 & 8

Metabolism IEnergy & Enzymes

Roadmap 8

In this chapter you will learn how

looking at energy,asking

looking at enzymes,asking

8.1

8.2

8.3

8.4

8.5

What happens toenergy in chemicalreactions?

How do enzymes help speedchemical reaction rates?

Can chemical energydrive nonspontaneousreactions?

What factors affect enzyme function?

How do enzymes work togetherin metabolic pathways?

Enzymes use energy to drive the chemistry of life

Key Concept: Energy conversions in metabolism are accompanied by changes in entropy (ΔS) and potential energy (enthalpy; ΔH).

Thermodynamics

Glucose

Cellulose (polymer of glucose)

Valley Complex (Bitterroot National Forest, Montana)

Jul 31, 2000

The Catabolism of Glucose

C6H12O6 + 6 O2 + 38 ADP + 38 Pi 6 CO2 + 6 H2O + 38 ATP

glucose oxygen carbon dioxide

water+ heat

Energy content Breakdown

– Fire– Series of reactions in a

metabolic pathway

Metabolism

Metabolism – controlled, enzyme-mediated chemical reactions by which cells acquire and use energy to perform work

Energy

Energy – the capacity to do work (bring about change) or supply heat

Forms of energy roughly fall into two categories (many are a mix of both!)

Potential Energy - energy of position, stored energy Chemical energy

Kinetic Energy - energy of motion Solar energy Mechanical energy Thermal energy (molecular motion; temperature; heat)

Cellular Work? – What is that?

Chemical work - store, build, rearrange, break apart molecules

Mechanical work - movement (within cells, cells, organisms)

Electrochemical work - move charged substances against concentration gradient

Energy Transformation

Thermodynamics: the study of energy transformations 1st Law: Conservation of Energy

Energy cannot be created or destroyed, but it can be transferred and transformed

2nd Law: Increasing Entropy (S)Energy cannot be changed from one form to another without a loss of usable energy (increase in disorder)

Moving Toward Disorder (Entropy)

Glucose• More organized• Less entropy (less stable)

Carbon Dioxide + Water• Less organized• More entropy (more

stable)

ΔS = S(products) – S(reactants)

Entropy, Order, and Energy

Cells, tissues, organs, organisms… are highly ordered!

How is this order maintained when things are spontaneously moving toward a state of disorder?

To maintain order… need energy!

“A living cell is a temporary respository of order purchased at the cost of a constant

flow of energy.”

Potential Energy (H)

Remember: Potential energy is the energy of position… look at the position of the electrons!

Which has more potential energy (H), the reactants or the products?

How do you know? Energy(heat/light)

Potential Energy (Enthalpy, H)

What is the change in potential energy (ΔH) of this reaction?

Energy(heat/light)

ΔH = H(products) – H(reactants)

Chemical ReactionsSpontaneous

Spontaneous Reactions – reactions that will proceed on their own without any continuous external influence (such as energy)

Reactions tend to be spontaneous if products have…

lower potential energy

more entropy (less order)

…than reactants

Spontaneous reactions are not necessarily fast!

Chemical ReactionsSpontaneous

Spontaneous Reactions – reactions that will proceed on their own without any continuous external influence (such as energy)

Reactions tend to be spontaneous if products have…

lower potential energy - ΔH is ?

more entropy (less order) - ΔS is ?

…than reactants

Spontaneous reactions are not necessarily fast!

Spontaneous Reaction: Example

Glucose• More organized (less entropy)• More potential energy

Carbon Dioxide + Water• Less organized (more entropy)• Less potential energy

ΔH < 0ΔS > 0

Gibbs Free-Energy

Gibbs Free-Energy (G) – a measure of the energy found in a molecule; includes the combined contributions of potential energy and entropy

Can use information about the free energy of reactants and products to determine if a reaction will proceed spontaneously

∆G = ∆H - T∆S

1 Methane(CH4)

2 Oxygens (O2)

Potential energy drops 1 Carbon dioxide

(CO2)2 Waters (H2O)

Gibbs Free-Energy Change (∆G)

∆G = G(products) – G(reactants)

Exergonic Reaction – can occur spontaneously; releases energy; ∆G < 0

Endergonic Reaction – cannot occur spontaneously; requires an input of energy; ∆G > 0

Exergonic and Endergonic Reactions

Exergonic Reaction Rxn releases energy

and thus occursspontaneously.

EXAMPLE: respiration Endergonic Reaction

Rxn won’t proceedunless energy issupplied.

EXAMPLE: biosynthesis

Key Concepts:• Exergonic reactions can be coupled to

endergonic reactions to make the overall process spontaneous.

• Coupling of oxidation and reduction reactions enables electron transfer (a form of energy conservation).

Reaction Coupling

Reaction Coupling

Enzymes link energy release from exergonic rxns (DG < 0) to energy demand of endergonic rxns (DG > 0)

Reaction Coupling

Enzymes link energy release from exergonic rxns (DG < 0) to energy demand of endergonic rxns (DG > 0)

Coupling makes the net rxn spontaneous!

ATP hydrolysis releases energy…

…but that energy is useless unless captured in some form, such as this:

ATP: Understanding its “power”

Redox Reactions

OIL – Oxidation Is Loss… of electrons. RIG – Reduction Is Gain… of electrons. Coupling oxidation & reduction transfers energy!

Often involve the transfer of protons (H+) along with electrons; hence, dehydrogenation

Metabolic Redox Reactions

So, what are the clues that this is a redox reaction? Which compound is being oxidized and which is

being reduced in the reaction?

Metabolic Redox Reactions

C6H12O6 + 6O2 6CO2 + 6H2O

From __________ to ___________

From __________ to __________

How do I figure this out? Method 1: Which atoms hold electrons where?

Method 2: Since protons (H+) accompany electrons, oxidation is loss of H’s; reduction is gain of H’s!

Metabolic Redox Reactions

Need to know relative electronegativity

Key Concepts:• Enzymes catalyze reactions by…

• Bringing reactants together in a precise orientation so that “effective collisions” are more likely.

• Stabilizing the transition state (high energy state halfway between reactants and products).

Enzymes

Enzymes

Enzyme – protein (usually) catalyst used to speed up and control biological reactions

Catalyst – substance that increases the rate of a chemical reaction without undergoing any permanent change itself

Transition state

ProductsSubstrates

Enzyme

How Do Enzymes Speed Reactions?

Transition State

Reactants

Products

Activation Energy

Why is this beneficial?

Transition State and Activation Energy

Transition State – high-energy intermediate state of reactants (combination of old and new bonds) that must be achieved for a reaction to proceed

Activation Energy – amount of energy required to reach the transition state; Ea

A + B—C Substrates(Reactants)

A—B + C Products

A- - B- -C Transition State

Enzymes

Enzymes (catalysts) lower the activation energy of a reaction

Without enzymes, reactions proceed very slowly

The net change in energy between the products and reactants (∆G) is the same with or without an enzyme

Reaction with High Activation Energy

Transition State

Substrates (Reactants)

Products

Ea without enzyme

Enzyme-Catalyzed Reaction

Products

Transition State

Ea with enzyme

∆G does not change

Substrates (Reactants)

Enzyme Features

Enzymes…

1. do not make anything happen that could not happen on its own (just make it faster)

2. are not used up in the reaction (are recycled)

3. usually work in the forward and reverse directions

4. are specific for their substrate(s)

Enzymes Catalyze Chemical Reactions

Induced-Fit Model

Specific substrate binds to an enzyme active site

Substrate(glucose)

Substrate(ATP)

Enzyme(hexokinase)

When the ATPand glucose bindto the active site,the enzymechanges shape.This “inducedfit” reorients thesubstrates andpushes them towardsthe transition state.

Steps in an Enzyme-Catalyzed Reaction

Substrates

Enzyme

Transition state

Shape changes

ProductsSubstrates

Enzyme

Transition state Products

Shapechanges

1. Initiation: Reactants bind tothe active site in a specificorientation, forming anenzyme-substrate complex.

2. Transition state facilitation: Interactions between enzymeand substrate lower theactivation energy required.

3. Termination: Products havelower affinity for active siteand are released. Enzyme isunchanged after the reaction.

high energy

H2OH2O - H

HO -

Substrates bindto active site

Enzyme interactionsbend substrates towards

transition state

Products are formed

and released

Mechanisms of Enzyme Catalysis

Mechanisms by which enzymes facilitate chemical reactions: Increase “effective concentration” of substrates

Help substrates get together in active site Orient substrates correctly (so that collisions are “effective”) Shuttle out water

Transfer energy Conserve energy as it is transferred (in the form of groups of

atoms, such as phosphate groups)

Stabilize the transition state Help substrates achieve activation energy via many weak

interactions in the active site

Factors That Affect Reaction Rates

Reaction rate (amount of product generated per unit time) is dependent upon…

Substrate Concentration Enzyme Concentration Temperature pH Cofactors Inhibitors/Activators

Concentration of SubstrateEnzyme-Catalyzed Reaction

Why does it reach a

maximum?

Temperature

pH

pH

Cystic Fibrosis• Trypsin transport to duodenum is

deficient• Meconium ileus (infant’s first stools –

obstruction)

Cofactors

Cofactors - non-protein molecules that help an enzyme function properly (provide functional groups that amino acids lack) Inorganic ions - metals (Cu++, Zn++, Fe++) – aid in transfer

of electrons and oxygen Organic molecules – coenzymes – aid in transfer of

electrons and functional groups Coenzyme A NAD+ and FAD Vitamins

Enzyme Inhibition

Enzyme Inhibition – decrease in enzyme activity resulting from the binding of an inhibitor to the enzyme

Enzyme Inhibition

Types of Enzyme Inhibition Competitive Inhibition – inhibitor resembles the

enzymes’ normal substrate – it competes with substrate for binding to the active site

Allosteric regulation – regulator molecule binds to enzyme at an allosteric site, causing a conformation change that affects substrate binding to the active site

Enzyme Inhibition:Competitive Inhibition

Competitive inhibitor

Substrate

Enzyme

When the regulatory molecule binds to the enzyme’s active site, the substrate cannot bind

Enzyme Inhibition:Competitive Inhibition - Example

HIV Protease Inhibitor

Active Site of HIV Protease

Enzyme Inhibition:Non-competitive Inhibition

Substrate

Enzyme

Regulatorymolecule

When the regulatory molecule binds to a different site on the enzyme (allosteric site), it induces a shape change that makes the active site unavailable

Enzyme Inhibition:Non-competitive Inhibition - Example

Feedback Inhibition Enzyme 1 has allosteric

binding site to which the end-product of the pathway can bind.

When the end-product is abundant, it binds to this allosteric site thereby inhibiting Enzyme 1.

When Enzyme 1 is inactivated, the entire pathway stops.

Enzyme Inhibition

0 50 100 150 200 250 300 350 400 450 5000

5

10

15

20

25

Enzyme Inhibition: Graph B

no inhibitor

10 mM inhibitor

50 mM inhibitor

Substrate Concentration (mM)

Rate

of P

rodu

ct F

orm

ation

(min

-1)

0 50 100 150 200 250 300 350 400 450 5000

5

10

15

20

25

Enzyme Inhibition: Graph A

no inhibitor

10 mM inhibitor

50 mM inhibitor

Substrate Concentration (mM)

Rate

of P

rodu

ct F

orm

ation

(min

-1)

competitive non-competitive

Applying what we’ve learned to understand the structure and function of metabolic pathways.

Putting Things Together

Coming back to metabolism…

Some concepts we can now understand1. Enzymes are specific in their substrate binding.2. Enzymes catalyze metabolic reactions by lowering

activation energy (stabilizing transition states).3. Enzyme activities can be regulated by molecules that

affect substrate binding or catalytic functions.

But how do these pathways work together?

1. Each step is catalyzed by a specific enzyme.

2. Products of one enzymatic reaction become the substrates for the next one.

3. Substrates/products of middle steps are called intermediates.

Metabolic Pathways: Key Concepts

4. End products often act as feedback inhibitors of the first enzyme in the pathway.

What type of inhibition is this?

5. Pathways can be arranged on scaffolding or as multienzyme complexes.

Metabolic PathwaysKey Concepts (cont’d)

4 connected pathways Glycolysis Pyruvate processing

(prep rxn) Citric acid (Krebs) cycle Electron transport

chain/chemiosmosis

2 types of electron carriers

2 types of ATP synthesis 2 compartments

Which leads us to cell respiration…

More about this next week…

Energy Flow in Cell RespirationElectron transfer proton gradient ATP synthesis

Electron Transport Chain

Chemiosmosis

Where did these electrons come from?

These electrons + some protons + O2 combine to form H2O