biological science - sisd.net · pdf filebiological science fifth edition 8 lecture...

Freeman Quillin Allison

© 2014 Pearson Education, Inc.

BIOLOGICAL SCIENCEFIFTH EDITION

8

Lecture Presentation by

Cindy S. Malone, PhD, California State University Northridge


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 to

energy in chemical

reactions?

How do enzymes help speed

chemical reaction rates?

Can chemical energy

drive nonspontaneous

reactions?

What factors affect enzyme function?

How do enzymes work together

in metabolic pathways?

Enzymes use energy to drive the chemistry of life


▪ Two types of energy exist

▪ Kinetic energy

– Energy of motion

– Molecular level is thermal energy

▪ Potential energy

– Energy of position or configuration

– Molecular level chemical energy is stored

▪ The free energy of a reaction is the amount of

energy available to do work


Figure 8.1

1. Potential energy 2. Kinetic energy 3. Other forms of energy

Conclusion: Energy is neither created nor destroyed; it simply changes form.

Heat

Ep (higher)

Ep (lower)

Ek

Mechanical

energySound


▪ Chemical potential energy

– In cells

– Electrons are the most important source

▪ Amount of potential energy in an electron is based on

– Its position relative to positive and negative charges

▪ Electrons closer to negative charges and farther from

positive charges

– Have higher potential energy

▪ Molecular potential energy

– Is a function of electron configuration and position


Figure 8.2

(a) The potential energy of an electron is related to its position.

(b)

Electrons have the

greatest potential energy

in the outermost electron

shells

Nucleus 1st 2nd 3rd Electron shells

Ep (higher)

Ep (lower)

Ek

Heat

or light

1. Potential energy 2. Kinetic energy 3. Other forms of energy

Conclusion: Energy is neither created nor destroyed; it simply changes form.


▪ Energy is conserved

▪ Energy cannot be created or destroyed

▪ Energy can only be transferred and transformed

▪ Enthalpy (H) includes

– The potential energy of the molecule (heat content)

– Effect of the molecule on surrounding pressure and

volume

– Changes in enthalpy are represented by H

– Difference in heat content


▪ Exothermic reaction

– Releases heat energy

– G 0

– Products have less

potential energy than

reactants

▪ Endothermic reaction

– Heat energy is taken up

– G 0

– Products have higher

potential energy than

reactants


▪ Amount of disorder

▪ When the products of a chemical reaction become less

ordered than the reactant molecules

– Entropy increases

– S 0

▪ Second law of thermodynamics

– Total entropy always increases in isolated systems


▪ Determines whether a reaction is spontaneous or

requires added energy to proceed

G H TS

▪ G Gibbs free energy change

▪ H change in enthalpy

– A measure of chemical potential energy

▪ S change in entropy

– A measure of disorder

▪ T temperature in degrees Kelvin


▪ G 0 a spontaneous reaction

an exergonic reaction

▪ G 0 a reaction that requires energy input to

occur and is not spontaneous

an endergonic reaction

▪ G 0 a reaction that is at equilibrium


▪ For most reactions to proceed

– One or more chemical bonds have to break

– Others have to form

▪ Substances must collide in a specific orientation that

brings the electrons involved near each other

▪ When the concentration of reactants is high

– More collisions should occur

– Reactions should proceed more quickly


Figure 8.5

Lower energyproducts

Lower energyreactants

Higher energyproducts

Higher energyreactants

Endergonic reaction(requires energy)

Exergonic reaction(releases energy)

Energy

Energy

Energy


▪ Reduction–oxidation reactions (redox reactions)

– Are chemical reactions that involve electron

transfer

▪ When an atom or molecule gains an electron

– It is reduced

– Reduction gain of one or more e and a hydrogen

ion (H+)


▪ ATP adenosine triphosphate

– ATP is the cellular currency for energy

– It provides the fuel for most cellular activities

▪ ATP

– Has high potential energy

– Allows cells to do work

▪ ATP works by

– Phosphorylating target molecules

– Transferring a phosphate group


▪ Hydrolysis of ATP is exergonic because

– The entropy of the product molecules is much higher

than that of the reactants

▪ Energy released during ATP hydrolysis

– Is transferred to a protein during phosphorylation

– Usually causes a change in the protein’s shape


Figure 8.8

(a) ATP stores a large amount of potential energy.

(b) Energy is released when ATP is hydrolyzed.

Phosphate groups

Adenine

Ribose

Clustered negative chargesraise the potential energyof linked phosphate groups

WaterADP

Inorganicphosphate

EnergyATP


▪ Enzymes

– Are protein catalysts

– Typically catalyze only one reaction

▪ Most biological chemical reactions occur at meaningful rates only in the presence of an enzyme

▪ Enzymes

– Bring reactants together in precise orientations

– Stabilize transition states

▪ Protein catalysts are important

– Because they speed up the chemical reactions that are required for life


▪ Enzymes bring substrates together

– In specific positions that facilitate reactions

– Are very specific in which reactions they catalyze

▪ Substrates bind to the enzyme’s active site

▪ Many enzymes undergo a conformational change

– When the substrates are bound to the active site

– This change is called an induced fit


Figure 8.10

Substrate(glucose)

Substrate(ATP)

Enzyme(hexokinase)

When the ATP

and glucose bind

to the active site,

the enzyme

changes shape.

This “induced

fit” reorients the

substrates and

binds them tighter

to the active site.


▪ The activation energy (Ea) of a reaction

– Is the amount of free energy required to reach the

intermediate condition, or transition state

▪ Reactions occur when

– Reactants have enough kinetic energy to reach the

transition state

– The kinetic energy of molecules is a function of their

temperature


Figure 8.11

Reactants

Products

Transition state

Progress of reaction

Fre

e e

nerg

y

Ea

G

Activation energy


▪ Interactions between the enzyme and the substrate

– Stabilize the transition state

– Lower the activation energy required for the reaction

to proceed


Figure 8.12

Reactants

Products

Transition state

Progress of reaction

Fre

e e

ne

rgy

Ea

G

Activation energywith enzyme

G doesnot change


▪ Enzyme catalysis has three steps:

1. Initiation

– Substrates are precisely oriented as they bind to the

active site

2. Transition state facilitation

– Interactions between the substrate and active site R-

groups lower the activation energy

3. Termination

– Reaction products are released from the enzyme


Figure 8.13

Substrates

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.


▪ A catalyst

– Is a substance that lowers the activation energy of a

reaction

– And increases the rate of the reaction

▪ Catalysts

– Lower the activation energy of a reaction by

– Lowering the free energy of the transition state

– Do not change G

– Are not consumed in the reaction


▪ Enzymes are saturable

▪ The rate of a reaction is limited by the amounts of

– Substrate present

– Enzyme available

▪ The speed of an enzyme-catalyzed reaction

– Increases linearly at low substrate concentrations

– Slows as substrate concentration increases

– Reaches maximum speed at high substrate

concentrations


Figure 8.14

Substrate concentration

Rate

of

pro

du

ct f

orm

ati

on

Maximum speed of reaction

Catalyzedreaction

Uncatalyzedreaction


▪ Enzymes are regulated by molecules that are not part

of the enzyme itself

1. Cofactors are inorganic ions

– Such as the metal ions Zn2+, Mg2+, and Fe2+

– Reversibly interact with enzymes

2. Coenzymes are organic molecules

– That interact with enzymes

– Such as the electron carriers NADH or FADH2


▪ Enzymes function best

– At some particular temperature and pH

▪ Temperature affects

– The movement of the substrates and enzyme

▪ pH affects

– The enzyme’s shape and reactivity


Figure 8.15

Temperature (C) pH

Rela

tiv

e c

hit

inase

act

ivit

y (

%)

Rela

tiv

e c

hit

inase

act

ivit

y (

%)

(a) Enzymes from different organisms may function bestat different temperatures.

(b) Enzymes from different organisms may function bestat different pHs.

From bacteriathat live in a

cool and neutralenvironment


cool and neutralenvironment


hot and acidicenvironment


hot and acidicenvironment


▪ The rate of an enzyme-catalyzed reaction depends on

– Substrate concentration

– The enzyme’s intrinsic affinity for the substrate

– Temperature

– pH


▪ Competitive inhibition occurs when

– A molecule similar in size and shape to the substrate

competes with the substrate for access to the active site

▪ Allosteric regulation occurs when

– A molecule causes a change in enzyme shape

– By binding to the enzyme

– At a location other than the active site

▪ Allosteric regulation can activate or deactivate the

enzyme


Figure 8.16

Substrates

Enzyme

Regulatorymolecule

Regulatorymolecule

Regulatorymolecule

Shapechanges

Shapechanges

or or or

(a) Competitive inhibition (b) Allosteric regulation

Enzyme in absenceof regulation

Competitive inhibitionThe substrates cannotbind when a regulatorymolecule binds to theenzyme’s active site.

Allosteric activationThe active site becomesavailable to the substrateswhen a regulatory moleculebinds to a different site onthe enzyme.

Allosteric inhibitionThe active site becomesunavailable to the substrateswhen a regulatory moleculebinds to a different site onthe enzyme.

biological science - sisd.net · pdf filebiological science fifth edition 8 lecture...

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