an introduction to metabolism · an introduction to metabolism ... slide 4 metabolism •...

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

PowerPoint® Lecture Presentations for

BiologyEighth Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp

Chapter 8

An Introduction to Metabolism

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Slide 2

• Metabolism in an organism is about energy and enzymes.

• It is the sum of all of the chemical reactions performed by the cells of the organism.

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Slide 3

1. Free Energy

2. Kinetic energy

3. Heat (thermal energy)

4. Potential energy

5. Chemical energy


Energy cannot be converted from one form to another.

True or False?

a) E that matter possesses because of its location or structure

b) Kinetic E associated with random movement of atoms or molecules

c) Potential E available for release in a chemical reaction

d) E associated with motion

e) The capacity to cause change (can be used to do work)

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Metabolism

• Thermodynamics says that the flow of chemical energy into an organism is equal to the energy used as work, the energy lost as heat and the chemical potential energy stored by the organism.

– true for organisms

– true for the tissues in an organism

– true for the individual cells in a tissue

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Slide 5 Fig. 8-UN1

Enzyme 1 Enzyme 2 Enzyme 3DCBA

Reaction 1 Reaction 3Reaction 2

Starting moleculesproteins

carbohydrateslipids

polymers v. monomers

Productswork energyheat energy

chemical energy

storage = potential

Enzymes make it ALL happen!

Metabolic Pathway

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Slide 6 Fig. 8-6

Reactants

Energy

Free

ene

rgy

Products

Amount ofenergy

released(∆G < 0)

Progress of the reaction

Products

ReactantsEnergy

Free

ene

rgy

Amount ofenergy

required(∆G > 0)


• An exergonicreaction proceeds with a net releaseof energy

• An endergonicreaction absorbsenergy from its surroundings

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Metabolic pathways

• Catabolic pathways release energy (exergonic) by breaking down complex molecules into simpler compounds

– Examples?

• Anabolic pathways consume energy (endergonic) to build complex molecules from simpler ones

– Examples?

• Work pathways couple the energy derived from exergonic reactions to perform an endergonic activity – together the whole is always exergonic

– Do we harness all of the exergonic energy?

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Slide 8 Fill in the Chart

Reaction 1:ATP + H2O ADP + Pi

Reaction 2:6CO2 + 6H2O C6H12O6 + 6O2

Exergonic/Endergonic

Anabolic/Catabolic

Energy Requirements

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Slide 9 Fill in the Chart

Reaction 1:ATP + H2O ADP + Pi

Reaction 2:6CO2 + 6H2O C6H12O6 + 6O2

Exergonic/Endergonic Exergonic Endergonic

Anabolic/Catabolic Catabolic – break down Anabolic – synthesis reaction

Energy Requirements E is released requires the input of E

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ATP powers cellular work

• A cell does three main kinds of work:– Chemical:

– Transport:

– Mechanical:

• To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an endergonic one

• Most energy coupling in cells is mediated by ATP


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Slide 11 Fig. 8-8

Phosphate groupsRibose

Adenine

ATP is composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups

What are the two cellular roles of ATP?

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Slide 12 Fig. 8-9

Inorganic phosphate

Energy

Adenosine triphosphate (ATP)

Adenosine diphosphate (ADP)

P P

P P P

P ++

H2O

i

What is the name of this

reaction?

Is this an exergonic or endogonic reaction?

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Fig. 8-11

(b) Mechanical work: ATP binds noncovalentlyto motor proteins, then is hydrolyzed

Membrane protein

P i

ADP+

P

Solute Solute transported

P i

Vesicle Cytoskeletal track

Motor protein Protein moved

(a) Transport work: ATP phosphorylatestransport proteins

ATP

ATP

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Slide 14 Fig. 8-12

Energy for cellularwork (endergonic,energy-consumingprocesses)

P iADP +

Energy fromcatabolism (exergonic,energy-releasingprocesses)

ATP + H2O

ATP is a renewable resource that is regenerated by addition of a phosphate group to adenosine diphosphate (ADP)

The energy to phosphorylate ADP comes from catabolic reactions in the cell

Where in the cell does this occur?

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Slide 15 Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers

• A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction

• An enzyme is a catalytic protein


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Fig. 8-13

Sucrose (C12H22O11)

Glucose (C6H12O6) Fructose (C6H12O6)

Sucrase

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Slide 17 Fig. 8-15


Glucoseandfructose

sucroseand H2O

Course ofreactionwithoutenzyme

EAwithoutenzyme EA with

enzymeis lower

Course ofreactionwith enzyme

Enzymes catalyze reactions by lowering the EA barrier

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Slide 18 Substrate Specificity of Enzymes

• The reactant that an enzyme acts on is called the enzyme’s ___________.

• The ________is the region on the enzyme where it binds

• This binding is extremely specific (sucrase –recognizes sucrose, not maltose). Why?


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Fig. 8-16

Substrate

Active site

Enzyme Enzyme-substratecomplex

(b)(a)

Hexokinase - first enzyme in the breakdown of glucose (glycolysis)

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Slide 20 Catalysis in the Enzyme’s Active Site

• In an enzymatic reaction, the substrate binds to the active site of the enzyme

• The active site can lower an EA barrier by:– Orienting substrates correctly

– Straining substrate bonds– Providing a favorable microenvironment

– Covalently bonding to the substrate


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Slide 21 Fig. 8-17

Substrates

Enzyme

Products arereleased.

Products

Substrates areconverted toproducts.

Active site can lower EAand speed up a reaction.

Substrates held in active site by weakinteractions, such as hydrogen bonds andionic bonds.

Substrates enter active site; enzyme changes shape such that its active siteenfolds the substrates (induced fit).

Activesite is

availablefor two new

substratemolecules.

Enzyme-substratecomplex

5

3

21

6

4

Induced fit of a substrate brings

chemical groups of the active site into

positions that enhance their

ability to catalyze the reaction

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Effects of Local Conditions on Enzyme Activity

• An enzyme’s activity can be affected by– General environmental factors, such as

temperature and pH

– Chemicals that specifically influence the enzyme


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Slide 23 Fig. 8-18

Rat

e of

rea

ctio

n

Optimal temperature forenzyme of thermophilic

(heat-tolerant) bacteria

Optimal temperature fortypical human enzyme

(a) Optimal temperature for two enzymes

(b) Optimal pH for two enzymes

Rat

e of

rea

ctio

n

Optimal pH for pepsin(stomach enzyme)

Optimal pHfor trypsin(intestinalenzyme)

Temperature (ºC)

pH543210 6 7 8 9 10

0 20 40 80 60 100

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Slide 24 Cofactors

• Cofactors are nonprotein enzyme helpers

• Cofactors may be inorganic (such as a metal in ionic form) or organic

• An organic cofactor is called a coenzyme

• Coenzymes include vitamins


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Enzyme Inhibitors

• Competitive inhibitors bind to the active site of an enzyme, competing with the substrate

• Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective

• Examples of inhibitors include toxins, poisons, pesticides, and antibiotics


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Slide 26 Fig. 8-19

(a) Normal binding (c) ___________ inhibition(b) ___________ inhibition

inhibitor

Active siteinhibitor

Substrate

Enzyme

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Slide 27 CONNECTION

• Many poisons, pesticides, and drugs are enzyme inhibitors

Cyanide - binds competitively and irreversibly to cytochrome oxidase, an enzyme involved in ATP production during cellular respiration

Penicillin - binds competitively and irreversibly to transpeptidase, an enzymeneeded for the growth of bacterial cell walls - humans lack this enzyme

Ibuprofen and aspirin - bind competitively to cyclooxygenases, the enzymes thatmake prostaglandins, chemicals that promote inflammation (and can lead to pain and fever)

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Concept 8.5: Regulation of enzyme activity helps control metabolism

• Metabolic pathways must be tightly regulated

• A cell does this by switching on or off the genes that encode specific enzymes or by regulating the activity of enzymes (once they are made)


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Slide 29 Allosteric Regulation of Enzymes

• Allosteric regulation occurs when a regulatory molecule binds to a protein at one site and affects the protein’s function at another site

• Allosteric regulation may either inhibit or stimulate an enzyme’s activity


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Slide 30 Fig. 8-20a

(a) Allosteric activators and inhibitors

InhibitorNon-functionalactivesite

Stabilized inactiveform

Inactive form

Oscillation

ActivatorActive form Stabilized active form

Regulatorysite (oneof four)

Allosteric enzymewith four subunits

Active site(one of four)

Allosterically regulated enzymes:

• Usually polypeptidesubunits

• have active and inactiveforms

• binding of eitheractivators or inhibitors stabilize the enzyme

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Fig. 8-20b

(b) Cooperativity: another type of allosteric activation

Stabilized activeform

Substrate

Inactive form

• Cooperativity is a form of allosteric regulation that can amplify enzyme activity

• In cooperativity, binding by a substrate to one active site stabilizes favorable conformational changes at all other subunits

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Slide 32 Cooperativity and Hemoglobin

http://en.wikibooks.org/wiki/Structural_Biochemistry/Protein_function/Heme_group/Hemoglobin

When one oxygen binds a heme, the rest bind more easily. When the first oxygen is released, it stimulates the release of the others. This is due to conformational changes in the hemoglobin subunits.

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Slide 33 Identification of Allosteric Regulators

• Allosteric regulators are attractive drug candidates for enzyme regulation

• Inhibition of proteolytic enzymes called caspases may help management of inappropriate inflammatory responses


Animation: http://bcs.whfreeman.com/thelifewire/content/chp06/0602002.html

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Fig. 8-21a

SH

Substrate

Hypothesis: allostericinhibitor locks enzymein inactive form

Active form canbind substrate

S–SSH

SH

Activesite

Caspase 1

Known active form

Known inactive form

Allostericbinding site

Allostericinhibitor

EXPERIMENT

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Slide 35 Fig. 8-21b

Caspase 1

RESULTS

Active formInhibitor

Allostericallyinhibited form

Inactive form

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Slide 36 Feedback Inhibition

• In feedback inhibition, the end product of a metabolic pathway shuts down the pathway

• Feedback inhibition prevents a cell from wasting chemical resources by synthesizing more product than is needed


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Fig. 8-22

Intermediate C

Feedbackinhibition

Isoleucineused up bycell

Enzyme 1(threoninedeaminase)

End product(isoleucine)

Enzyme 5

Intermediate D

Intermediate B

Intermediate A

Enzyme 4

Enzyme 2

Enzyme 3

Initial substrate(threonine)

Threoninein active site

Active siteavailable

Active site ofenzyme 1 nolonger bindsthreonine;pathway isswitched off.

Isoleucinebinds toallostericsite

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Slide 38 Specific Localization of Enzymes Within the Cell

• Cellular structures help bring order to metabolic pathways

• Some enzymes act as structural components of membranes

• In eukaryotic cells, some enzymes reside in specific organelles; for example, enzymes for cellular respiration are located in mitochondria


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