CHAPTER 6 CAMPBELL BIO

CAMPBELL BIOLOGY IN FOCUS

© 2014 Pearson Education, Inc.

Urry • Cain • Wasserman • Minorsky • Jackson • Reece

Lecture Presentations by Kathleen Fitzpatrick and Nicole Tunbridge

6An Introduction to Metabolism

Edited by Rena Quinlan, Ph.D.

Overview: The Energy of Life

The living cell is a miniature chemical factory where thousands of reactions occur

The cell extracts energy and applies energy to perform work

Some organisms even convert energy to light, as in bioluminescence (Fig. 6.1)


Figure 6.1. Two firefly squid (Watasenia scintillans) convert energy stored in organic molecules to light, a process called bioluminescence that aids in mate recognition

Concept 6.1: An organism’s metabolism transforms matter and energy

Metabolism is the totality of an organism’s chemical reactions

Metabolism is an emergent property of life that arises from interactions between molecules within the cell


Metabolic Pathways

A metabolic pathway begins with a specific molecule and ends with a product

Each step is catalyzed by a specific enzyme


Enzyme 1Startingmolecule

Enzyme 2 Enzyme 3

Reaction 1 Reaction 2 Reaction 3ProductDCBA

Figure 6.UN01

Catabolic pathways release energy by breaking down complex molecules into simpler compounds

Cellular respiration, the breakdown of glucose in the presence of oxygen, is an example of a pathway of catabolism

Anabolic pathways consume energy to build complex molecules from simpler ones

The synthesis of protein from amino acids is an example of anabolism


Forms of Energy

Energy is the capacity to cause change

Energy exists in various forms, some of which can perform work

Like cellular work


Kinetic energy is energy associated with motion

Thermal energy is kinetic energy associated with random movement of atoms or molecules

Heat is thermal energy in transfer from one object to another

Potential energy is energy that matter possesses because of its location or structure

Chemical energy is potential energy available for release in a chemical reaction

Energy can be converted from one form to another


A diver has more potentialenergy on the platform.

A diver has less potential energy in the water.

Diving convertspotential energy tokinetic energy.

Climbing up converts the kinetic energy of muscle movement to potential energy.

Figure 6.2

The Laws of Energy Transformation

Thermodynamics is the study of energy transformations


The First Law of Thermodynamics


Fig. 6.3(a). First Law of Thermodynamics: Energy can be transferred or transformed but neither created nor destroyed. For example, chemical reactions in this brown bear will convert the chemical (potential) energy in the fish to the kinetic energy of running.

Chemicalenergy

According to the first law of thermodynamics, the energy of the universe is constant:

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

The first law is also called the principle of conservation of energy

The Second Law of Thermodynamics

During every energy transfer or transformation, some energy is unusable and is often lost as heat

a logical consequence of the loss of usable energy during energy transfer or transformation is that each such event makes the universe more disordered – scientists use a quantity called Entropy as a measure of disorder, or randomness

According to the second law of thermodynamics:

Every energy transfer or transformation increases the entropy of the universe


Heat

Fig. 6.3(b). Second Law of Thermodynamics: Everyenergy transfer or transformation increases the disorder(entropy) of the universe. For example, as the bear runs,disorder is increased around the bear by the release of heat and small molecules that are the by-products of metabolism.

Concept 6.3: ATP powers cellular work by coupling exergonic reactions to endergonic reactions

A cell does three main kinds of work (all 3 are powered by ATP): Chemical work: the pushing of endergonic reactions (absorbs free

energy from surroundings) that would not occur spontaneously, such as the synthesis of polymers from monomers (eg., photosynthesis converts light to chemical energy to convert CO2 and water to glucose and oxygen) (discussed further in chapters 7 and 8).

Transport work: the pumping of substances across membranes against the direction of spontaneous movement (eg., Active Transport) (see chapter 5).

Mechanical work: such as the beating of cilia (see chapter 4), the contraction of muscle cells, and the movement of chromosomes during cellular reproduction.



Fig. 6.8 (b) The hydrolysis of ATP – energy is released from ATP when the terminal phosphate bond is broken

Energy

Adenosine triphosphate (ATP)

Adenosine diphosphate (ADP)Inorganic

phosphate

The bonds between the phosphate groups of ATPcan be broken by hydrolysis – releases energy

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

In addition to its role in energy coupling, ATP is also used to make RNA

The Regeneration of ATP


Energy fromcatabolism(exergonic, energy-releasing processes)

Energy for cellularwork (endergonic, energy-consuming processes)

Fig. 6.11. The ATP cycle. Energy released by breakdown reactions (catabolism) in the cell is used to phosphorylate ADP, regenerating ATP. Chemical potential energy stored in ATP drives most cellular work.

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

• The ATP cycle is a revolving door through which energy passes during its transfer from catabolic to anabolic pathways

Concept 6.4: 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

Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction


Sucrase

Sucrose(C12H22O11)

Fructose(C6H12O6)

Glucose(C6H12O6)

Figure 6.UN02

Every chemical reaction between molecules involves bond breaking and bond forming

Enzymes catalyze reactions by lowering the activation energy (EA) barrier (also called the free energy of activation), which is the initial energy required to start a chemical reaction

How Enzymes Speed Up Reactions


Products

G is unaffectedby enzyme

Reactants

Progress of the reaction

Fre

e en

erg

y

EA withenzymeis lower

EA

withoutenzyme

Course of reactionwithoutenzyme

Course of reactionwith enzyme

Fig. 6.13. The effect of an enzyme on activation energy. Withoutaffecting the free energy change (G ) for a reaction, an enzyme speeds the reaction by reducing its activation energy (EA)

Substrate Specificity of Enzymes


Enzyme-substratecomplex

Enzyme

Substrate

Active site

Fig. 6.14. Induced fit between an enzyme and its substrate.

The reactant that an enzyme acts on is called the enzyme’s substrate The enzyme binds to its substrate, forming an enzyme-substrate complex The active site is the region on the enzyme where the substrate binds

Enzyme specificity results from the complementary fit between the shape of its active site and the substrate shape

Enzymes change shape due to chemical interactions with the substrate

This induced fit of the enzyme to the substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction


Substrates

Enzyme

Substrates areconverted toproducts.

Products arereleased.

Products


Substrates areheld in active site byweak interactions, suchas H bonds and ionic bonds.

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

Activesite is

availablefor new

substrates.

5

4

3

2

1

Fig. 6.15. The active site and catalytic cycle of an enzyme.

Catalysis in the Enzyme’s Active Site – in an enzymatic reaction, the substrate binds to the enzyme’s active site

Substrates

Enzyme

Substrates areconverted toproducts.

Products arereleased.

Products


Substrates areheld in active site byweak interactions.

Substrates enteractive site.

Activesite is

availablefor new

substrates.

5

4

3

21

Fig. 6.15. The active site and catalytic cycle of an enzyme.

The active site of an enzyme can lower an EA barrier by:

Orienting substrates correctly

Enzyme may stretch substrate molecules thereby bending critical chemical bonds that must be broken during the reaction.

Providing a favorable microenvironment (eg., if active site has amino acids with acidic R groups – may provide pocket of low pH in an otherwise neutral cell

Brief covalent bonding to the substrate

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


Effects of Temperature and pH

Each enzyme has an optimal temperature in which it can function

Each enzyme has an optimal pH in which it can function

Optimal conditions favor the most active shape for the enzyme molecule


Temperature (C)

Optimal temperature forenzyme of thermophilic

(heat-tolerant) bacteria (77C)

Optimal temperature fortypical human enzyme(37C)

Optimal pH for pepsin(stomach enzyme)

Optimal pH for trypsin(intestinalenzyme)

(a) Optimal temperature for two enzymes

(b) Optimal pH for two enzymespH

120100806040200

9 1086420 7531

Rat

e o

f re

acti

on

Rat

e o

f re

acti

on

Figure 6.16

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


Enzyme Inhibitors


(b) Competitive inhibition (c) Noncompetitive inhibition

(a) Normal binding

Competitiveinhibitor

Noncompetitiveinhibitor

Substrate

Enzyme

Active site

Fig. 6.17. Inhibition of enzyme activity.

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

Allosteric Regulation of Enzymes – may either inhibit or stimulate an enzyme’s activity

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

Regulatory molecules, either activators or inhibitors, bind to specific regulatory sites, affecting the shape and function of the enzyme.

(b) Cooperativity: another type of allosteric activation that can amplify enzyme activity

(a) Allosteric activators and inhibitors

Substrate

Inactive form Stabilizedactive form

Stabilizedactive form

Active form

Active site(one of four)

Allosteric enzymewith four subunits

Regulatorysite (one of four)

Activator

Oscillation

Stabilizedinactive form

Inactiveform

InhibitorNon-functionalactive site

Figure 6.18. Allosteric regulation of enzyme activity.

Feedback Inhibition


Active site available

Intermediate A

End product(isoleucine)

Intermediate B

Intermediate C

Intermediate D

Enzyme 2

Enzyme 3

Enzyme 4

Enzyme 5

Feedbackinhibition

Isoleucinebinds toallostericsite.

Isoleucineused up bycell

Enzyme 1(threoninedeaminase)

Threoninein active site

Fig. 6.19. Feedback inhibition in isoleucine synthesis.

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

Specific Localization of Enzymes Within the Cell

Mitochondria

The matrix containsenzymes in solutionthat are involved in

one stage of cellularrespiration.

Enzymes for anotherstage of cellular

respiration areembedded in theinner membrane.

1 m

Figure 6.20. Organelles and structural order in metabolism. Organelles such as the mitochondrion (TEM) contain enzymes that carry out specific functions, in this case cellular respiration.

Structures within the cell help bring order to metabolic pathways

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

CHAPTER 6 CAMPBELL BIO

Documents

energy transfer

motionthermal energy

free energy

energy of lifethe

anotherpotential energy

structure chemical energy

potential energy available

loss of usable energy