Chapter 8(1)

Chapter 8

An Introduction to
Metabolism

PowerPoint® Lecture Presentations for

Biology
Eighth Edition
Neil Campbell and Jane Reece

Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

An organism’s metabolism transforms matter and
energy, subject to the laws of thermodynamics
• Metabolism is the totality of an organism’s chemical
reactions
– Example of an emergent property that arises from
interactions between molecules within the cell
• A metabolic pathway begins with a specific molecule and
ends with a product
– Each step is catalyzed by a specific enzyme

Enzyme 1 Enzyme 2 Enzyme 3
A B C D
Reaction 1 Reaction 2 Reaction 3
Starting Product
molecule

Catabolic vs Anabolic Pathways
• Catabolic pathways

– release energy by breaking down complex molecules
into simpler compounds
– Ex: Cellular respiration (the breakdown of glucose in
the presence of oxygen)
• Anabolic pathways

– consume energy to build complex molecules from
simpler ones
– Ex: The synthesis of protein from amino acids


Forms of Energy

• Energy is the capacity to cause change or do
work and can be converted from one form to
another
– Kinetic energy (energy of movement)

– Heat (thermal energy)

– Potential energy

– Chemical energy


A diver has more potential energy Diving converts potential energy
on the platform than in the water. to kinetic energy.

Climbing up converts the kinetic energy A diver has less potential energy
of muscle movement to potential energy. in the water than on the platform.

The Laws of Energy Transformation

• Thermodynamics is the study of energy
transformations
• A closed system is isolated from its surroundings
– Ex: liquid in a thermos

• In an open system, energy and matter can be
transferred between the system and its
surroundings
– Ex: Organisms absorb energy and release heat


The Laws of Thermodynamics
• First Law of Thermodynamics
– Energy can be transferred and transformed, but it cannot be
created or destroyed
– The energy of the universe is constant
– “principle of conservation of energy”
– Ex: plants convert sunlight to chemical energy

• Second Law of Thermodynamics
– During every energy transfer or transformation, some energy is
unusable, and is often lost as heat
– Every energy transfer or transformation increases the entropy
(disorder) of the universe


Fig. 8-3

First and Second law of Thermodynamics

Heat CO2
+
Chemical H2O
energy

(a) First law of thermodynamics (b) Second law of thermodynamics

Biological Order and Disorder

• Cells create ordered structures from less ordered materials

– Ex: amino acids make proteins

• Organisms also replace ordered forms of matter and
energy with less ordered forms
– Ex: the break down of food molecules produces
water, heat and CO2

• Entropy (disorder) may decrease in an organism, but the
universe’s total entropy increases
• Energy flows into an ecosystem in the form of light and
exits as heat


The free-energy change of a reaction tells us
whether or not the reaction occurs spontaneously

• Free Energy
– energy that can do work when temperature and
pressure are uniform, as in a living cell
– measure of a system’s instability

• The change in free energy (∆G) during a process is related
to the change in enthalpy, or change in total energy (∆H),
change in entropy (∆S), and temperature in Kelvin (T):

Free energy change: ∆G = ∆H – T∆S

Free Total Temp Entropy
energy energy (K)


Free-Energy Change, ∆G

• Equilibrium is a state of maximum stability

• Spontaneous reactions:
• ∆G < 0 (a negative ∆G)
• can be harnessed to perform work when it is moving
towards equillibrium
• free energy decreases and the stability of a system
increases
• ∆G represents the difference between the free energy
of the final state and the free energy of the initial state

∆G = Gfinal state – Ginitial state


Relationship of free energy to stability, work capacity and
spontaneous change

Unstable systems

• More free energy (higher G)
• Less stable
• Greater work capacity

In a spontaneous change:

• The free energy of the system
decreases (∆G < 0)
• The system becomes more
stable
• The released free energy can
be harnessed to do work

• Less free energy (lower G)
• More stable
• Less work capacity
(c) Chemical reaction
(a) Gravitational motion (b) Diffusion

Exergonic and Endergonic Reactions in Metabolism

• An exergonic reaction (downhill) proceeds with a net
release of free energy and is spontaneous
– ∆G is negative
– The greater the decrease in free energy, the more work can
be done

• An endergonic reaction (uphill) absorbs free energy from
its surroundings and is nonspontaneous
– ∆G is positive and is the energy required to drive the reaction

• If a chemical process is exergonic/downhill then the
opposite reaction must be endergonic/uphill


Fig. 8-6

Reactants

Amount of
energy
released
Exergonic reation:

Free energy
(∆G < 0)

energy released Energy
Products

Progress of the reaction

Products

Amount of
energy
Endergonic reation: required
Free energy

(∆G > 0)
energy required Energy
Reactants


Equilibrium and Metabolism

• Reactions in a closed system eventually reach equilibrium
and then do no work
– Cells are not in equilibrium; they are open systems
experiencing a constant flow of materials
• In life metabolism is never at equilibrium
• A catabolic pathway in a cell releases free energy in a
series of reactions
• Closed and open hydroelectric systems can serve as
analogies

What would happen if a cell was in equilibrium?


Fig. 8-7

∆G < 0 ∆G = 0
An isolated hydroelectric system
•Downhill flow of water turns a turbine
•turbine drives a generator
•Electricity turns on a light bulb
•Eventually equilibrium will be reached Spontaneous reaction

(b) An open hydroelectric system ∆G < 0

•Running water powers the generator
•Intake and outflow of water keeps equilibrium from occurring
•Electricity turns on a light bulb

c) A multistep open hydroelectric ∆G < 0
system ∆G < 0
•Running water powers the generator ∆G < 0
•The product becomes the reactant in the
next reaction
•Equilibrium will not be reached
•Ex: cellular respiration

Similar to a catabolic pathway that releases energy

Practice Quiz A B
• Which one of these is the best example of a
spontaneous reaction?
• Which one is more unstable?
• Which reaction is uphill? Which is downhill?
• Which reaction is endergonic? Exergonic?
• Which one will require more energy for work?
Stable Unstable
• Which one has a +∆G? Uphill Downhill
Less work More work
• Which one has a -∆G? Low ∆G High ∆G
∆G increases ∆G decreases
• In B, is the ∆G going to decrease or increase? Nonspontaneous Spontaneous
Endergonic Exergonic
Absorbs energy Releases energy

ATP powers cellular work by coupling exergonic
reactions to endergonic reactions

• A cell does three main kinds of work which all require
energy:
– Chemical – the pushing of endergonic rxns that require energy
– Transport – pump substances across membranes against a
gradient
– Mechanical – ex: muscle contraction, beating of cilia

• 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


The Structure and Hydrolysis of ATP

• ATP (adenosine triphosphate)
– is the cell’s energy shuttle
– composed of ribose (a sugar), adenine (a nitrogenous
base), and three phosphate groups
Adenine

Phosphate groups
Ribose


The Structure and Hydrolysis of ATP

• Hydrolysis can break the bonds between
phosphate groups of ATP
• Energy is released from ATP when the terminal
phosphate bond is broken (exergonic rxn)
• This release of energy comes from the chemical
change to a state of lower free energy, not from
the phosphate bonds themselves

ATP  ADP + Pi + Energy
Higher ∆G  Lower ∆G (more stable)


Fig. 8-9

P P P

Adenosine triphosphate (ATP)

H2O

Pi + P P + Energy
Inorganic
phosphate
Adenosine diphosphate (ADP)

ATP + H20  ADP + Pi
∆G = -7.3 kcal/mol
Exergonic

How ATP Performs Work

• Mechanical, transport, and chemical work are
powered by the hydrolysis of ATP
• The energy from the exergonic reaction of ATP
hydrolysis can be used to drive an endergonic
reaction
• Overall, the coupled reactions are exergonic

• ATP drives endergonic reactions by
phosphorylation


NH2

(a) Endergonic reaction
NH3
Glu + ∆G = +3.4 kcal/mol
Glu
Glutamic Ammonia Glutamine
acid

P
(b) Coupled with ATP hydrolysis, + + ADP
an exergonic reaction ATP
Glu Glu

1 ATP phosphorylates glutamic acid,
making the amino acid less stable NH2
(exergonic).
P
2 Ammonia displaces the phosphate + NH3
+ Pi
group, forming glutamine. Glu Glu

(c) Overall free-energy change

Overall exergonic
reaction with energy
coupling

Fig. 8-11

Membrane protein
) Transport work:
TP phosphorylates
ansport proteins

P Pi

Solute Solute transported

ADP
ATP +
Pi
Cytoskeletal track

(b) Mechanical work:
ATP binds non-
covalently to motor ATP
proteins, then is
hydrolyzed
Motor protein Protein moved

The Regeneration of ATP

• ATP is a renewable resource

– ADP + Pi  ATP

• The energy to phosphorylate ADP comes from
catabolic reactions in the cell (those that release
energy)
• The chemical potential energy temporarily stored
in ATP drives most cellular work


Fig. 8-12

Energy coupling and the renewal of ATP

ATP synthesis ATP hydrolysis
requires energy releases energy
(endergonic) (exergonic)
ATP + H2O

Energy from Energy for cellular
catabolism work (endergonic,
(exergonic, energy-consuming
energy-releasing ADP + P i
processes)
processes)

Exergonic reactions  drive the formation of ATP (endergonic)
Endergonic reactions driven by hydrolysis of ATP (exergonic)

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 Sucrose

reaction
• An enzyme is a Sucrase

catalytic protein
– Ex: Sucrase
hydrolyzes sucrose
molecules Glucose Fructose


The Activation Energy Barrier

• Every chemical reaction between molecules
involves bond breaking and bond forming
• Free energy of activation
– AKA activation energy (EA)

– The initial energy needed to start a chemical reaction

– Often supplied in the form of heat from the surroundings

– Enzymes decrease EA
• Do not affect the change in free energy (∆G)

• Hasten reactions that would occur eventually


Fig. 8-14

Energy profile of an exergonic reaction (spontaneous)
AB + CD  AC + BD

A B
Unstable transition state
C D

A B EA Determines the rate of the rxn
Free energy

C D
Reactants
A B
∆G < O
C D

Products


Fig. 8-15

The effect of an enzyme on activation energy

Course of
reaction EA
without
enzyme without EA with
enzyme enzyme
is lower
Free energy

Reactants
Course of ∆G is unaffected
reaction by enzyme
with enzyme

Products


Substrate Specificity of Enzymes

• The reactant that an enzyme acts on is called
the enzyme’s substrate
• The enzyme binds to its substrate, forming an
enzyme-substrate complex
Enzyme + Enzyme-Substrate Enzyme +
Substrate(s) complex Product(s)


Fig. 8-16

Induced fit between an enzyme and its substrate

Substrate

Active site

Enzyme Enzyme-substrate
complex

The active site is the region on the enzyme where the substrate binds.
An enzyme’s recognition of a substrate is very specific due to it AA sequence.

Catalysis in the Enzyme’s Active Site

• substrate binds to the active site

• The active site can lower an EA barrier by
– Orienting substrates correctly
– Straining substrate bonds
– Providing a favorable microenvironment
– Covalently bonding to the substrate


Fig. 8-17
1 Substrates enter active site; enzyme
changes shape such that its active site 2 Substrates held in
enfolds the substrates (induced fit). active site by weak
interactions, such as
hydrogen bonds and
ionic bonds.

Substrates
Enzyme-substrate
complex
3 Active site can lower EA
and speed up a reaction.

6 Active
site is
available
for two new
substrate
molecules.

Enzyme

5 Products are 4 Substrates are
released. converted to
products.

Products

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

• Ex: vitamins


Effects of Local Conditions on Enzyme Activity

• An enzyme’s activity can be affected by
– pH
– Temperature
• Each enzyme has an optimal temperature in which it can
function
• Each enzyme has an optimal pH in which it can function

– Chemicals that specifically influence the
enzyme
• Competitive vs noncompetitive inhibitors


Fig. 8-18
Optimal temperature for Optimal temperature for
typical human enzyme enzyme of thermophilic
(heat-tolerant)

Rate of reaction
bacteria

40
0 60
20 80 100
Temperature (ºC)
(a) Optimal temperature for two enzymes

Optimal pH for pepsin Optimal pH
(stomach enzyme) for trypsin
(intestinal
Rate of reaction

enzyme)

0 41 5 2 3 6 7 8 9 10
pH
(b) Optimal pH for two enzymes

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
• Ex: toxins, poisons, pesticides, and antibiotics


Fig. 8-19

Types of Enzyme Inhibition

Substrate

Active site
Competitive
inhibitor

Enzyme

Noncompetitive inhibitor
(a) Normal binding (b) Competitive inhibition (c) Noncompetitive inhibition –
The shape of the enzyme is
changed

Regulation of enzyme activity helps control
metabolism

• Metabolic pathways are tightly regulated
– Allosteric regulation  can inhibit or stimulate an
enzyme’s activity
– Feedback inhibition  end product of a metabolic
pathway shuts down the pathway (ie: negative
feedback mechanism)
• prevents a cell from wasting chemical resources by
synthesizing more product than is needed


Allosteric Regulation of Enzymes
• Enzymes are made up of several subunits or polypeptide
chains which have there own active site

• Regulatory molecule binds to a protein at one site and
affects the protein’s function at another site
– Activator  stabilizes the active form of the enzyme
– Inhibitor  stabilizes the inactive form of the enzyme

• Cooperativity
– Can amplify enzyme activity
– Binding of a substrate to one active site stabilizes favorable
conformational changes at all other subunits
– One substrate molecule primes the enzyme to accept additional
substrate molecules more readily

Allosteric Regulation of Enzymes

tors and inhibitors (bind to regulatory sites)

Allosteric enyzme Active site
with four subunits (one of four)

(b) Cooperativity
Regulatory (substrate binds to active site)
site (one
of four) Activator Substrate
Active form Stabilized active form

Oscillation

Inactive form Stabilized active
form

Non-
functional Inactive form Inhibitor Stabilized inactive
active form
site
Fig. 8-20

Initial substrate

Feedback Active site
(threonine)

Inhibition available
Threonine
in active site

in isoleucine Enzyme 1
(threonine
synthesis Isoleucine
used up by
deaminase)

cell
Intermediate A
As isoleucine
Feedback
accumulates, inhibition Enzyme 2

it slows down its
Intermediate B
own synthesis by
allosterically Enzyme 3
inhibiting the
enzyme for the first Active site of
Intermediate C
Isoleucine
step of the pathway binds to enzyme 1 no Enzyme 4
allosteric longer binds
site threonine;
pathway is Intermediate D
switched off.
Enzyme 5

End product
(isoleucine)

Fig. 8-22

Practice Quiz
1. Lists the three components of ATP.

2. ________ reactions release energy while ________ reactions absorb
energy

3. Cells get energy from __________ to synthesize ATP from ADP and Pi.
– Anabolic pathways, catabolic pathways, feedback inhibition, regeneration

1. Explain how energy coupling works.

2. True of False: ATP hydrolysis is exergonic and spontaneous.

3. Enzymes lower the ________ of a chemical reaction.

4. True or False: ∆G is decreased when an enzyme is present.

5. When a protein is __________ it can become more unstable. Thus the
energy from its removal can drive endergonic reactions.

6. List the three types of work that ATP does.

Chapter 8(1)

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