Chapter 8

download report

Transcript Chapter 8

Chapter 8:
An Introduction to Metabolism
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Marie Maynard Daly – American biochemist who did
extensive research into the metabolism of the arterial
wall in the 1960s. Her worked helped to uncover
several processes related to aging.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
© 2011 Pearson Education, Inc.
Bioluminescence by a fungus
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 8.1: An organism’s metabolism
transforms matter and energy, subject to the
laws of thermodynamics
• Metabolism is the totality of an organism’s
chemical reactions
© 2011 Pearson Education, Inc.
Organization of the Chemistry of Life into
Metabolic Pathways
• A metabolic pathway begins with a specific
molecule and ends with a product
• Each step is catalyzed by a specific enzyme
© 2011 Pearson Education, Inc.
Figure 8.UN01
Enzyme 2
Enzyme 1
A
Reaction 1
Starting
molecule
Enzyme 3
D
C
B
Reaction 2
Reaction 3
Product
• 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
© 2011 Pearson Education, Inc.
• Anabolic pathways consume energy to build
complex molecules from simpler ones
• The synthesis of protein from amino acids is an
example of anabolism
© 2011 Pearson Education, Inc.
Forms of Energy
• Energy is the capacity to cause change
• Energy exists in various forms, some of which can
perform work
• Kinetic energy is energy associated with motion
• Heat (thermal energy) is kinetic energy associated
with random movement of atoms or molecules
• 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
© 2011 Pearson Education, Inc.
Figure 8.2 Transformations between kinetic and
potential energy
On the platform, a diver
has more potential energy.
Climbing up converts kinetic
energy of muscle movement
to potential energy.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Diving converts potential
energy to kinetic energy.
In the water, a diver has
less potential energy.
Figure 8.3
The two laws of thermodynamics
Heat
Chemical
energy
(a) First law of thermodynamics
(b) Second law of thermodynamics
The First Law of Thermodynamics
• 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
© 2011 Pearson Education, Inc.
The Second Law of Thermodynamics
• During every energy transfer or transformation,
some energy is unusable, and is often lost as
heat
• According to the second law of
thermodynamics
– Every energy transfer or transformation
increases the entropy (disorder) of the
universe
© 2011 Pearson Education, Inc.
Exergonic and Endergonic Reactions in
Metabolism
• An exergonic reaction proceeds with a net
release of free energy and is spontaneous
• An endergonic reaction absorbs free energy
from its surroundings and is nonspontaneous
© 2011 Pearson Education, Inc.
Figure 8.6 Free energy changes (G) in exergonic
and endergonic reactions
Reactants
Free energy
Amount of
energy
released (∆G<0)
Energy
Products
Progress of the reaction
(a) Exergonic reaction: energy released
Free energy
Products
Amount of
energy
released (∆G>0)
Energy
Reactants
Progress of the reaction
(b) Endergonic reaction: energy required
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 8.3: ATP powers cellular work by coupling
exergonic reactions to endergonic reactions
• 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
© 2011 Pearson Education, Inc.
Figure 8.8 The structure of adenosine triphosphate
(ATP)
Adenine
N
O
–O
P
O–
O
P
O–
O
P
C
C
N
C
CH
HC
O
O
O
N
CH2
O
O–
H
Phosphate groups
H
H Ribose
H
OH
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
NH2
OH
N
Figure 8.9 The hydrolysis of ATP
P
P
P
Adenosine triphosphate (ATP)
H2O
Pi
+
Inorganic phosphate
P
P
Adenosine diphosphate (ADP)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Energy
Figure 8.11 How ATP drives cellular work
P
i
P
Motor protein
Protein moved
(a) Mechanical work: ATP phosphorylates motor proteins
Membrane
protein
ADP
+
ATP
P
Pi
P
Solute
Solute transported
(b) Transport work: ATP phosphorylates transport proteins
P
Glu + NH3
Reactants: Glutamic acid
and ammonia
NH2
+
P
i
Glu
Product (glutamine)
made
(c) Chemical work: ATP phosphorylates key reactants
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
i
Figure 8.12 The ATP cycle
ATP hydrolysis to
ADP + P i yields energy
ATP synthesis from
ADP + P i requires energy
ATP
Energy from catabolism
(exergonic, energy yielding
processes)
ADP + P
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
i
Energy for cellular work
(endergonic, energyconsuming processes)
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
• Hydrolysis of sucrose by the enzyme sucrase is an
example of an enzyme-catalyzed reaction
© 2011 Pearson Education, Inc.
Figure 8.13 The effect of enzymes on reaction rate.
Course of
reaction
without
enzyme
EA
without
enzyme
Free energy
EA with
enzyme
is lower
Reactants
∆G is unaffected
by enzyme
Course of
reaction
with enzyme
Products
Progress of the reaction
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.14 Induced fit between an enzyme and
its substrate
Substrate
Active site
Enzyme- substrate
complex
Enzyme
(a)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
(b)
Figure 8.15 The active site and catalytic cycle of
an enzyme
1 Substrates enter active site; enzyme
changes shape so its active site
embraces the substrates (induced fit).
Substrates
Enzyme-substrate
complex
6 Active site
is available for
two new substrate
molecules.
Enzyme
5 Products are
Released.
Products
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
2 Substrates held in
active site by weak
interactions, such as
hydrogen bonds and
ionic bonds.
3 Active site (and R groups of
its amino acids) can lower EA
and speed up a reaction by
• acting as a template for
substrate orientation,
• stressing the substrates
and stabilizing the
transition state,
• providing a favorable
microenvironment,
• participating directly in the
catalytic reaction.
4 Substrates are
Converted into
Products.
Figure 8.16 Environmental factors affecting
enzyme activity
Optimal temperature for
enzyme of thermophilic
(heat-tolerant)
bacteria
Rate of reaction
Optimal temperature for
typical human enzyme
0
60
40
20
80
100
Temperature (Cº)
(a) Optimal temperature for two enzymes
Rate of reaction
Optimal pH for pepsin
(stomach enzyme)
0
1
2
3
4 5
pH
Optimal pH
for trypsin
(intestinal
enzyme)
6
OptimalpH
for two
two enzymes
enzymes
(b) Optimal
pH for
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
7
8
9 10
Cofactors & Coenzymes
• Cofactors are nonprotein enzyme helpers
• Cofactors may be inorganic (such as a mineral in
ionic form) or organic
• An organic cofactor is called a coenzyme
• Coenzymes include vitamins
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Figure 8.17 Inhibition of enzyme activity
A substrate can
bind normally to the
active site of an
enzyme.
Substrate
Active site
Enzyme
(a) Normal binding
A competitive
inhibitor mimics the
substrate, competing
for the active site.
A noncompetitive
inhibitor binds to the
enzyme away from
the active site, altering
the conformation of
the enzyme so that its
active site no longer
functions.
Competitive
inhibitor
(b) Competitive inhibition
Noncompetitive inhibitor
(c) Noncompetitive inhibition
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Allosteric Regulation of Enzymes
• Allosteric regulation 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
© 2011 Pearson Education, Inc.
Figure 8.19 Allosteric regulation of enzyme activity
Allosteric enyzme
with four subunits
Regulatory
site (one
of four)
Active site
(one of four)
Allosteric activater
stabilizes active from
Activator
Active form
Stabilized active form
Oscillation
Allosteric activater
stabilizes active form
NonInactive form Inhibitor
functional
active
site
Stabilized inactive
form
(a) Allosteric activators and inhibitors. In the cell, activators and inhibitors
dissociate when at low concentrations. The enzyme can then oscillate again.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Binding of one substrate molecule to
active site of one subunit locks
all subunits in active conformation.
Substrate
Inactive form
Stabilized active form
(b) Cooperativity: another type of allosteric activation. Note that the
inactive form shown on the left oscillates back and forth with the active
form when the active form is not stabilized by substrate.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings