Ch 8 Cellular Metabolism

1
Ch 8 Cellular Metabolism How cells utilize
energy
2
LE 8-2
On the platform, the diver has more
potential energy.
Diving converts potential energy to kinetic
energy.
Climbing up converts kinetic energy of muscle
movement to potential energy.
In the water, the diver has less potential
energy.
3
LE 8-3
CO2
Heat
Chemical energy
H2O
First law of thermodynamics
Second law of thermodynamics
4
The First Law of Thermodynamics

• Energy cannot be created or destroyed
• Energy can be transferred and transformed
• Principle of conservation of energy

5
The Second Law of Thermodynamics

• Every energy transfer or transformation increases
  the entropy (disorder) of the universe
• Because some energy is lost as heat (unusable)

6

• Metabolism
• an organisms (or cells) total chemical
  reactions

Name a common cellular reaction.
Two kinds of reactions Catabolism (catabolic
rxn) Breakdown of a larger molecule into
smaller lower energy products Releases of
energy Exergonic rxn
Anabolism (anabolic rxn) Synthesis of larger
high energy molecules from lower energy
reactants Requires input of energy Endergonic
reactions
7
LE 8-12
ATP
Energy for cellular work (endergonic,
energy- consuming processes)
Energy from catabolism (exergonic,
energy- yielding processes)
P
ADP

i
Cellular energy used for transport (across
membranes) mechanical work (motility,
contraction) enzymatic activity (catalysis of
reactions)
8
Examples
Catabolic rxn C6H12O6 6O2 ----gt 6CO2 6H2O
ATP glucose
Exergonic
9
Biological rxns -Catalyzed by enzymes -Often
arranged in multiple steps called pathways
10
LE 8-UN141
Enzymatic Pathway
Enzyme 1
Enzyme 2
Enzyme 3
A
B
C
D
Reaction 1
Reaction 2
Reaction 3
Starting molecule
Product
11
Enzymes
Biological catalysts Increase rate of
reactions by lowering activation energy
(EA) Spontaneous reactions can take a long
time! Need enzymes to speed reactions for cell
survival
12
Activation Energy (EA)

• Needed to destabilize bonds of reactants

13
LE 8-14
A
B
C
D
Transition state
Could raise temp. to break bonds
EA
A
B
Free energy
C
D
Reactants
A
B
DG lt O
C
D
Products
Progress of the reaction
14
Why dont cells rely on increases in temperature
to break bonds?
Denaturation of proteins and damage to the cell.
15
LE 8-15
Course of reaction without enzyme
EA without enzyme
EA with enzyme is lower
Reactants
Free energy
Course of reaction with enzyme
DG is unaffected by enzyme
Products
Progress of the reaction
16
LE 8-13
Example
Sucrose C12H22O11
Glucose C6H12O6
Fructose C6H12O6
17
Structure Function of Enzyme DRAW

• Enzymes bind substrate molecules (the reactant)
• Substrates bind to active site on enzyme
• Binding induces conformational change in
  enzyme--better fit for substrate
• Active sites are highly specific and
  discriminatory i.e. sucrase does not accept
  lactose

18
LE 8-16
Substrate
Active site
Enzyme-substrate complex
Enzyme
19
How does enzyme lower activation energy of
reaction?

• Orients substrates for optimal interaction

• Strains substrate bonds

• Provides a favorable microenvironment

-Covalently bonds to the substrate
20
LE 8-17
Substrates enter active site enzyme changes
shape so its active site embraces the substrates
(induced fit).
Substrates held in active site by
weak interactions, such as hydrogen bonds
and ionic bonds.

• 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.

Substrates
Enzyme-substrate complex
Active site is available for two
new substrate molecules.
Enzyme
Products are released.
Substrates are converted into products.
Products
21
How do we know when a reaction is exergonic or
endergonic?
Measure the systems ability to perform work
(usable energy) at uniform temperature and
pressure.
Change in Gibbs free energy (G) ?G
?H-T?S Where ?H change in total energy of the
system, or enthalpy Tabsolute temperature in
Kelvin (oC273) ?S change in entropy (a measure
of disorder)
22
Another way to think about the state of energy in
a cell is before and after a particular reaction
occurs ?G G final state - G initial state
(reactants)
(products)
If the reaction gives final products that have
less energy than the initial reactants, is ?G
negative or positive? The reverse?
When ?G lt 0, the reaction is exergonic and
spontaneous.
When ?G gt 0, the reaction is endergonic and not
spontaneous.
23
LE 8-6a
Reactants
Amount of energy released (?G lt 0)
Free energy
Products
Energy
Progress of the reaction
Exergonic reaction energy released
Catabolic rxn C6H12O6 6O2 ----gt 6CO2 6H2O
ATP glucose
24
LE 8-6b
25
Relationship among Free Energy, Instability, and
Equilibrium

• Free energy
• a measure of a systems instability, its
  tendency to change to a more stable state
• During a spontaneous change
• free energy decreases and the stability of a
  system increases
• Equilibrium is a state of maximum stability
  (?G0)
• If the metabolism of a cell is at equilibrium,
  what has occurred?

RIP
26
LE 8-12
ATP
Energy for cellular work (endergonic,
energy- consuming processes)
Energy from catabolism (exergonic,
energy- yielding processes)
P
ADP

i
Cellular energy used for transport (across
membranes) mechanical work (motility,
contraction) enzymatic activity (catalysis of
reactions)
27
ATP structure

• ATP
• adenosine triphosphate
• cellular energy carrier

28
LE 8-8
ATP structure Adenosine triphosphate Cellular
energy currency
Adenine (base)
a
g
b
Phosphate groups
Ribose (sugar)
29
LE 8-9
P
P
P
Adenosine triphosphate (ATP)
H2O

P
P
P
Energy

i
Adenosine diphosphate (ADP)
Inorganic phosphate
30

• Terminal phosphate bond (ATP--gt ADP Pi)
• Hydrolysis of high energy phosphate bond
• Energy is released (exergonic)
• ADP lower energy than ATP
• Why?
• Is ADP more stable than ATP? Explain.

31
LE 8-8
Adenine
a
g
b
Phosphate groups
Ribose
32

• Energy from ATP hydrolysis
• drives endergonic reactions
• Overall, coupled reactions are exergonic

33
LE 8-10
Endergonic reaction DG is positive, reaction is
not spontaneous
NH2
NH3
DG 3.4 kcal/mol

Glu
Glu
Ammonia
Glutamine
Glutamic acid
Exergonic reaction DG is negative, reaction is
spontaneous
P
DG 7.3 kcal/mol
ATP
ADP
H2O


i
Coupled reactions Overall DG is
negative together, reactions are spontaneous
DG 3.9 kcal/mol
34
How ATP Performs Work

• Inorganic phosphate from ATP hydrolysis
• Transferred to target molecule
• Called phosphorylation
• Creates highly reactive, unstable target molecule
• More prone to do work or change (conformation)
• Mechanical, transport, enzymatic

35
LE 8-11
P
i
P
Motor protein
Protein moved
Mechanical work ATP phosphorylates motor proteins
Membrane protein
ADP
ATP

P
i
P
P
i
Solute transported
Solute
Transport work ATP phosphorylates transport
proteins
P
NH2
NH3
P


Glu
i
Glu
Reactants Glutamic acid and ammonia
Product (glutamine) made
Chemical work ATP phosphorylates key reactants
36
Regeneration of ATP

• ADP P i--gt ATP
• Energy for ADP phosphorylation from catabolic
  reactions

37
LE 8-12
ATP
Energy for cellular work (endergonic,
energy- consuming processes)
Energy from catabolism (exergonic,
energy- yielding processes)
P
ADP

i
38
Environmental Conditions Affect Enzyme Function
?
Temperature cold--gtdecreased chance of bumping
into substrate hot--gt good chance of substrate
interaction but chance of denaturation at some
point pH-gtchange in charge (H or OH-) can
denature proteins and substrate Examples of pH
sensitive enzymes?
39
LE 8-18
Optimal temperature for typical human enzyme
Optimal temperature for enzyme of thermophilic
(heat-tolerant
bacteria)
What is your normal body temp.?
Rate of reaction
0
20
40
60
80
100
Temperature (C)
Optimal temperature for two enzymes
Optimal pH for pepsin (stomach enzyme)
Optimal pH for trypsin (intestinal enzyme)
Rate of reaction
0
1
2
3
4
5
6
7
8
9
10
pH
Optimal pH for two enzymes
40
Cofactors

• Non-protein enzyme helpers (like metals (Fe))

• Coenzymes
• organic cofactors
• Vitamins
• e.g. Vitamin K required for blood clotting
• Required in certain carboxylation reactions

41
Regulation of Enzymes Enzyme Inhibitors

• Competitive inhibitor
• binds to active site of enzyme
• blocks substrate binding by competition

• Noncompetitive inhibitor
• binds to another part of enzyme
• causes enzyme to change shape
• prevents active site from binding substrate
• Allosteric effect

DRAW
42
LE 8-19
Substrate
A substrate can bind normally to the active site
of an enzyme.
Active site
Enzyme
Normal binding
A competitive inhibitor mimics the substrate,
competing for the active site.
Competitive inhibitor
Competitive inhibition
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.
Noncompetitive inhibitor
Noncompetitive inhibition
43
Allosteric Regulation of Enzymes

• Where protein function at one site is affected by
  binding of a regulatory molecule at another site
• May inhibit or stimulate enzyme activity

44
Allosteric Activation and Inhibition

• Most allosterically regulated enzymes are made
  from polypeptide subunits
• active and inactive forms
• binding of activator stabilizes active form of
  enzyme
• binding of inhibitor stabilizes inactive form of
  enzyme

45
LE 8-20a
Allosteric activator stabilizes active form.
Allosteric enzyme with four subunits
Active site (one of four)
Regulatory site (one of four)
Activator
Active form
Stabilized active form
Oscillation
Allosteric inhibitor stabilizes inactive form.
Non- functional active site
Inhibitor
Stabilized inactive form
Inactive form
Allosteric activators and inhibitors
46

• Cooperativity
• form of allosteric regulation that can amplify
  enzyme activity
• binding of substrate to one active site
  stabilizes favorable conformational changes at
  all other subunits

47
LE 8-20b
Binding of one substrate molecule to active site
of one subunit locks all subunits in active
conformation.
Substrate
Stabilized active form
Inactive form
Cooperativity another type of allosteric
activation
48
Feedback Inhibition

• End product of a metabolic pathway shuts down the
  pathway
• Prevents over-production of unneededmolecules

49
LE 8-21
Initial substrate (threonine)
Active site available
Threonine in active site
Enzyme 1 (threonine deaminase)
Isoleucine used up by cell
Intermediate A
Feedback inhibition
Enzyme 2
Active site of enzyme 1 cant bind theonine pathwa
y off
Intermediate B
Enzyme 3
Intermediate C
Isoleucine binds to allosteric site
Enzyme 4
Intermediate D
Enzyme 5
End product (isoleucine)
50
Metabolic regulation influenced by cellular
localization

• Cellular structures organize and concentrate
  enzymes in pathways
• Membranes, organelles (mitochondria, chloroplast)

51
LE 8-22
Mitochondria, sites of cellular respiration
1 µm
52
LE 8-22
Its nice to get so much attention!