Metabolism & Enzymes
AP Biology
From food webs to the life of a cell
energy
energy
energy
AP Biology
Flow of energy through life
 Life is built on chemical reactions

transforming energy from one form to
organic molecules  ATP
another
& organic molecules
sun
solar energy 
AP
Biology
ATP
& organic molecules
organic molecules 
ATP & organic molecules
The First Law of Thermodynamics
 Energy cannot be created or destroyed,
only transformed.

Living systems need to continually
acquire and transform energy in order
to remain alive.
 “Free energy”: The energy available in
a system to do work.
AP Biology
The Second Law of Thermodynamics
 Every time energy is transformed, the
entropy (“disorder”) of the universe
increases.
 In order to increase/maintain their
internal order, living systems must
process more ordered forms of matter
in to less ordered ones
AP Biology
Living Systems are “Open” Systems
 Matter and energy move in to living
systems from the environment. Living
systems transform matter and energy
and return it to the environment
AP Biology
Multi-Step Metabolism
 To increase control, living systems produce free
energy in multiple-step pathways, mediated by
enzyme catalysts
AP Biology
Metabolism
 Chemical reactions of life

forming bonds between molecules
 dehydration synthesis
 synthesis
 anabolic reactions

breaking bonds between molecules
 hydrolysis
 digestion
 catabolic reactions
AP Biology
That’s why
they’re called
anabolic steroids!
Examples
 dehydration synthesis (synthesis)
+
enzyme
H2O
 hydrolysis (digestion)
enzyme
H2O
AP Biology
+
Examples
 dehydration synthesis (synthesis)
enzyme
 hydrolysis (digestion)
enzyme
AP Biology
Chemical reactions & energy
 Some chemical reactions release energy
exergonic
 digesting polymers
 hydrolysis = catabolism

digesting molecules=
LESS organization=
lower energy state
 Some chemical reactions require
input of energy
building molecules=
MORE organization=
higher energy state
endergonic
 building polymers
 dehydration synthesis = anabolism

AP Biology
Endergonic vs. exergonic reactions
exergonic
endergonic
- energy released
- digestion
- energy invested
- synthesis
+G
-G
AP Biology
G = change in free energy = ability to do work
Energy & life
 Organisms require energy to live

where does that energy come from?
 coupling exergonic reactions (releasing energy)
with endergonic reactions (needing energy)
+
digestion
synthesis
+
AP Biology
+
energy
+
energy
What drives reactions?
 If reactions are “downhill”, why don’t they
just happen spontaneously?

because covalent bonds are stable bonds
starch
AP Biology
Why don’t
stable polymers
spontaneously
digest into their
monomers?
Activation energy
 Breaking down large molecules
requires an initial input of energy
activation energy
 large biomolecules are stable
 must absorb energy to break bonds

AP Biology
cellulose
energy
CO2 + H2O + heat
Too much activation energy for life
 Activation energy
amount of energy needed to destabilize
the bonds of a molecule
 moves the reaction over an “energy hill”

glucose
AP Biology
Not a match!
That’s too much
energy to expose
living cells to!
Reducing Activation energy
 Catalysts

reducing the amount of energy to
start a reaction
uncatalyzed reaction
Pheeew…
that takes a lot
less energy!
catalyzed reaction
NEW activation energy
reactant
product
AP Biology
Catalysts
 So what’s a cell got to do to reduce
activation energy?

get help! … chemical help… ENZYMES
Call in the
ENZYMES!
G
AP Biology
Enzymes
 Biological catalysts


proteins (& RNA)
facilitate chemical reactions
 increase rate of reaction without being consumed
 reduce activation energy
 don’t change free energy (G) released or required


required for most biological reactions
highly specific
 thousands of different enzymes in cells

AP Biology
control reactions
of life
Enzymes vocabulary
substrate
 reactant which binds to enzyme
 enzyme-substrate complex: temporary association
product
 end result of reaction
active site
 enzyme’s catalytic site; substrate fits into active site
active site
substrate
enzyme
AP Biology
products
Properties of enzymes
 Reaction specific

each enzyme works with a specific substrate
 chemical fit between active site & substrate
 H bonds & ionic bonds
 Not consumed in reaction

single enzyme molecule can catalyze
thousands or more reactions per second
 enzymes unaffected by the reaction
 Affected by cellular conditions

any condition that affects protein structure
 temperature, pH, salinity
AP Biology
Naming conventions
 Enzymes named for reaction they catalyze




sucrase breaks down sucrose
proteases break down proteins
lipases break
down lipids
DNA polymerase builds DNA
 adds nucleotides
to DNA strand

pepsin breaks down
proteins (polypeptides)
AP Biology
Lock and Key model
 Simplistic model of
enzyme action

substrate fits into 3-D
structure of enzyme’
active site
 H bonds between
substrate & enzyme

AP Biology
like “key fits into lock”
In biology…
Size
doesn’t matter…
Shape matters!
Induced fit model
 More accurate model of enzyme action
3-D structure of enzyme fits substrate
 substrate binding cause enzyme to
change shape leading to a tighter fit

 “conformational change”
 bring chemical groups in position to catalyze
reaction
AP Biology
How does it work?
 Variety of mechanisms to lower
activation energy & speed up reaction

synthesis
 active site orients substrates in correct
position for reaction
 enzyme brings substrate closer together

digestion
 active site binds substrate & puts stress on
bonds that must be broken, making it easier
to separate molecules
AP Biology
Math Skills: Gibbs Free Energy
3.1: All living systems require constant input of free energy
Be able to use and interpret the Gibbs Free Energy Equation to
determine if a particular process will occur spontaneously or nonspontaneously.
 ΔG= change in free energy

(- = exergonic, + = endergonic)
 ΔH= change in enthalpy for the reaction
(- = exothermic, + = endothermic)
 T = kelvin temperature
 ΔS = change in entropy
 (+ = entropy increase, - = entropy decrease)

AP Biology
Spontaneity
Spontaneous reactions continue once they are
initiated. Non-spontaneous reactions require
continual input of energy to continue.
AP Biology
Using the Equation
To use the equation, you’ll need to be given values.
 Exothermic reactions that increase entropy are
always spontaneous/exergonic
 Endothermic reactions that decrease entropy are
always non-spontaneous/endergonic.
 Other reactions will be spontaneous or not
depending on the temperature at which they
occur.
AP Biology
Sample Problem
 Determine which of the following reactions will occur
spontaneously at a temperature of 298K, justify your answer
mathematically:
 Reaction 1:
A + B → AB
Δ H: +245 KJ/mol
Δ S: -.02 KJ / K
 Reaction 2:
BC→ B + C
Δ H: -334 KJ/mol
Δ S: +.12 KJ/K
AP Biology
Got any Questions?!
AP Biology
2007-2008
Factors that Affect Enzymes
AP Biology
2007-2008
Factors Affecting Enzyme Function
 Enzyme concentration
 Substrate concentration
 Temperature
 pH
 Salinity
 Activators
 Inhibitors
AP Biology
catalase
Enzyme concentration
reaction rate
What’s
happening here?!
enzyme concentration
AP Biology
Factors affecting enzyme function
 Enzyme concentration

as  enzyme =  reaction rate
 more enzymes = more frequently collide with
substrate

reaction rate levels off
reaction rate
 substrate becomes limiting factor
 not all enzyme molecules can find substrate
AP Biology
enzyme concentration
Substrate concentration
reaction rate
What’s
happening here?!
substrate concentration
AP Biology
Factors affecting enzyme function
 Substrate concentration

as  substrate =  reaction rate
 more substrate = more frequently collide with
enzyme

reaction rate levels off
reaction rate
 all enzymes have active site engaged
 enzyme is saturated
 maximum rate of reaction
AP Biology
substrate concentration
Temperature
reaction rate
What’s
happening here?!
37°
temperature
AP Biology
Factors affecting enzyme function
 Temperature

Optimum T°
 greatest number of molecular collisions
 human enzymes = 35°- 40°C
 body temp = 37°C

Heat: increase beyond optimum T°
 increased energy level of molecules disrupts
bonds in enzyme & between enzyme & substrate
 H, ionic = weak bonds
 denaturation = lose 3D shape (3° structure)

Cold: decrease T°
 molecules move slower
 decrease collisions between enzyme & substrate
AP Biology
Enzymes and temperature
 Different enzymes function in different
organisms in different environments
reaction rate
human enzyme
hot spring
bacteria enzyme
37°C
AP Biology
temperature
70°C
(158°F)
How do ectotherms do it?
AP Biology
Metabolic Strategies
 Ectothermy: Conform internal temperature to
environmental temperature.
 Endothermy: Regulate internal temperature
within a narrow range.
 Both strategies
have advantages and
tradeoffs.
AP Biology
Body Size Considerations
 Smaller animals need to produce more
energy per unit of mass due to
increased radiation of heat into the
environment.
AP Biology
Free Energy Considerations & Reproduction
 Reproductive strategies are optimized
for free energy considerations.

AP Biology
Ex. Seasonal Reproduction.
Insufficient Free Energy Production: Individuals
 Insufficient free energy production by
individuals will lead to disease and
death.
AP Biology
Insufficient Free Energy Production: Populations
 If the individuals in the population are
unable to survive, the growth rate of the
population will decline
AP Biology
Insufficient Free Energy Production: Ecosystems
 If the populations in an ecosystem
decline, the ecosystems will decrease
in complexity
AP Biology
Math Skills: Coefficient Q10
3.1: All living systems require constant input
 Be able to use and interpret the Coefficient Q10
equation:





t2 = higher temperature
t1 = lower temperature
k2= metabolic rate at higher temperature
k1= metabolic rate at lower temperature
Q10 = the factor by which the reaction rate increases when
the temperature is raised by ten degrees of free energy
AP Biology
 Q10 tells us how a particular process
will be affected by a 10 degree change
in temperature.
 Most biological processes have a Q10
value between 2 and 3
AP Biology
Sample Problem
 Data taken to determine the effect of
temperature on the rate of respiration in
a goldfish is given in the table below.
Calculate the Q10 value for this data. (°C
AP Biology
Temperature (°C)
Heartbeats per
minute
20
18
25
42
pH
What’s
happening here?!
trypsin
reaction rate
pepsin
pepsin
trypsin
0
AP Biology
1
2
3
4
5
6
pH
7
8
9
10
11
12
13
14
Factors affecting enzyme function
 pH

changes in pH
 adds or remove H+
 disrupts bonds, disrupts 3D shape
 disrupts attractions between charged amino acids
 affect 2° & 3° structure
 denatures protein

optimal pH?
 most human enzymes = pH 6-8
 depends on localized conditions
 pepsin (stomach) = pH 2-3
 trypsin (small intestines) = pH 8
AP Biology
Salinity
reaction rate
What’s
happening here?!
salt concentration
AP Biology
Factors affecting enzyme function
 Salt concentration

changes in salinity
 adds or removes cations (+) & anions (–)
 disrupts bonds, disrupts 3D shape
 disrupts attractions between charged amino acids
 affect 2° & 3° structure
 denatures protein

enzymes intolerant of extreme salinity
 Dead Sea is called dead for a reason!
AP Biology
Compounds which help enzymes
Fe in
 Activators
hemoglobin

cofactors
 non-protein, small inorganic
compounds & ions
 Mg, K, Ca, Zn, Fe, Cu
 bound within enzyme molecule

coenzymes
 non-protein, organic molecules
 bind temporarily or permanently to
enzyme near active site
AP Biology
 many vitamins
 NAD (niacin; B3)
 FAD (riboflavin; B2)
 Coenzyme A
Mg in
chlorophyll
Compounds which regulate enzymes
 Inhibitors
molecules that reduce enzyme activity
 competitive inhibition
 noncompetitive inhibition
 irreversible inhibition
 feedback inhibition

AP Biology
Competitive Inhibitor
 Inhibitor & substrate “compete” for active site


penicillin
blocks enzyme bacteria use to build cell walls
disulfiram (Antabuse)
treats chronic alcoholism
 blocks enzyme that
breaks down alcohol
 severe hangover & vomiting
5-10 minutes after drinking
 Overcome by increasing substrate
concentration

AP Biology
saturate solution with substrate
so it out-competes inhibitor
for active site on enzyme
Non-Competitive Inhibitor
 Inhibitor binds to site other than active site


allosteric inhibitor binds to allosteric site
causes enzyme to change shape
 conformational change
 active site is no longer functional binding site
 keeps enzyme inactive

some anti-cancer drugs
inhibit enzymes involved in DNA synthesis
 stop DNA production
 stop division of more cancer cells

cyanide poisoning
irreversible inhibitor of Cytochrome C,
an enzyme in cellular respiration
 stops production of ATP
AP Biology
Irreversible inhibition
 Inhibitor permanently binds to enzyme

competitor
 permanently binds to active site

allosteric
 permanently binds to allosteric site
 permanently changes shape of enzyme
 nerve gas, sarin, many insecticides
(malathion, parathion…)
 cholinesterase inhibitors

AP Biology
doesn’t breakdown the neurotransmitter,
acetylcholine
Allosteric regulation
 Conformational changes by regulatory
molecules

inhibitors
 keeps enzyme in inactive form

activators
 keeps enzyme in active form
AP Biology Conformational
changes
Allosteric regulation
Cooperativity
 Substrate acts as an activator



substrate causes conformational
change in enzyme
 induced fit
favors binding of substrate at 2nd site
makes enzyme more active & effective
 hemoglobin
Hemoglobin
 4 polypeptide chains
 can bind 4 O2;
 1st O2 binds
 now easier for other
O2 to bind
AP3Biology
Metabolic pathways







ABCDEFG
5
6
enzyme enzyme enzyme
enzyme enzyme enzyme
enzyme
1
2
3
4
 Chemical reactions of life
are organized in pathways

AP Biology
divide chemical reaction
into many small steps
 artifact of evolution
  efficiency
 intermediate branching points
  control = regulation
Efficiency
 Organized groups of enzymes

enzymes are embedded in membrane
and arranged sequentially
 Link endergonic & exergonic reactions
Whoa!
All that going on
in those little
mitochondria!
AP Biology
Feedback Inhibition
 Regulation & coordination of production


product is used by next step in pathway
final product is inhibitor of earlier step
 allosteric inhibitor of earlier enzyme
 feedback inhibition

no unnecessary accumulation of product






ABCDEFG
1
2
3
4
5
6
X
enzyme enzyme enzyme enzyme enzyme enzyme
AP Biology
allosteric inhibitor of enzyme 1
threonine
Feedback inhibition
 Example
synthesis of amino
acid, isoleucine from
amino acid, threonine
 isoleucine becomes
the allosteric
inhibitor of the first
step in the pathway

 as product
accumulates it
collides with enzyme
more often than
substrate does
AP Biology
isoleucin
e
Don’t be inhibited!
Ask Questions!
AP Biology
2007-2008