Chapters 3
and 7:
Enzymes
and Cellular
Respiration
Metabolic Pathways and Enzymes
• Cellular reactions are usually part of a metabolic
pathway, a series of linked reactions
• Many reactions have molecules in common
• Energy can be released in small amounts
rather than all at once
• Illustrated as follows:
E1
E2
E3
E4 E5 E6
A → B → C → D → E →F → G
• Letters A-F are reactants or substrates, B-G are
the products in the various reactions, and E1-E6
are enzymes.
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter8/animations.html
• Enzymes – speed up the rate of chemical
reactions
• Substrates – molecules which react with
enzymes
• Only one small part of an enzyme, called the
active site, reacts with the substrate(s).
• Active site may undergo a slight change in
shape in order to fit with the substrate
• The enzyme is not changed by the reaction
(active site returns to its original state), and it is
free to act again.
E1
E2
E3
E4 E5 E6
A → B → C → D → E →F → G
• Energy of activation (Ea) - the energy that
must be added to cause molecules to react
with one another
• Enzyme lowers the amount of energy
required for reaction to occur
• Enzymes allow reactions to take place at
lower temperatures – otherwise,
reactions would not be able to occur at
normal body temperatures
Energy of activation (Ea)
When no enzyme is
present – more
energy required
When an enzyme is
added – less energy
required
Induced fit model
•Because the enzyme must undergo a slight change
in shape to fit with the substrate, this is known as
the induced fit model.
Enzymatic reaction
Substrate is broken down
into smaller products
Substrates are combined into
a larger product
• Every reaction in a cell requires a specific
enzyme.
• Enzymes are named for their substrates:
Substrate
Enzyme
Lipid
Lipase
Urea
Urease
Maltose
Maltase
Ribonucleic acid
Ribonuclease
http://www.lewport.wnyric.org/JWANAMAKER/animations/Enzyme
activity.html
Factors Affecting Enzymatic Speed
• Temperature and pH
• Substrate concentration
• Enzyme concentration
• Temperature and pH:
• As the temperature rises, enzyme activity
increases.
• If the temperature is too high, enzyme activity
slows rapidly because the enzyme is denatured.
• When enzyme is denatured, its shape changes and
it can no longer attach to the substrate.
• Each enzyme has an ideal temperature and pH at
which the rate of reaction is highest.
• Change in pH can alter the structure of the
enzyme, and can eventually cause enzyme to
denature.
Rate of an enzymatic reaction as a function
of temperature and pH
•Rates and concentration:
•Reaction rate depends
on the number of
enzyme-substrate
complexes that can be
formed.
•When all available
enzymes and active sites
are filled, the rate of
activity cannot increase
further.
•Substrate concentration
•Enzyme activity increases as substrate
concentration increases because there are
more collisions between substrate
molecules and the enzyme.
•Enzyme concentration
•Enzyme activity increases as enzyme
concentration increases because there are
more collisions between substrate
molecules and the enzyme.
Overview of Cellular Respiration
• Makes ATP molecules
• Releases energy in 4 reactions
• Glycolysis, Transition reaction, Citric acid cycle
(Kreb’s cycle), and Electron transport system
• An aerobic process that requires O2
• If oxygen is not available (anaerobic), glycolysis is
followed by fermentation
The four phases of complete glucose
breakdown
•
•
•
•
ATP (adenosine triphosphate)
The energy currency of cells.
A nucleotide made of the following:
• Adenine (a base)
• Ribose (a sugar)
• Three phosphate groups
Constantly regenerated from ADP (adenosine
diphosphate) after energy is expended by the cell.
Pneumonic devices – ATP – a triple phosphate
- ADP – a double phosphate
http://www.stolaf.edu/people/giannini/flashanimat/metabolism/
atpsyn2.swf
The ATP cycle
•Oxidation is the loss of electrons; hydrogen atoms
are removed from glucose.
•Reduction is the gain of electrons; oxygen atoms
gain electrons.
•Remember OIL RIG (oxidation is loss, reduction
is gain)
Enzymes involved:
• NAD+
• Nicotinamide adenine dinucleotide
• Accepts H+ to become NADH
• FAD
• Flavin adenine dinucleotide (sometimes
used instead of NAD+)
• Accepts 2H+ to become FADH2
The NAD+ cycle
•Structure of mitochondria:
•Has a double membrane, with an
intermembrane space between the two
layers.
•Cristae are folds of inner membrane
•The matrix, the innermost compartment,
which is filled with a gel-like fluid.
Where does each step occur?
•Outside the mitochondria
•Step 1 - Glycolysis
•Inside the mitochondria
•Step 2 - Transition reaction (matrix)
•Step 3 – Citric acid cycle (matrix)
•Step 4 – Electron transport system
(cristae)
•
•
•
•
Step 1. Glycolysis
Occurs in the cytoplasm (outside the
mitochondria)
Glucose  2 pyruvate molecules.
Universally found in all organisms
Does not require oxygen (anaerobic).
http://www.science.smith.edu/departments/Biology/Bio231/gly
colysis.html
Glycolysis summary
• Inputs:
•
•
•
•
Glucose
2 NAD+
2 ATP
4 ADP + 2 P
• Outputs:
•
•
•
•
2 pyruvate
2 NADH
2 ADP
2 ATP (net gain)
•When oxygen is available, pyruvate enters the
mitochondria, where it is further broken down
•If oxygen is not available, fermentation occurs
Step 2 - Transition Reaction
• Requires oxygen (aerobic)
• Occurs in the matrix of the mitochondria
• Pyruvate (made during glycolysis) is converted to
acetyl CoA, and CO2 is released
• NAD+ is converted to NADH
• The transition reaction occurs twice per glucose
molecule.
Transition reaction inputs and outputs
per glucose molecule
• Inputs:
• 2 pyruvate
• 2 NAD+
• Outputs:
• 2 acetyl groups
• 2 CO2
• 2 NADH
http://www.science.smith.edu/departments/Biology/Bio231/krebs.ht
ml
Step 3 - Citric Acid Cycle (aka Kreb’s Cycle)
• Occurs in the matrix of the mitochondria.
• Requires oxygen (aerobic)
• Cycle occurs twice (each of the following occurs
twice)
• C2 acetyl group is converted to a C6 citrate.
• NAD+ accepts electrons 3 times
• FAD accepts electrons once.
• Produces four CO2
• Results in a gain of one ATP
Citric acid cycle inputs and outputs per
glucose molecule
• Inputs:
• 2 acetyl groups
• 6 NAD+
• 2 FAD
• 2 ADP + 2 P
• Outputs:
• 4 CO2
• 6 NADH
• 2 FADH2
• 2 ATP
http://www.science.smith.edu/departments/Biology/Bio231/krebs.ht
ml
•
•
•
•
Step 4 - Electron Transport System (ETS)
Requires oxygen (aerobic)
Located in the cristae of mitochondria
NADH and FADH2 carry electrons picked up during
glycolysis, transition reaction, & citric acid cycle
and enter the ETS.
The ETS consists of:
– protein complexes that pump H+
– mobile carriers that transport electrons
– ATP synthase complex - H+ flow through it, making ATP
• H+ flow through from high to low concentration
• For every 3 H+ that flow through, one ATP is made
http://vcell.ndsu.nodak.edu/animations/atpgradient/movie.htm
http://www.sp.uconn.edu/%7Eterry/images/movs/synthase.mov
http://www.science.smith.edu/departments/Biology/Bio231/etc.html
http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter9/animations.ht
ml
Energy Yield from Glucose Metabolism
• Per glucose molecule:
– 10 NADH take electrons to the ETS  3
ATP from each
– 2 FADH2 take electrons to the ETS  2 ATP
from each
Accounting of energy yield per glucose
molecule breakdown
•
•
•
•
•
Fermentation
Occurs when oxygen is not available.
Follows glycolysis
Pyruvate formed by glycolysis is reduced to
alcohol and CO2, or to lactate.
Fermentation uses NADH and regenerates NAD+,
which can be used during glycolysis.
Occurs in anaerobic bacteria, fungus, & human
muscle cells.
http://instruct1.cit.cornell.edu/Courses/biomi290/MOVIES/GLYCO
LYSIS.HTML
Glycolysis and Fermentation inputs
and outputs per glucose molecule
• Inputs (all into
glycolysis):
• Glucose
• 2 ATP
• 4 ADP + 2 P
• Outputs:
• 2 lactate or
• 2 alcohol & 2 CO2
• 2 ADP (glycolysis)
• 2 ATP (net gain)
(glycolysis)
Advantages and Disadvantages of
Fermentation
• Fermentation can provide a rapid burst of ATP
in muscle cells, even when oxygen is in
limited supply.
• Lactate, however, is toxic to cells.
• Initially, blood carries away lactate as it forms;
eventually lactate builds up, lowering cell pH,
and causing muscles to fatigue.
• Oxygen debt occurs, and the liver must
reconvert lactate to pyruvate.
Efficiency of Fermentation
• Two ATP produced during fermentation are
equivalent to 14.6 kcal; complete oxidation of
glucose to CO2 and H2O represents a yield of
686 kcal per molecule of glucose.
• Thus, fermentation is only 2.1% efficient
compared to cellular respiration (which is 39%
efficient).
• (14.6/686) x 100 = 2.1%