Metabolic Pathways and Enzymes
• Cellular reactions are usually part of a metabolic
pathway, a series of linked reactions
• 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
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.
Activation Energy
• 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
Enzymatic Reaction
Substrate is broken down
into smaller products
Substrates are combined into
a larger product
Enzyme Names
• Every reaction in a cell requires a specific
enzyme.
• Enzymes are named for their substrates:
Substrate
Lipid
Enzyme
Lipase
Ureas
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
declines 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 (potential energy) from glucose
(chemical energy)
• 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
Coupled Reaction
The four phases of complete glucose
breakdown
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)
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.
Reaction that Occurs in Cellular Respiration
•It is an oxidation-reduction reaction, or redox reaction for short.
•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
•
•
•
•
•
Step 1. Glycolysis
Occurs in the cytoplasm (outside the
mitochondria)
Glucose  2 pyruvate molecules.
Universally found in all organisms
Does not require oxygen (anaerobic).
Main energy source for prokaryotes
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
• Occurs in the matrix of the mitochondria
• Is the transition between glycolysis and the citric
acid cycle.
• Pyruvate (made during glycolysis) is converted to
acetyl CoA, and CO2 is released
• NAD+ is converted to NADH + H+
• 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.
C2 acetyl group is converted to a C6 citrate.
Each acetyl group gives off 2 CO2 molecules.
NAD+ accepts electrons 3 times
FAD accepts electrons once.
Results in a gain of one ATP per every turn of the
cycle; there are two cycles per glucose, so a net of
2 ATP are produced.
• The citric acid cycle produces four CO2 per
molecule of glucose.
Citric acid cycle
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
•
•
•
•
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
Overview of the electron transport
system
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 Electron Transport Chain
• 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
• Electrons carried by NADH produced during
glycolysis are shuttled to the electron transport chain
by an organic molecule (mechanism of delivery may
vary # of ATP produced by ETS).
Accounting of energy yield per glucose
molecule breakdown
Fermentation
• Occurs when oxygen is not available.
• During fermentation, the pyruvate formed by
glycolysis is reduced to lactic acid .
• Fermentation uses NADH and regenerates
NAD+.
• Occurs in anaerobic bacteria, fungus, & human
muscle cells.
http://instruct1.cit.cornell.edu/Courses/biomi290/MOVIES/GLYCO
LYSIS.HTML
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.
Fermentation inputs and outputs per
glucose molecule
• Inputs:
• Glucose
• 2 ATP
• 4 ADP + 2 P
• Outputs:
• 2 lactate or
• 2 alcohol & 2 CO2
• 2 ADP
• 2 ATP (net gain)