Title: Chemical Reactions in Cells
1Chemical Reactions in Cells
- Energetics, Enzymes and Metabolic Reactions
2Energy
- Energy is the capacity for work or change.
- Kinetic Energy energy of movement
- Potential Energy stored energy
- 1st Law of Thermodynamics
- Energy can be transferred and transformed from
one form to another but it cannot be created or
destroyed.
3Energy
- 2nd Law of Thermodynamics
- Energy transfer or transformation increases the
entropy of the universe - Increase in entropy randomness
- Energy conversions result in a loss of useful
energy
4Free Energy Energy Useful for Change
5Free Energy Energy Useful for Change
6Chemical Reactions
- Involve the breaking and formation of chemical
bonds - Reactants are converted to products.
- Two types of reactions based on energy use
- Exergonic free energy released
- Endergonic free energy consumed
7Exergonic Reactions
Burning glucose (sugar)an exergonic reaction
Activation energy neededto ignite glucose
Glucose O2
Energy released byburning glucose
C O2 H2O
8Endergonic Reactions
Photosynthesisan endergonic reaction
Glucose
Net energycaptured bysynthesizingglucose
Activationenergy fromlight capturedby
photosynthesis
CO2 H2O
9Applying Your Knowledge
- Endergonic Reaction
- Exergonic Reaction
A. Which type of reaction would be spontaneous?
B. For which type of reaction will the products
have a higher energy than the reactants?
C. Which type of reaction releases energy?
10ATP Provides Energy for Cellular Reactions
11Short-Term Energy Storage
- Chemical Energy is stored in the bonds of ATP
- ATP adenosine triphosphate
- ADP adenosine diphosphate
- to store energy
- ADP Phosphate Energy? ATP
- to release energy
- ATP ? ADP Phosphate Energy
12Coupled Reactions
- Pairing of an Exergonic reaction, often
involving ATP, with an Endergonic reaction
Note that overall free energy change is negative
13Metabolic Reactions
- Anabolic
- link simple molecules to produce complex
molecules (eg. dehydration synthesis of starch) - require energy
- Catabolic
- break down complex molecules to release simple
ones (eg. hydrolysis of starch? sugars) - release energy stored in chemical bonds
14Metabolic Pathways
InitialReactants
Intermediates
FinalProducts
B
C
A
Enzyme 1
Enzyme 2
Enzyme 3
Enzyme 4
Pathway 1
F
G
Pathway 2
Enzyme 5
Enzyme 6
15Enzymes Assist in Biological Reactions
- Enzymes are biological catalysts.
- biological composed of protein or, rarely, RNA
- catalyst speeds up a reaction without being
changed by the reaction
16Properties of Enzymes
- Enzymes speed up biological reactions by lowering
the activation energy for the reaction. - Enzymes provide a surface where the catalysis
takes place - The reaction reaches equilibrium more rapidly
17Activation Energy Controls Rate of Reaction
Amount of energy required for reaction to occur
transition state
Activationenergy withoutcatalyst
Activationenergy withcatalyst
18Properties of Enzymes
- Enzymes are SPECIFIC for the reactants
(substrates) in the reactions that they catalyze.
- Only substrates that fit the active site of the
enzyme can bind and complete the reaction - active site region on enzyme where substrates
bind
19Enzyme-Substrate Interactions
Substrate
Substrate
1 Substrates enter active site
ActiveSite
2 Shape change promotes reaction
Enzyme
- Product releasedenzyme ready again
20Chemical Events at Active Sites
- Enzymes hold substrates in the proper orientation
for the reaction to take place
21Chemical Events at Active Sites
- Enzymes induce strain in the substrate to produce
a transition state favorable to reaction - Active site provides a microenvironment that
favors the chemical reaction
22Chemical Events at Active Sites
- Active site directly participates in the reaction
- covalent bonding can occur between enzyme and
substrate - R groups of the enzymes amino acids can
temporarily add chemical groups to the substrates
23Molecules that Assist Enzymes
- Cofactors inorganic ions that bind to enzymes,
eg. zinc - Coenzymes small organic factors that temporarily
bind to enzymes, eg. biotin, NAD, ATP - Prosthetic groups non-protein factors that are
permanently bound an enzyme, eg. heme
24Factors Influencing Reaction Rate
Rate no longer increases since the active sites
of all enzymes are saturated with substrate
Rate is more rapid
Rate is proportional to substrate concentration
25Factors Influencing Reaction Rate
- Competitive Inhibitors Bind at the active site,
compete for binding with substrate - Irreversible form covalent bond with amino acids
in the active site
26Factors Influencing Reaction Rate
- Competitive Inhibitors Bind at the active site,
compete for binding with substrate - Reversible molecule similar to substrate
occupies active site but does not undergo reaction
27Factors Influencing Reaction Rate
- Non-Competitive Inhibitors Bind to a different
site, cause a conformational change in the enzyme
that alters the active site - Reversible
28Factors Influencing Reaction Rate
- Allosteric Regulation
- Conversion between active and inactive forms of
an enzyme due to binding of regulatory molecules
at an allosteric site - Activators stabilize the active form
- Allosteric inhibitors stabilize the inactive form
29Factors Influencing Reaction Rate
- Allosteric Regulation
- Cooperativity a substrate causing induced fit in
one enzyme subunit can cause a change to the
active form in all the other subunits
30Enzyme Regulation Feedback Inhibition
Commitment step
D
C
B
A
Enz. 5
Enz. 4
Enz. 3
Enz. 2
Enz. 1
Threonine(substrate)
Isoleucine(end product)
Feedback Inhibition The product of a pathway
inhibits an initial step in the pathway to
decrease its own production
31Properties of Enzymes
- Three dimensional structure of an enzyme
preserves its ACTIVE SITE - Conditions that can affect three dimensional
structure include heat, pH (acid/base balance)
and other chemicals (salt, charged ions)
32Effects of Temperature and pH on Enzymatic
Activity
33Applying Your Knowledge
- Active Site
- Activation Energy
- Allosteric Site
- Commitment step
- Induced fit
- Where can an inhibitor bind to stabilize the
inactive form of an enzyme? - Where do the substrates bind?
- Enzymes (raise or lower) the (1, 2, 3, 4 or 5) of
a reaction. - What is the model for a shape change caused by
substrate binding to the enzyme?