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Lecture 7
Reactions and Enzymes
Chemical Reactions in a Cell
The Terminology
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The sum total of all the chemical conversions in a cell is
called metabolism.
 Organic compounds participating in metabolism are called
metabolites.
 Two types of metabolic reactions: anabolic and catabolic
reactions.
 Anabolic reactions are those that link simple molecules
together to make complex ones. These are energy-storing
reactions (endergonic).
 Catabolic reactions are those that break down complex
molecules into simpler ones. Some of these reactions
provide the energy for anabolic reactions. These are energy
releasing reactions (exergonic).
Thermodynamics
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All chemical changes are governed by the laws of
thermodynamics.
1st Law – Conservation/ transformation of energy
2nd Law – Tendency toward disorder/randomness
Ordered systems need energy to maintain their
order or they will deteriorate.
Energy changes in molecules occur when bonds
are broken and new ones formed.
Organic compounds are a store of potential energy
Activation Energy
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The energy required to initiate a chemical
reaction.
Once this barrier is overcome the reaction will
proceed, generating enough energy to occur
spontaneously.
E.g Petrol + air
The activation energy is important in living
organisms as it prevents breakdown of
components in the presence of air/oxygen.
The biological alternative of overcoming the
activation energy is less lethal.
Free energy for endergonic reactions
Respiration
Catabolism (exergonic) reaction
Sugar (source of potential energy)
Photosynthesis
Electrons excited to a higher energy level
Anabolic (endergonic) reaction
Sun (source of energy)
Enzymes
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Globular proteins
Encoded in DNA sequence
Enzymes speed up the rate of metabolic reactions
in cells, they are biological catalysts.
Enzymes are not used up by the reaction, and they
cannot change the basic nature/equilibrium of a
reaction.
Specific
Activity is affected by pH, temperature, substrate
concentration and enzyme concentration.
Efficient
Example of a Catalytic
Reaction.
A
B
Hydrogen peroxide
2H2O2
Hydrogen peroxide
2H2O2
Heat to 150 oC
water + oxygen
2H2 O + O2
Rapid decomposition/explosive
reaction clearly not suitable for living
organisms
Catalase
37 oC
water + oxygen
2H2 O + O2
Breakdown of a toxic compound
into less harmful components at a
temperature better suited for living organisms
Components of Enzyme
Catalysed Reactions
E+S
ES
EP
E+P
S
Enzyme Structure
Active site
E
Amino acids
3
4
5
Substrate
Enzyme ActionMechanism of Catalysis
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Enzymes are thought to accomplish this by reducing the
activation energy required for a reaction to occur.
The activation energy is reduced when the substrate
attaches (by weak ionic attractions and hydrogen bonds) to
the active site of the enzyme.
The enzyme-substrate complex is known as the transition
state.
In the transition state, internal bonds of the substrate
molecule become distorted and strained.
Hence less energy is required to break the internal covalent
bonds of the substrate.
Enzyme ActionMechanism of Catalysis
Rate of Enzyme Reaction
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Affected by:– A) Enzyme concentration-more active sites for
substrates increases reaction.
– B) Substrate concentration-rate will increase however
eventually all active sites will be occupied so rate will
reach plateau (saturation).
– C) Temperature-increase in collision of enzyme and
substrate so rate will inc as temp is inc. However above
optimum temperature enzyme denatures.
– D) pH
Rate of Enzyme Reaction
A
B
Rate
Rate
Conc. of Enzyme
C
Rate
Conc. of Substrate
D
optimum temp
denaturation
Temperature
Rate
pH
optimum pH
Enzyme activity can be
inhibited
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Competitive Inhibition – action of antibiotics such as
penicillin. Prevents cell wall synthesis by competing with
amino acid for active site. Bacterial cell bursts because of
an incomplete cell wall.
 Non competitive reversible inhibition-rate of
reaction decreases as inhibitor conc. increases, at saturation
rate of reaction will be nil.
 Non competitive irreversible inhibition e.g heavy
metals, nerve gas contains a compound that combines with
serine in active site of acetylcholinesterase, and stops
acetylcholine from binding. Causes accumulation of
actelycholine. Nerve impulses cannot be stopped.
Prolonged muscle contraction.
Enzymes are also able to
regulation Metabolic Pathways
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Commonest form of regulation in cells.
 Allosteric Enzymes
 Regulated by compounds that act as non
competitive inhibitors.
 Bind well away from the active site but
causes a change in active site such that it no
longer binds substrate.
Assignment 3
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Read pp 125-127
 Q1. Give an example of an enzyme cofactor.
 Q2. How are metabolic pathways in a cell
regulated (controlled)?
Term Paper
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Apply scientific method
 Introduction
 Data
 Analysis
 Conclusion
 Word limit excl. diagrams 2000