Enzymes
A catalyst is a substance that speeds up a chemical reaction without being used up or chemically altered
in the reaction itself.
Catalysts are needed in order to make chemical reactions happen at a quicker pace, and produce a
sufficient concentration of product molecules.
Conditions that may include a rate of reaction include:
■ High temperature/pressure.
■ Extreme pH.
■ High concentrations of the respective reactants.
Enzymes are proteins that take the form of biological catalysts in cells, and allow chemical reactions to
occur at suitable rates.
★ They are reusable so only a small number is needed to catalyse reactions.
★ They remain unchanged by any of the reactions that they catalyse.
Without the presence of enzymes, the rate of chemical reactions in organisms would be too low to support
life.
→ To form specific product molecules, the reactants are needed to collide at an exact angle and speed,
for the reaction to occur.
→ The chances of this occurring under normal conditions is so low, the event would be insignificant, so it is
the enzymes’ function to ensure this.
Metabolism is the complex network of interdependent and interacting chemical reactions occurring in living
organisms. The molecules involved are metabolites.
○ Many of the reactions take place in multiple stages.
○ Each stage is catalysed by a separate enzyme.
○ They are broadly categorised as anabolic or catabolic.
Enzymes are globular proteins.
The active site is critical to the enzyme function, where the substrate binds. Enzymes are specific to the
substrate. The properties of the enzyme and substrate are complementary, and the two fit together like
jigsaw pieces. This is called the enzyme-substrate specificity.
Because of this, thousands of enzymes are needed throughout an organism, to carry out individual
chemical reactions.
Anabolic and Catabolic Reactions:
Anabolic Reactions are involved with the building of large molecules from smaller molecules, examples
include:-
1) Photosynthesis: where CO2 and water are built into complex sugars.
2) Protein Synthesis: where amino acids are joined together in a sequence.
3) The formation of glycogen, by linking glucose molecules together.
Anabolic reactions often include condensation reactions, as they are endergonic and require an input of
energy to take place.
Catabolic reactions are involved with breaking down large molecules into smaller, simpler ones.
These reactions are often carried out to release energy for cellular processes and for the excretion of
waste, examples include;
1) Respiration, where CO2 and water are produced from the oxidation of sugars.
2) Deamination of proteins to release urea.
3) Breakdown of macromolecules into monomers during digestion.
Catabolic reactions often include hydrolysis reactions, as they are exergonic, and free energy is released
for cellular processes.
Anabolism. Catabolism.
Requires an input of energy. Releases energy.
Builds large molecules from small ones. Breaks down large molecules into smaller ones.
Used to store energy in chemical form. Used to release chemical energy as heat and for
other activities such as movement and active
transport.
Involves condensation reaction. Involves hydrolysis reactions.
Used for growth, repair and energy storage. Performs several activities such as digestion,
excretion and energy supply.
Both are made up of enzyme-catalysed reactions.
Both are coupled to ATP, the principle energy carrier in cells.
The structure of Enzymes:
→ Enzyme catalysis involves molecular motion and the collision of substrates with the active site.
→ Substrates collide at random with the active site for the enzyme-catalysed reaction to take place.
→ This has to happen at the correct orientation and speed.
→ Substrates bind to enzymes, forming a temporary enzyme-substrate complex.
→ The reaction occurs within the enzyme-substrate complex which leads to changes in the properties to
bind with a specific substrate.
Induced-Fit Hypothesis:
Originally known as the lock-and-key model. It is an explanation of how the enzyme and substrate interact
with each other. The enzyme and active site can change shape as the substrate molecule enters the
enzyme. These changes are known as conformational changes. Is done to ensure an ideal binding
arrangement, and maximise the ability of the enzyme to catalyze the reaction.
Denaturation: Enzymes.
Enzymes can be denatured when it is exposed to high temperature or extreme pH. The hydrogen bonds
holding the enzyme molecule in its 3D shape break. This causes a change in the 3D shape of the protein,
permanently, preventing the substrate from binding. The optimum temperature for enzymes is 37C in
humans, so temperatures exceeding 40 C will cause enzymes to denature.
