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OCR Biology AS Level (4)
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Chapter 4: EnzymesChapter 4.1: Enzyme action
Why are enzymes important?
Most of the processes necessary to life involve chemical reactions, and these reactions need to happen very
fast.
In the laboratory or in industry this would demand very high temperatures and pressures.
These extreme conditions are not possible in living cells - they would damage the cell components.
Instead, the reactions are catalysed by enzymes.
Enzymes are biological catalysts.
They are globular proteins that interact with substrate molecules causing them to react at much faster rates
without the need for harsh environmental conditions.
Without enzymes many of the processes necessary to life would not be possible.
The role of enzymes in reactions
Living organisms need to be built and maintained.
This involves the synthesis of large polymer-based components.
For example, cellulose forms the walls of plants cells and long protein molecules form the contractile
filaments of muscles in animals.
The different cell components are synthesised and assembled into cells, which then form tissues, organs, and
eventually the whole organism.
The chemical reactions required for growth are anabolic reactions and they are all catalysed by enzymes.
Energy is constantly required for the majority of living processes, including growth.
Energy is released from large organic molecules, like glucose, in metabolic pathways consisting of many
catabolic reactions.
Catabolic reactions are also catalysed by enzymes.
These large organic molecules are obtained from the digestion of food, made up of even larger organic
molecules, like starch.
Digestion is also catalysed by a range of enzymes.
Reactions rarely happen in isolation but as part of multi-step pathways.
Metabolism is the sum of all the different reactions and reaction pathways happening in a cell or an
organism, and it can only happen because of the control and order imposed by enzymes.
Just like reactions in a laboratory, the speed at which different cellular reactions proceed varies considerably
and is usually dependent on environmental conditions.
The temperature, pressure and pH may all influence the rate of a chemical reaction.
Enzymes can only increase the rates of reaction up to a certain point called the Vmax
Mechanism of enzyme action
Molecules in a solution move and collide randomly.
For a reaction to happen, molecules need to collide in the right orientation.
When high temperatures and pressures are applied the speed of the molecules will increase, therefore so
will the number of successful collisions and the overall rate of reaction
Many different enzymes are produced by living organisms. as each enzyme catalyses one biochemical
reaction, of which there are thousands in any given cell.
This is termed the specificity of the enzyme.
Energy needs to be supplied for most reactions to start.
This is called the activation energy.
Sometimes, the amount of energy needed is so large it prevents the reaction from happening under normal
conditions.
Enzymes help the molecules collide successfully, and therefore reduce the activation energy required.
There are two hypotheses for how enzymes do this.
Lock and key hypothesis
An area within the tertiary structure of the enzyme has a shape that is complementary to the shape of a
specific substrate molecule.
This area is called the active site.
In the same way that only the right key will fit into a lock, only a specific substrate will fit the active site of an
enzyme.
This is the lock and key hypothesis.
, When the substrate is bound to the active site an enzyme-substrate complex is formed.
The substrate or substrates then react, and the product or products are formed in an enzyme-product
complex.
The product or products are then released, leaving the enzyme unchanged and able to take part in
subsequent reactions.
The substrate is held in such a way by the enzyme that the right atom-groups are close enough to react.
The R-groups within the active site of the enzyme will also interact with the substrate, forming temporary
bonds.
These put strain on the bonds within the substrate, which also helps the reaction along.
Induced-fit hypothesis
Evidence from research into enzyme action suggests the active site of the enzyme changes shapes slightly as
the substrate enters.
This is called the induced-fit hypothesis and is a modified version of the lock and key hypothesis.
The initial interaction between the enzyme and substrate is relatively weak, but these weak interactions
rapidly induce changes in the enzyme's tertiary structure that strengthen binding, putting strain on the
substrate molecule.
This can weaken a particular bond or bonds in the substrate, therefore lowering the activation energy for the
reaction.
Intracellular enzymes
Enzymes have an essential role in both the structure and the function of cells and whole organisms.
The synthesis of polymers from monomers, for example, making polysaccharides from glucose, requires
enzymes.
Enzymes that ac within cells are called intracellular enzymes.
Hydrogen peroxide is a toxic product of many metabolic pathways.
The enzyme catalysed ensures hydrogen peroxide is broken down to oxygen and water quickly, therefore
preventing its accumulation.
It is found in both plant and animal tissues.
Extracellular enzymes
All the reactions happening within cells need substrates to make products needed by the organism.
These raw materials need to be constantly supplied to cells to keep up with the demand.
Nutrients present in the diet or environment of the organism supplies these materials.
Nutrients are often in the form of polymers such as proteins and polysaccharides.
These large molecules cannot enter cells directly through the cell-surface membrane.
They need to be broken down into smaller components first.
Enzymes are released from cells to break down these large nutrient molecules into smaller molecules in the
process of digestion.
These enzymes are called extracellular enzymes.
They work outside the cell that made them.
In some organisms, for example lungi. they work outside the body
Both single-celled and multicellular organisms rely on extracellular enzymes to make use of polymers for
nutrition
Single-celled organisms, such as bacteria and yeast, release enzymes into their immediate environment.
The enzymes break down larger molecules, such as proteins, and the smaller molecules produced, such as
amino acids and glucose, are then absorbed by the cells.
Many multicellular organisms eat food to gain nutrients.
Although the nutrients are taken into the digestive system the large molecules still must be digested so
smaller molecules can be absorbed into the bloodstream.
From there they are transported around the body to be used as substrates in cellular reactions.
Examples of extracellular enzymes involved in digestion in humans are amylase and trypsin.
Digestion of starch
The digestion of starch begins in the mouth and continues in the small intestine.
Starch is digested in two steps, involving two different enzymes.
Different enzymes are needed because each enzyme only catalyses one specific reaction.
1) Starch polymers are partially broken down into maltose, which is a disaccharide. The enzyme involved in this
stage is called amylase. Amylase is produced by the salivary glands and the pancreas. It is released in saliva
into the mouth, and in pancreatic juice into the small intestine.
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