AS Bio notes Raida 12D
Enzymes
3.1 What is an enzyme?
> Enzyme: a protein that acts as a biological catalyst in a chemical reaction by reducing
activation energy
An enzyme is a biological catalyst; because all enzymes are proteins
Catalyst because it speeds up a chemical reaction but remains unchanged at the end of
the reaction
Enzymes are globular proteins, they fold up into precise shapes
Almost all metabolic reactions which take place in living organisms are catalysed by
enzymes
Many enzyme names end in -ase; for example, amylase and ATPase
Intracellular and extracellular enzymes:
Not all enzymes work inside cells, those that do are described as intracellular
Enzymes that are secreted by cells and catalyse reactions outside cells are described as
extracellular
Digestive enzymes in the gut are extracellular enzymes
Some organisms secrete enzymes out their bodies, fungi, for example, often do this in
order to digest the food on which they are growing
3.2 Mode of action of enzymes
> Active site: area on an enzyme where the substrate can bind
> Lock-and-key hypothesis: the substrate is a complementary shape to the active site of the
enzyme, and fits exactly into the site; the enzyme shows specificity for the substrate
> Induced-fit hypothesis: the substrate is a complementary shape to the active site of the
enzyme, but not an exact fit - the enzyme, or sometimes the substrate, can change shape
slightly to ensure a perfect fit but is still described as showing specificity
The lock-and-key hypothesis and the induced-fit hypothesis:
Like all globular proteins, enzyme molecules are coiled into a precise three-dimensional
shape
Hydrophilic R groups on the outside of the molecule make them soluble in the water in
the cytoplasm
Enzyme molecules have an Active site
The active site of an enzyme is a region to which another molecule/s can bind
This molecule is the substrate of the enzyme
The shape of the active site allows the substrate to fit perfectly
The idea that the enzyme has a particular shape into which the substrate fits exactly is
known as the Lock-and-key hypothesis
(The substrate is the key whose shape fits the lock of the enzyme)
The substrate is held in place by temporary bonds which form between the substrate
and some of the R groups of the enzyme’s amino acids
This combined structure is the Enzyme-substrate complex
Each enzyme will act on only one type of substrate molecule
, This is because the shape of the active site will only allow one shape of molecule to fit
The enzyme is said to be specific for this substrate (enzyme is showing specificity)
Induced-fit hypothesis is the same as the lock-and-key hypothesis, but adds the idea
that the enzyme, and sometimes the substrate, can change shape slightly as the
substrate molecule enters the enzyme, in order to ensure a perfect fit (this makes the
catalysis even more efficient)
An enzyme may catalyse a reaction in which the substrate molecule is split into two or
more molecules
Alternatively, it may catalyse the joining together of two molecules (ex. Making a
dipeptide from two amino acids)
Enzyme-product complex is briefly formed before the release of the product
When the reaction is complete, the product/s leave the active site
The enzyme is unchanged by this process, so it is now available to receive another
substrate molecule
The rate of the overall reaction can be high
The example of lysozyme:
Enzymes
3.1 What is an enzyme?
> Enzyme: a protein that acts as a biological catalyst in a chemical reaction by reducing
activation energy
An enzyme is a biological catalyst; because all enzymes are proteins
Catalyst because it speeds up a chemical reaction but remains unchanged at the end of
the reaction
Enzymes are globular proteins, they fold up into precise shapes
Almost all metabolic reactions which take place in living organisms are catalysed by
enzymes
Many enzyme names end in -ase; for example, amylase and ATPase
Intracellular and extracellular enzymes:
Not all enzymes work inside cells, those that do are described as intracellular
Enzymes that are secreted by cells and catalyse reactions outside cells are described as
extracellular
Digestive enzymes in the gut are extracellular enzymes
Some organisms secrete enzymes out their bodies, fungi, for example, often do this in
order to digest the food on which they are growing
3.2 Mode of action of enzymes
> Active site: area on an enzyme where the substrate can bind
> Lock-and-key hypothesis: the substrate is a complementary shape to the active site of the
enzyme, and fits exactly into the site; the enzyme shows specificity for the substrate
> Induced-fit hypothesis: the substrate is a complementary shape to the active site of the
enzyme, but not an exact fit - the enzyme, or sometimes the substrate, can change shape
slightly to ensure a perfect fit but is still described as showing specificity
The lock-and-key hypothesis and the induced-fit hypothesis:
Like all globular proteins, enzyme molecules are coiled into a precise three-dimensional
shape
Hydrophilic R groups on the outside of the molecule make them soluble in the water in
the cytoplasm
Enzyme molecules have an Active site
The active site of an enzyme is a region to which another molecule/s can bind
This molecule is the substrate of the enzyme
The shape of the active site allows the substrate to fit perfectly
The idea that the enzyme has a particular shape into which the substrate fits exactly is
known as the Lock-and-key hypothesis
(The substrate is the key whose shape fits the lock of the enzyme)
The substrate is held in place by temporary bonds which form between the substrate
and some of the R groups of the enzyme’s amino acids
This combined structure is the Enzyme-substrate complex
Each enzyme will act on only one type of substrate molecule
, This is because the shape of the active site will only allow one shape of molecule to fit
The enzyme is said to be specific for this substrate (enzyme is showing specificity)
Induced-fit hypothesis is the same as the lock-and-key hypothesis, but adds the idea
that the enzyme, and sometimes the substrate, can change shape slightly as the
substrate molecule enters the enzyme, in order to ensure a perfect fit (this makes the
catalysis even more efficient)
An enzyme may catalyse a reaction in which the substrate molecule is split into two or
more molecules
Alternatively, it may catalyse the joining together of two molecules (ex. Making a
dipeptide from two amino acids)
Enzyme-product complex is briefly formed before the release of the product
When the reaction is complete, the product/s leave the active site
The enzyme is unchanged by this process, so it is now available to receive another
substrate molecule
The rate of the overall reaction can be high
The example of lysozyme:
