Proteins
Protein molecules are large and are made from smaller amino acid molecules.
The base sequence of a gene’s DNA controls the amino acid sequence of a protein.
The amino acid sequence of a protein decides the eventual shape of the protein molecule.
The shape of the protein molecule determines its function.
Functions of proteins in living organisms




Enzymes – these are catalysts that speed up chemical reactions but do not get
changed by the reaction
Hormones- these are chemical messengers that carry messages from one part of the
body to another
Antibodies- these are made by white blood cells and they destroy germs
Structural proteins- make up the body’s structures, e.g. collagen in skin and keratin
in hair.
Enzymes
Enzymes are proteins.
They are catalysts (a catalyst is a chemical that speeds up a chemical reaction but is not
changed by it)
Enzymes are made by all living cells.
Enzymes and substrates
The substance an enzyme changes is called its substrate.
Each enzyme has its own substrate, e.g.
Enzyme
amylase
lipase
pepsin
catalase
Substrate
starch
fat
protein
hydrogen peroxide
Shape of the enzyme molecule
The shape of an enzyme molecule is vital to how the enzyme works. Each enzyme molecule
has chains of amino acids folded together into a roughly spherical shape like a tangled ball
of string. One part of the protein molecule, called the active site, has a particular shape that
allows the enzyme to function as a catalyst.
Active site
Enzyme molecule consists of amino acid
chains folded like a tangled ball of string
Enzyme molecule
How enzymes work
Enzymes are specific. Each enzyme has a particular substance, called the substrate of the
enzyme, which it works on. The shape of the substrate is complimentary to (matches) the
shape of the enzyme active site. The substrate fits on to the enzyme like a key fits into a
lock. While the substrate is joined to the enzyme, the reaction takes place.
Substrate
molecule
products
Substrate molecule fits
on to active site
Active
site
Enzyme
molecule
While the substrate is
joined to the active site,
the reaction takes place
making the products
Products separate
from enzyme leaving
it unchanged by the
reaction and free to
accept another
substrate molecule
Factors Affecting Enzyme Activity: Temperature
At very low temperatures, enzyme molecules are inactive but undamaged. The enzyme and
substrate molecules move around slowly and meet only rarely. Therefore, the rate of
enzyme activity is low.
As the temperature increases, the enzyme and substrate molecules move around at a faster
rate, more molecular collisions occur and more enzyme-substrate complexes are formed. So
the rate of reaction increases.
Enzyme activity
The temperature at which the enzyme works at its fastest rate is called the optimum
temperature. The optimum temperature for most human body enzymes is 37oC.
optimum
temperature
temperature
At high temperature, an enzyme is denatured. The shape of the active site is changed so
that the substrate molecule no longer first onto it – this change is permanent and the enzyme
will not work again even if the temperature is lowered.
substrate
Substrate fits
enzyme active site
Substrate no longer
matches enzyme
active site
enzyme
denatured
enzyme
active site
Factors Affecting Enzyme Activity: pH
The pH of a substance indicates whether it is acid, alkaline or neutral.
Enzymes work within a range of pH values. Each enzyme works best at its optimum pH.
Increasing rate of reaction
The shape of an enzyme, or protein, can be changed by changes in the pH of the solution it
is in. Outside its working pH range, an enzyme is denatured.
pepsin
pH
2
Optimum pH
most enzymes
7