Examples of biological
molecules mentioned in conjunction with enzymes
Carbohydrates like sugars and starch are a combination
of the elements carbon, hydrogen and oxygen (C, H and
Lipids (vegetable oils/animal fats) are mainly a
combination of the elements carbon, hydrogen and oxygen (C, H
and O), though sometimes phosphorus (P) too.
Proteins are mainly a combination of the elements
carbon, hydrogen, nitrogen and oxygen (C, H, N and O),
sometimes sulfur (S) too, enzyme proteins might contain metal
ions like Zinc Zn2+ which form part of the active
Enzymes are involved in all the reactions to synthesise
these larger molecules or break them down into smaller
Metabolism is the scientific term used for all the
chemical reactions that go on inside an organism's body - mainly
(i) synthesise molecules of all shapes
and sizes for specific uses in an organism,
(ii) break larger molecules down into smaller
ones (e.g. in digestion),
(iii) supply the energy needs for the chemistry
of every cell from respiration in the mitochondria.
Most metabolic reactions are controlled by specific enzymes.
What are enzymes?
and why are they so important in living systems?
Enzymes are complex protein molecules made of chains of
linked amino acids that catalyse most chemical
reactions that go on in cells.
Each of these biological catalysts has a
unique complex 3D protein structure, particularly the
shape of the active site, into which the specific substrate molecule's
shape 'fits in' to give the enzyme substrate complex, and be
(key and lock mechanism).
In forming this
enzyme-substrate complex, the enzymes provide a chemical
reaction pathway of lower activation energy, so, for any given temperature, a
greater proportion of reactant molecules have sufficient kinetic
energy to change when they collide with the active site on the
The activation energy for an enzyme
catalysed reaction (top of the green
hump shown on the diagram above) is the minimum kinetic energy the
reactant molecules must have to break bonds and undergo a chemical
The black higher activation energy curve
represents the non-catalysed reaction pathway.
For more details see
GCSE chemistry - rates of
reaction - effect of a catalyst
Because of their unique molecular structure, each enzyme catalyses
a specific reaction (rarely catalyse more than one reaction).
Most enzyme catalysed reactions involve:
Breaking large molecules
down into smaller ones.
Building small molecules
into large molecules.
Overall, always changing one molecule into
(see later in the details of 'key and lock'
Reminder - catalysts are substances that increase
the speed of reactions by lowering the activation energy needed, BUT, they
are not chemically changed overall or been used up after the reaction has taken place.
Most biochemical reactions have and
require, a specific enzyme to catalyse
This is referred to as the specificity of an enzyme.
Enzymes are true biological catalysts, speeding
up reactions without being used up in the chemical processes they
Every protein an organism requires, including enzymes,
for by a different gene in the DNA genome.
This specific gene not only
determines the sequence of the amino acids in an enzyme protein, but also
its unique shape, and it is the shape that largely determines what an
enzyme can do.
The enzyme shape is created by the folding and
coiling of chains of amino acids joined together in the protein
The 3D shape of the enzyme determines the
unique 3D shape of the active site.
Only one type of molecule can fit into
the active site - if denatured, it becomes inactive.
The rate of chemical reactions are increased by
increase in temperature, but higher temperatures may harm the structure and
function of complex biological molecules like the enzyme protein
Therefore the catalytic power of
enzymes speeds up the thousands of different chemical reactions which enable
most organisms to live specifically at relatively low
temperatures - in fact, to keep them alive at any temperature!
Extremophiles e.g. like bacteria growing near hot
volcanic undersea hydrothermal vents still have enzymes but their
activity is much less than in organisms at the typical lower
temperatures of the Earth's surface.
Without enzymes there would be no life - no photosynthesis in
plants, no protein synthesis and respiration in plants and animals, so
without these processes there would be no life!
e.g. mitochondria contain all the enzymes needed
for the chemical reactions involved in respiration - the source of
energy to power cells,
and plant cells have all the enzymes needed for
photosynthesis - chlorophyll (not an enzyme) is just one molecule in the many required
for the process.
Moreover, all the chemical processes of life must well
controlled to keep things in balance so any organism can function properly.
This involves enzymes and hormones for example e.g. the right levels of
sugar in the blood (hormone control), what the organism can do with the
sugars (enzyme controlled).
Hormones may control the appropriate concentrations of substrates and
products as well as temperature - all important variables that need
BUT, if the rates of so many chemical reactions are not controlled
in harmony with each other, then cells may be damaged beyond repair hence
endangering the whole organism,
AND many of the reactions involved are
facilitated by enzymes.
All of the descriptions of enzyme action on this
page are greatly simplified, and most biochemical reactions are multi-stage
initial substrate reactant 1 == enzyme 1
==> product 1 == enzyme 2 ==> product 2 etc. == last enzyme ==>
so, please remember this when studying my
'pretty' simplified diagrams!
Be able to explain what an enzyme is and they do as
Be able to draw and understand a diagram showing the profile
for an uncatalysed reaction, and catalysed reaction, and showing the 'hump'
of the activation energy and its significance.
Using a diagram for an enzyme reaction explain the
importance of lowering the activation energy for the biochemistry of living