Cells and Energy - Part 3
Enzymes accelerate the rate of
Normally most of the reactions in metabolic
pathways would occur very slowly if at all without the help of
enzymes. Spontaneous reactions (such as explosions) are not useful to
living things because
- they cannot be controlled
- they release too much energy too quickly
- they are ineffecient wasting much of the
potential energy as heat
How does life cope with energy barriers and
- exergonic reactions with high activation
energy are tamed by enzymes which provide an alternative path with
lower energy of activation.
- Arrow A represents free energy
- Arrow B is the energy of activation of the catalyzed
- Arrow C is the energy of activaton of the uncatalyzed
- endergonic reactions, which by themselves are
impossible at biologically acceptable temperatures can be coupled
to exergonic reactions. Enzymes can take the phosphate from ATP
and use it to "charge" a molecule in an endergonic pathway. This
phosphorylation step is used several times in
Characteristics of Enzymes
- Enzymes are biological catalysts and are
usually globular proteins. Some RNA molecules made by the cell
have catalytic properties and are appropriately called
- Enzymes catalyze reactions by lowering a
thermodynamic barrier called the energy of activation. Activation
energy is the energy that must be absorbed by reactants for their
bonds to break. Enzymes provide a less energetic path to a
transition state in which bonds can be made or broken more
frequently and using less energy at a given temperature. Even an
exergonic reaction, in which delta G is negative, energy must
first be absorbed to reach the transition state. The activation
energy is a barrier essential to life because it prevents the
energy-rich molecules of the cell from decomposing
- Enzymes and their substrates are induced to
fit together. The electric charge and shape of the enzyme's active
site puts stress on one of the substrate's covalent
- Enzymes are not consumed in the reaction they
catalyze. Because they remain unchanged they can be used over and
over again. Eventually the enzyme will lose its function, either
by natural decay or by other enzymes within the cell. This is an
important regulatory function since few metabolic processes used
continuously would simply waste energy.
- Each enzyme may catalyze hundreds of reactions
every second. The speed of the enzyme can vary dramatically.
Lysozyme, which breaks down bacterial cell walls, processes one
substrate molecule every two seconds whereas another enzyme,
carboanhydrase can change over a million substrate molecules
during the same 2 seconds.
- Enzyme activity is usually controlled by
feedback inhibition. This generally involves one of the products
of the reaction reducing or stopping the activity of the enzyme as
its concentration of that product increases.
product D combines with Enzyme1 preventing formation of
For a general discussion of
and feedback systems you should visit
the Fundamentals of Geology site.
- Enzymes catalyze both forward and reverse
reactions equally well. Only by quickly removing products from the
reaction site can a reaction go to completion. For instance if one
of the products is a gas it can escape and by so doing prevent the
HCl + Na2CO3
NaCl + H2O + CO2 (gas)
notice that the reverse reaction is impossible if carbon
dioxide is not supplied.
NaCl + H2O +
HCl + Na2CO3
- Enzymes are highly selective combining with a
specific substrate. Substrates are 3-dimensional objects
(molecule) which are complementary to the enzyme's active site.
The substrate is temporarily bound to its enzyme. The bond between
enzyme and substrate is weak, either hydrogen or ionic in nature.
Enzymes can be poisoned by molecules which are similar to the
normal substrate but which remain in the active site for
unacceptably long periods of time.
Illustration of temporary Enzyme-Substrate bond
verses inhibition with a poison.
- Enzyme activity is sensitive to environmental
changes in pH, temperature, or salt concentrations. (See next
Modified July 10, 2005