Enzymes and Energy 103- Some enzymes are produced as inactive forms that are
later activated within the cell.
a. Activation may be achieved by phosphorylation of
the enzyme, in which case the enzyme can later be
inactivated by dephosphorylation.
b. Phosphorylation of enzymes is catalyzed by an
enzyme called protein kinase.
c. Protein kinase itself may be inactive and require the
binding of a second messenger called cyclic AMP in
order to become activated. - The rate of enzymatic reactions increases when either
the substrate concentration or the enzyme concentration
is increased.
a. If the enzyme concentration remains constant,
the rate of the reaction increases as the substrate
concentration is raised, up to a maximum rate.
b. When the rate of the reaction does not increase upon
further addition of substrate, the enzyme is said to be
saturated.
B. Metabolic pathways involve a number of enzyme-catalyzed
reactions. - A number of enzymes usually cooperate to convert an
initial substrate to a final product by way of several
intermediates. - Metabolic pathways are produced by multienzyme
systems in which the product of one enzyme becomes
the substrate of the next. - If an enzyme is defective due to an abnormal gene,
the intermediates that are formed following the step
catalyzed by the defective enzyme will decrease, and the
intermediates that are formed prior to the defective step
will accumulate.
a. Diseases that result from defective enzymes are
called inborn errors of metabolism.
b. Accumulation of intermediates often results in damage
to the organ in which the defective enzyme is found. - Many metabolic pathways are branched, so that one
intermediate can serve as the substrate for two different
enzymes. - The activity of a particular pathway can be regulated by
end-product inhibition.
a. In end-product inhibition, one of the products of the
pathway inhibits the activity of a key enzyme.
b. This is an example of allosteric inhibition, in which
the product combines with its specific site on the
enzyme, changing the conformation of the active site.
4.3 Bioenergetics 97
A. The flow of energy in the cell is called bioenergetics.
- According to the first law of thermodynamics,
energy can neither be created nor destroyed but only
transformed from one form to another. - According to the second law of thermodynamics, all
energy transformation reactions result in an increase in
entropy (disorder).
a. As a result of the increase in entropy, there is a
decrease in free (usable) energy.
b. Atoms that are organized into large organic
molecules contain more free energy than the more
disorganized, smaller molecules.
3. In order to produce glucose from carbon dioxide and
water, energy must be added.
a. Plants use energy from the sun for this conversion, in
a process called photosynthesis.
b. Reactions that require the input of energy to produce
molecules with more free energy than the reactants
are called endergonic reactions.
4. The combustion of glucose to carbon dioxide and water
releases energy in the form of heat.
a. A reaction that releases energy, thus forming
products that contain less free energy than the
reactants, is called an exergonic reaction.
b. The same total amount of energy is released when
glucose is converted into carbon dioxide and water
within cells, even though this process occurs in many
small steps.
5. The exergonic reactions that convert food molecules
into carbon dioxide and water in cells are coupled
to endergonic reactions that form adenosine
triphosphate (ATP).
a. Some of the chemical-bond energy in glucose is
therefore transferred to the “high energy” bonds
o f AT P.
b. The breakdown of ATP into adenosine diphosphate
(ADP) and inorganic phosphate results in the
liberation of energy.
c. The energy liberated by the breakdown of ATP is
used to power all of the energy-requiring processes
of the cell. ATP is thus the “universal energy carrier”
of the cell.
B. Oxidation-reduction reactions are coupled and usually
involve the transfer of hydrogen atoms.
1. A molecule is said to be oxidized when it loses electrons;
it is said to be reduced when it gains electrons.
2. A reducing agent is thus an electron donor; an oxidizing
agent is an electron acceptor.
3. Although oxygen is the final electron acceptor in the
cell, other molecules can act as oxidizing agents.
4. A single molecule can be an electron acceptor in one
reaction and an electron donor in another.
a. NAD and FAD can become reduced by accepting
electrons from hydrogen atoms removed from other
molecules.
b. NADH 1 H^1 , and FADH 2 , in turn, donate these
electrons to other molecules in other locations within
the cells.
c. Oxygen is the final electron acceptor (oxidizing
agent) in a chain of oxidation-reduction reactions
that provide energy for ATP production.