Download Enzymes: Organic Catalysts What are Enzymes?

Enzymes: Organic Catalysts
Biology 12
A. Allen
What are Enzymes?
• Enzymes are proteins.
• Enzymes allow many chemical reactions to occur
within the homeostasis constraints of a living system.
Enzymes function as organic catalysts.
• A catalyst is a chemical involved in, but not changed
by, a chemical reaction.
• Many enzymes function by lowering the activation
energy of reactions. By bringing the reactants closer
together, chemical bonds may be weakened and
reactions will proceed faster than without the catalyst.
The use of enzymes can lower the activation energy
of a reaction (Ea).
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… What are Enzymes?
• Enzymes can act rapidly, as in the case of carbonic
anhydrase (enzymes typically end in the -ase suffix),
which causes the chemicals to react 107 times faster
than without the enzyme present. Carbonic anhydrase
speeds up the transfer of carbon dioxide from cells to
the blood.
• There are over 2000 known enzymes, each of which
is involved with one specific chemical reaction.
• Enzymes are substrate specific. The enzyme
peptidase (which breaks peptide bonds in proteins)
will not work on starch (which is broken down by
human-produced amylase in the mouth).
How Do Enzymes Work?
• The functioning of the enzyme is determined by the shape of the protein
(primary, secondary tertiary & quaternary structure)
• active site-the part of the enzyme onto which the substrate ‘fits’. It recognizes,
confines and orients the substrate in a particular direction.
The induced fit hypothesis:
• As the substrate binds to the enzyme, functional groups from each react which
alters the structure of the enzyme. The altered shape of the enzyme placing some
strain on the substrate molecules Æ substrate is joined/broken.
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• Cofactors are
nonproteins essential
for enzyme activity.
Ions such as K+ and
Ca+2 are cofactors.
• Coenzymes are
nonprotein organic
molecules bound to
enzymes near the
active site. NAD+ is
made from dietary
niacin (vitamin B3)
(nicotinamide adenine
dinucleotide).
Factors that Affect Enzyme Efficacy
Temperature
• Increases in temperature Æ increase in molecular movement
Æ will speed up the rate of nonenzyme mediated reactions,
and so temperature increase speeds up enzyme mediated
reactions, but only to a point.
• When heated too much, enzymes (since they are proteins
dependent on their shape) become denatured. When the
temperature drops, the enzyme regains its shape.
• Thermolabile enzymes, such as those responsible for the
color distribution in Siamese cats and color camouflage of the
Arctic fox, work better (or work at all) at lower temperatures
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…Factors that Affect Enzyme Efficacy
• Concentration of substrate and product also
control the rate of reaction, providing a
biofeedback mechanism.
…Factors that Affect Enzyme Efficacy
• Changes in pH will also denature the enzyme
by changing the shape of the enzyme.
• Remember: the shape of a protein (in this case,
an enzyme determines its function). Enzymes
are also adapted to operate at a specific pH or
pH range.
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Enzyme Inhibition
• There are a number of substances that can inhibit enzyme
activity.
• Competitive Inhibitors are so similar to the enzyme’s substrate
that they are able to enter the enzyme’s active site and block the
normal substrate from binding.
• A drug, sulfanilamide resembles PABA, a chemical essential to
a bacteria that infects animals. sulfanilamide competes with
PABA, shutting down an essential bacterial (but not animal)
pathway.
…Enzyme Inhibition
• Non-competitive Inhibitors - substances that attach to a binding
site on an enzyme other than the active site, causing a change in the
enzyme’s shape and a loss of affinity for its substrate.
• Lead binds to SH groups in this fashion. Irreversible Inhibition occurs
when the chemical either permanently binds to or massively denatures
the enzyme so that the tertiary structure cannot be restored. Nerve gas
permanently blocks pathways involved in nerve message transmission,
resulting in death. Penicillin, the first of the "wonder drug" antibiotics,
permanently blocks the pathways certain bacteria use to assemble their
cell wall components.
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Allosteric Regulation
• Cells must control enzyme activity to coordinate cellular activities.
They may accomplish this in two ways: by restricting the
production of a particular enzyme, or by inhibiting the action of
an enzyme that has already been produced. Some enzymes possess
receptor sites, called allosteric sites, that are some distance away
from the active site.
• Substances that bind to the allosteric sites may inhibit or stimulate
an enzyme’s activity.
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