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Enzymes
B Y : ME R Y A GU IR RE
Enzymes
• Enzymes are globular proteins that catalyze metabolic
reactions
• Enzymes increase the rate of metabolic reactions
• The part of the enzyme that acts a catalyst is called the Active Site
• The active site is usually a cleft or depression which another molecule(s)
can bind to
• The rest of the enzyme is much larger and is involved in maintaining the
specific shape of the enzyme
• Enzymes are coiled into a three precise dimensional
shape, with hydrophilic R groups
• The hydrophilic R groups on the outside of the molecule ensure that the
enzyme is soluble
• A globular protein is a protein whose molecules are folded into a relatively
spherical shape, which is often water-soluble and metabolically active
Intracellular and Extracellular Enzymes
• Intracellular refers to enzymes that operate within the cell
• These enzymes are attached to the cell membrane or are in the
cytoplasm
• Extracellular refers to enzymes that are secreted by cells
and catalyze reactions outside the cell
• The products of these enzymes may be absorbed into the cell
• Lock and key hypothesis- a model for enzyme action, the
substrate is a complementary shape to the active site of
the enzyme, and it fits exactly into the site
• Induced fit hypothesis- a model for enzyme action, the
substrate is a complementary shape to the active site of
the enzyme, but is not an exact fit
• The enzyme or substrate can change shape slightly to ensure a perfect fit
• Activation energy- energy the substrates need for
changing themselves into products. Heating provides
activation energy
• Enzymes reduce activation energy needed, they do this by distorting the
shape of the substrate when it binds at the enzyme's active site
• The R groups of amino acids at the active site form temporary bonds with
the substrate molecule. This pulls the substrate slightly out of shape,
causing it to react and form products.
Michaelis-Menten constant
• The Michaelis-Menten equation describes the velocity of
enzymatic reactions (v) by relating it to [S] - concentration of a
substrate S
• It permits prediction of whether or not the rate of formation of
product will be affected by the availability of substrate
• Vmax- the theoretical maximum rate of an enzyme- controlled
reaction, obtained when all the active sites are occupied
• The constant Km is the substrate concentration at which the
reaction rate is half of Vmax.
• Km is an inverse measure of the affinity or strength of binding between the
enzyme and its substrate
• The lower the Km, the greater the affinity
Inhibitors
• Inhibitor- a substance that slows down the rate at which an
enzyme works
• Competitive inhibitors- bind the active site of an enzyme,
preventing a real substrate from binding and a product from
being formed
• Have similar shape to the enzyme's normal substrate, and can fit into the
enzyme's active site, preventing the substrate from binding
• Non-competitive inhibitors- can bind to an enzyme with or
without a substrate at different places at the same time
• It changes the conformation of an enzyme as well as its active site, which
makes the substrate unable to bind to the enzyme effectively so that the
efficiency decreases
• Have different shape than the substrate, do not bind to the active site
Immobilizing an Enzyme in Alginate
• The substrate that the enzyme acts upon is able to diffuse
through the gel, although this may be quite slow
• Immobilised enzymes are widely used in industry because it
allows the reaction to flow continuously and the product
will not be contaminated with the enzyme so will not need
to be purified
• Enzymes may be immobilized, or attached to an inert,
insoluble material such as calcium alginate to form a gel
capsule around them, this way the enzymes will be held in
place throughout the reaction, allowing it to be easily
separated from the products and may be used again