BC BIOLOGY 12
Enzymes
Metabolism: is the total of all the
chemical reactions in an organism.
• All of life’s metabolic reactions require
enzymes.
• Each of these metabolic reactions typically
occur in a series of chemical reactions
called Metabolic Pathways.
Metabolic pathways can be classified into
two groups:
1. Catabolic Pathways: involves the
breakdown or degradation of large
biological molecules usually with the
release of energy.
• Cellular respiration degrades glucose to
CO2 and H2O and releases energy.
2. Anabolic Pathways:
• involves the construction of biological
molecules using energy.
• Photosynthesis: uses CO2 and H2O
with light energy to make glucose.
Chemical Reactions between Plant and
Animal cells
Enzymes
• Enzymes allow metabolic reactions to occur
without investing a lot of energy.
• Enzymes lower the activation energy for
chemical reactions to take place.
• Enzymes help build & break bonds by
chemically bonding to the reactants to
form a transition “enzyme - substrate
complex” which is highly unstable and
therefore reactive.
• Enzymes act as catalysts which allow
lowering the Eact of metabolic
reactions.
Enzymes Structure
• They do this because enzymes have a highly
specific 3D shape that includes a place on
the enzyme which is exactly complementary
in shape to the shape of the substrate.
• This is called the “Active Site” and it is
much like a lock and key.
Enzymes Structure
• Enzymes are highly specific.
• One type of enzyme can catalyze only one type of
reaction.
• This is due to the highly specific shape of the
active site that will “fit” only one specifically
shaped substrate. Because the substrate
attachment is temporary only, enzymes can be reused thousands of time therefore they work in
very low concentrations in cells.
• This saves cells lots of energy they would have had
to spend building many enzyme copies.
Enzymes Structure
•
Enzymes are highly specific.
•
One type of enzyme can catalyze only one type of reaction due to
the highly specific shape of the active site that will “fit” only one
specifically shaped substrate.
•
Since the substrate attachment is temporary only, enzymes can be
re-used over and over again,saving cells a lot of energy of
remanufacturing of enzymes.
•
Many enzymes are named after the substrate.
 sucrose is broken down by sucrase
 lipids / lipase
 amylose (starch ) amylase
 fructose / fructase
 peptides / peptidase
 urea / urease
 lactose / lactase
 RNA / ribonuclease
DEGRADATIVE REACTION
Degradation or hydrolysis is when a substrate is broken
down into products, with the help of an enzyme and
water.
Catabolic Reaction
DEGRADATIVE REACTION
Below is an example of a HYDROLYTIC REACTION where water is added to
ENZYME SUBSTRATE COMPLEX to break the bond between sucrose
(dissacharide) to produce two (monosccharides) fructose and glucose.
Catabolic Reaction
SYNTHETIC REACTION
Synthesis is when two reactants are bonded together
to form a product.
Anabolic Reaction
Enzymes Structure
• Some enzymes are not constructed of
pure protein but include another piece.
1. Cofactors: these are non organic
atoms or molecules which complete the
shape and therefore reactivity of an
enzyme.
• ex. zinc, copper,
and other ions
Enzymes Structure
2. Coenzymes: these are organic
additions to the protein portion (called
the apoenzyme) that completes the
enzyme.
• Coenzymes are vital to life as they bring
numerous enzymes to full functional
ability. Vitamins act
as co-enzymes.
Enzyme Reaction Rates
• Numerous factors control the rate at which
enzyme catalyzed reactions occur.
1. Substrate concentration: reaction rate
increase as the concentration of the reactant
increase up to a point where the rate levels
off.
• This is because every active site of every
enzyme is occupied with a reactant molecule.
• The enzymes are working at full capacity.
Effect of Substrate Concentration
Enzyme Reaction Rates
2. Enzyme concentration: Reaction rate
will increase as enzyme concentration
increases until the reactants run out,
then the rate slows and finally stops.
3. Temperature: enzymes have an optimum
operating temperature near body
temperature.
• Too hot or too cold can either
temporarily or permanently denature
enzymes (proteins).
Effect of Temperature
Enzyme Reaction Rates
4. pH: Enzymes have an optimum pH
range which is usually quite narrow and
centers around pH 7 (are exceptions).
• Reaction rate slows as the pH changes
in either direction.
Effect of pH
Enzyme Reaction Rates
• 5. Competitive Inhibitors: these are
molecules that enter a cell which are 3D
shaped very near to an enzymes substrate.
These mimic the substrate and attach to the
enzymes active site but do nothing but
occupy it.
• The site is now unavailable to the real
substrate so the reaction slows.
Competitive Inhibitor
Enzyme Reaction Rates
6. Noncompetitive Inhibitors:
• These are molecules that attach to an enzyme at
some place other than the active site (Allosteric
site)
• By so doing, they distort the 3D shape of the
active site such that it can’t attach its
substrate.
• Ex. Heavy metals, antibiotics, poisons, pesticides
and herbicides.
Noncompetitive Inhibitor
Allosteric site
NEGATIVE FEEDBACK or FEEDBACK INHIBITION
ATP
• When a terminal phosphate breaks off, free energy is
released and available for cellular work (a catabolic
reaction).
• The product is now called ADP.
• ATP can be rebuilt by using chemical energy released
by the catabolism of glucose to rebuild the terminal
phosphate energy rich bond (anabolism).