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Biology
Enzymes
Enzymes
B11 – analyse the roles of enzymes in biochemical reactions
Textbook Chapter ___ pages _____.
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All organisms depend on their metabolism for survival.
Metabolism: particular biochemical reactions and pathways that occur in their
cells.
Enzymes catalyze (speed up) metabolism and increase the efficiency of these
reactions.
e.g. Thyroxin is a hormone that partially governs overall rate of metabolism; it
is a protein hormone released by thyroid cells into the blood.
Thyroxin attaches to receptor sites on surfaces of body cells where it increases
rate cells consume oxygen. This promotes ATP production by cellular
respiration making it available to increase rate of cell metabolism.
Re: thyroid gland accumulates iodine by ___ACTIVE TRANSPORT___ in order
to produce thyroxin.
Enzymatic activity of all of metabolism is dependent on efficient formation of a
complex between enzymes and their substrates.
Shape of molecules involved permits their combination into the complexes.
Anything that affects shape of molecules (___DENATURES___ molecules) will
affect potential for a reaction. (re: temperature, pH, ions, etc.)
Enzymatic Activity
Re: Enzymes are ___PROTEINS_______ with __3o or __4o___ structure.
these 3D structures are a characteristic of the unique sequence of amino acids
Substrates: molecules that react with enzymes
e.g. maltose is the substrate for the enzyme maltase. Unique structure of
maltase allows it to bind with maltose (substrate) and form a complex that
proceeds in the reaction forming two glucose molecules.
Biology
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Enzymes
e.g. Sucrose (___SUBSTRATE_) reacts with sucrase (___ENZYME_____) to
produce glucose and fructose.
IN BOOK
Catabolism: enzymatic reactions that involve the cleaving of a single substrate into two
products. The above hydrolysis reactions of maltose and sucrose are good examples.
(note: cata = down e.g. catastrophe)
Anabolism: enzymatic reactions where substrates are bonded together to from a
single product. e.g. maltase also catalyzes this reverse reaction between glucose
molecules to form maltose.
(note: ana = up, back, again e.g. anabolic)
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Many enzymatic reactions are reversible. However, conditions surrounding
reaction, whether environmental or relative concentrations of substrates
compared to products, determine which way a reaction will proceed. This is an
example of homeostasis.
Lock and Key Analogy clarifies the relationship bwtween substrates and enzymes.
According to this analogy, substrates and enzymes fit together like keys in locks. This is
a useful analogy because each component in reaction must have correct shape and fit
together for a reaction to occur.
Activated enzyme-substrate (E-S) complex: Combination of the substrate and enzyme
(“key” and “lock”).
Induced fit hypothesis: as E-S complex forms, it forces substrate into a slightly
different shape, bending the molecule, perhaps, which leads to reaction.
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Reaction proceeds when required energy exists.
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after reaction, enzymes are available to be used again.
Active site: portion or surface of enzyme that attaches to substrate and is involved in
reaction. Shape of active site must fit shape of substrate for E-S complex to form. If an
activated E-S complex can’t form, then no reaction can occur.
Summary
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Enzymes
Lock and Key analogy can be extended. Wrong key can sometimes be fit into a lock,
yet does not do anything. In “enzyme language”, it is possible for a substance other
than the substrate to fit into an active sit, but because configuration of this
combination is not exact, no reaction will proceed and reaction that could have
occurred would be blocked (inhibited).
Adenosine triphosphate (ATP) provides the energy for these reactions.
Activation energy: Amount of energy (ATP) required for a reaction to occur.
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Enzymes lower the activation energy – allows for reaction to occur with a
lowered energy requirement.
From book
Both substrates and products of a reaction exist at own energy level.
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Reaction is transition between these two energy levels.
Endothermic reaction: overall reaction requires net input of energy.
(note: endo = inside e.g. endocytosis)
Exothermic reaction: release of energy. Energy can be captured in form of ATP for
further use, or is given off as heat energy.
(note: exo = outside e.g. exterior)
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Regardless of type of reaction the energy of enzyme-controlled reactions occur at
such small amounts that other cell functions are not affected.
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Summary
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Enzymes
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Summary
Many enzymatic reactions exist as part of larger metabolic pathway, where one
product of one step becomes substrate for a subsequent reaction and so on.
Some metabolic pathways have several of these steps, each requiring a different
specific enzyme.
Sequential nature of a metabolic pathway means that energy and electron shifts
occur in smaller amounts.
Also, they allow cell to have greater regulatory control over reactions by
negative feedback mechanism. This occurs when concentration of a product of
one step increases high enough to begin to inhibit initial dedicated step that leads
to its production.
e.g.
To present enzymatic activity as purely a protein function is an
oversimplification. The physical structure that speeds up a reaction is a protein,
but this molecule doesn’t function in isolation.
e.g. The hydrolysis of maltose involves breaking of bond holding two
monosaccharides together, the addition of water and the formation of new
bonds attaching parts of a water molecule to each of the monosaccharide
products. Without maltase present, no such hydrolysis occurs.
As well, there is an energy consideration that is not reflected in the net equation.
Furthermore, there are other aspects like the impact in electron distribution in
maltose as the bond is being broken and again in the glucose molecules while
new bonds are being formed.
Regulating these additional features of an enzymatic reaction are roles of coenzymes and co-factors. Enzymatic reactions that require addition or release of
small particles like H+ use organic complexes called co-enzymes.
e.g. co-enzyme nicotinamide adenine dinucleotide (NAD+) which is derived
from vitamin B (niacin). NAD+ (and other molecules that function similarly)
transports hydrogen to or from a reaction. They are known simply as hydrogen
carriers (and sometimes are called reducing agents).
e.g. K1+ and Zn2+ help regulate other aspects of enzymatic reactions, such as
adding stability while electron distribution shifts.