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Topic 3.6: Enzymes
Why Should We Study Enzymes?All metabolic reactions such as
photosynthesis, digestion and respiration are catalysed by enzymes, allow them proceed faster at body temperature.
Understanding the nature of enzyme functions will enable us to understand why metabolic reactions such as photosynthesis and respiration are affected by factors such as pH and temperature
Use of enzymes in biotechnology for: food production e.g. lactose-free milk; manufacture of antibiotic penicillin and in biological washing powders.
Lesson’s objectives:(i) Define enzyme and active site;
(ii) Explain enzyme–substrate specificity;
(iii) Explain the effects of: temperature; pH & substrate concentration on enzyme activity
(iv) Define denaturation.
What is an enzyme? globular protein which functions as a biological catalyst, speeding up reaction rate by lowering activation energy without being affected by the reaction it catalyse
Active site
What is an Active Site? a position on the surface of an enzyme molecule to which substrate(s) bind.
Active Site
Mechanism of Enzyme Action
e.g. H2O2
e.g. O2 + H2O
Progress of Reaction
In an enzymatic reaction, the substrate binds to the active site forming an enzyme-substrate complex
The active site can lower activation energy (EA) barrier by:
(i) Orienting substrates correctly(ii) Straining substrate bonds thus weakening
them(iii)Providing a favorable microenvironment for
reaction to occur(iv)Allowing change in molecular shape of the
substrate to occur
Using the enzyme structure, explain the enzyme-substrate specificity.each globular enzyme includes an active site with a
specific, three-dimensional shape which is complementary to the shape of the substrate
the globular enzyme active site also includes a specific set of charges which are complementary to the charges of the substrate
through complementarity of shape and charge, the substrate is attracted to, and fits precisely into, the active site - this is known as the lock and key model
causes weakening of bonds in substrate to lower activation energy.
the enzyme’s active site is not rigid and substrate can induce slight changes in shape
State the factors that affects enzyme activity. Temperature;pH;Substrate Concentration;Inhibitors
Effect of Temperature on Enzyme Activity.
Temperature/o C
Enzyme Activity/%
X
a c
b
d
(a) Enzyme activity increase with increase in temperature, temperature increases the kinetic energy of the substrate and enzyme molecules causing more collisions of the substrate with the active site and the formation of activated complex's and product(s).
(b) The optimal temperature (X) is the temperature at which highest enzyme activity, the rate of reaction is at it best. There is a compromise between decreasing enzyme stability and kinetic energy of the reactants.
(c) Temperature above optimum temperature increases the kinetic energy of the enzyme molecules so much that they break bonds, change shape of the active site, enzyme become denatured.
(d) All enzymes are denatured and enzyme activity stops.
What is Denaturation?is a structural change in a protein/enzyme
that results in the loss (usually permanent) of its biological properties. Once denatured, enzymes are no longer capable of catalyzing biochemical reactions.
Denaturation is caused by excess heat and change in pH from optimum pH
Effect of pH on Enzyme Activity.
Enzyme Activity/%
pH
Pepsin
Trypsin
q r
pH also affects the rate of reaction of an enzyme catalyzed reaction.
Each enzyme has an optimal pH at which maximum rate of reaction is achieved.
At pH above or below the optimal pH the enzymatic activity decreases because bonds are made and broken which change the shape of the active site and therefore decrease the rate of reaction.
The two enzyme shown in the sketch graph illustrate the fact that different enzymes can have very different optimal pH e.g. Pepsin has an optimum pH of 1.8 while trypsin has an optimum pH of 7.8
Effect of Substrate Concentration on Enzyme Activity.
Substrate Concentration/arbitrary
Units
Enzyme activity/%
a
b
as substrate concentration increases enzyme activity increases because there are more collisions between the substrate and the enzyme molecules so that more activated complex's are formed and therefore product per unit time
at high substrate concentration, enzyme reaches maximum activity and enzyme activity remain constant with increase in substrate concentration
All active sites are occupied by substrate molecules at any one time such that adding more substrate molecules does not increase the rate of reaction.
Review Questions:
Q1 An enzyme __________________A is a source of energy for biochemical reactions
B is an organic catalystC increases the EA of a reaction
D is a inorganic catalyst
Q2 What name is given to the reactants in an enzymatically catalyzed reaction?
A productsB active sites C reactors D substrate
Q3 As a result of its involvement in a reaction, an enzyme _____.
A permanently alters its shapeB loses energyC is used upD is unchanged
Q4 The active site of an enzyme is the region that
A binds allosteric regulators of the enzyme.
B is involved in the catalytic reaction of the enzyme.
C binds the products of the catalytic reaction.
D is inhibited by the presence of a coenzyme or a cofactor.
Q5 Which curve represents the behavior of anenzyme taken from a bacterium that lives in hotsprings at temperatures of 70°C or higher?
A curve 1 B curve 2 C curve 3 D curve 4 E curve 5
Q6 Which curve was most likely generated from analysis of an enzyme from a human stomach where conditions are strongly acid?
A curve 1 B curve 2 C curve 3 D curve 4 E curve 5
Home Work: Read your textbook pp 66-69 plus the
lesson notes and answer the following questions:
(a) Outline how enzymes catalyze biochemical reactions. [3]
(b)Outline enzyme-substrate specificity. [5]
(c) Explain the effects of temperature, pH and substrate concentration on enzyme activity. [8]
Any Questions?Will this be in exam?!
Go and put digestive enzymes to work with some . . .
