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ENZYME
Characteristic Mechanism Factors
Definition
Enzymes are protein produced by the cells of living organism act as biological catalyst by controlling and accelerating the rate of biochemical reactions in the cell at fairly low temperatures.
Without enzymes, biochemical reactions become too slow to maintain the life supporting process in an organism.
EA
with enzyme
Figure 5.5B
Reactants
Products
EA
without
enzyme
Net change in energy
Important Terms…
The activation energy, EA
Is the initial amount of energy needed to start a chemical reaction
Is often supplied in the form of heat from the surroundings in a system
Cont…
The active site Is the region on the enzyme where the
substrate binds
Figure 8.16
Substate
Active site
Enzyme
(a)
Cont…
Cofactors Are nonprotein enzyme helpers
Coenzymes Are organic cofactors
Enzyme Inhibitors Competitive inhibitors
Bind to the active site of an enzyme, competing with the substrate
(b) Competitive inhibition
A competitiveinhibitor mimics the
substrate, competingfor the active site.
Competitiveinhibitor
A substrate canbind normally to the
active site of anenzyme.
Substrate
Active site
Enzyme
(a) Normal binding
Cont…
Noncompetitive inhibitors Bind to another part of an enzyme,
changing the function
A noncompetitiveinhibitor binds to the
enzyme away fromthe active site, altering
the conformation ofthe enzyme so that its
active site no longerfunctions.
Noncompetitive inhibitor
(c) Noncompetitive inhibition
Competitive and noncompetitive inhibition
Types of enzymes
1. Intracellular enzyme Enzymes that are produces in certain cells
and remain to react in the cell These enzymes could exist either in
cytoplasm (in organelles) or nucleus.
2. Extracellular enzyme Some enzymes are produced by cell but then
transported out of the cell for action outside the cell
Enzyme(sucrase)
Activesite
1
2
3
Substrate(sucrose)
Enzyme available with empty active site
Substrate binds to
enzyme with induced fit
Substrate is converted to
products
4
Products are released
Glucose Fructose
How an enzyme works
The enzyme is unchanged and can repeat the process
Figure 5.6
Characteristics
1. Enzymes generally act quickly the speed of reaction is usually stated in ‘turnover
number’ (refer to the number of substrates)
2. Enzymes are not damaged however, this does not mean that enzymes can be
used repeatedly forever without replacement
3. Enzymes can react in both direction
4. Enzymes are specific each enzymes limited to one specific reaction that
involved one specific substrate only
Cont…5. An enzymes molecule is usually bigger
than its substrates6. All enzymes are proteins and not all
protein are enzymes7. Enzymes are complex globular protein
and three dimensional.
Main function of enzymes Increase the rate of chemical reaction by
lowering activation energy
Mechanisms of EnzymesThere are 2 main hypothesis explaining the mechanism of enzyme action : The lock-and-key hypothesis The induced-fit hypothesis
Simple mechanism
Lock-and-key hypothesis
The hypothesis proposed that the active site and substrate are exactly complementary
An enzyme is a large globular protein with specific three dimensional shape.
It has a site called the active site. In the lock-and-key hypothesis, the shape of the substrate (‘key’) fits
into the active site of the enzyme (‘lock’) forming and enzyme-substrate complex
Reaction takes place and products are formed and released
Induced-fit hypothesis
It is a modified version of the lock-and-key hypothesis
The hypothesis suggested that active site is flexible and is not exactly complementary to the shape of the substrate
An enzyme collides with the substrate molecule. The substrate binds to the active site
The binding induces a slight change in the shape of the enzyme to enclose the substrate making the fit more precise.
factors
Any factors that affect the activity of an enzyme will change the rate of the reaction catalysed by that enzyme
Enzyme characteristic are affected by several factors such as temperature, pH, substrate and enzyme concentration.
The effect of Temperature At lower temperature, enzymes
are not active As the temperature rises, the
substrate and the enzyme molecules move rapidly and are more likely to collide
The increase in temperature to a specific level can increase the rate of the enzyme activity until the optimum temperature rate.
After the optimum temperature the enzymes activities decreased and totally stopped at temperature of ±60oC
The effect of pH
All enzymes have a specific optimum pH at which they function most efficiently
Most enzyme act at a pH in range of 5 – 9, and reactions most efficiently at a pH of 7
The small change in pH can produce a large effect on enzyme activity
A pH that is extreme usually destroy enzymes
Cont…
However, there are exceptions for certain enzymes such as:
Pepsin pH 1.5 – 2.5 (acidic) Rennin pH 8.5 (alkaline)
The effect of Enzyme Concentration
The rate of an enzyme-catalysed reaction is directly proportional to the concentration of the enzymes if substrates are present in excess concentration and no other factors are limiting
The effect of Substrate Concentration
Increasing the substrate concentration can give a increasing in reaction rate
At lower concentrations the rate increase in direct proportion to the substrate concentration
At higher substance
concentration the rate of
reaction becomes
constant
classification
Enzymes can be classified by the kind of chemical reaction catalyzed:
1. Hydrolases
2. Lyases
3. Isomerase
4. Ligases
5. Transferases
6. Oxidoreductase
This classification system introduced in 1961 by IUB (International Union of Biochemistry)