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.
E A with enzyme Figure 5.5B Reactants Products E A without enzyme Net change in energy
The activation energy, E A
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
The active site
Is the region on the enzyme where the substrate binds
Figure 8.16 Substate Active site Enzyme (a)
Are nonprotein enzyme helpers
Are organic cofactors
Bind to the active site of an enzyme, competing with the substrate
(b) Competitive inhibition A competitive inhibitor mimics the substrate, competing for the active site. Competitive inhibitor A substrate can bind normally to the active site of an enzyme. Substrate Active site Enzyme (a) Normal binding
Bind to another part of an enzyme, changing the function
A noncompetitive inhibitor binds to the enzyme away from the active site, altering the conformation of the enzyme so that its active site no longer functions. Noncompetitive inhibitor (c) Noncompetitive inhibition
Competitive and noncompetitive inhibition
Types of enzymes
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.
Some enzymes are produced by cell but then transported out of the cell for action outside the cell
How an enzyme works
Enzyme (sucrase) Active site 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
The enzyme is unchanged and can repeat the process
Enzymes generally act quickly
the speed of reaction is usually stated in ‘turnover number’ (refer to the number of substrates)
Enzymes are not damaged
however, this does not mean that enzymes can be used repeatedly forever without replacement
Enzymes can react in both direction
Enzymes are specific
each enzymes limited to one specific reaction that involved one specific substrate only
An enzymes molecule is usually bigger than its substrates
All enzymes are proteins and not all protein are enzymes
Enzymes are complex globular protein and three dimensional.
Main function of enzymes
Increase the rate of chemical reaction by lowering activation energy
Mechanisms of Enzymes
There are 2 main hypothesis explaining the mechanism of enzyme action :
The lock-and-key hypothesis
The induced-fit 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
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.
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 ±60 o C
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
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
Enzymes can be classified by the kind of chemical reaction catalyzed:
This classification system introduced in 1961 by IUB (International Union of Biochemistry)