enzymes Flashcards
Enzymes definition
Enzymes are biological catalysts. They speed up the rate of chemical reactions without themselves being chemically changed at the end of the chemical reactions. Hence, they can be reused and are effective in small concentrations.
Catabolic reactions
Consist of chemical reactions which break up complex molecules into simpler molecules.
Anabolic reactions
Consist of chemical reactions which build up simpler molecules into complex molecules
Structure of enzymes
Most enzymes are proteins with unique 3D structure.
Each enzyme has an active site, where substrate bind.
- substrate is the molecule which an enzyme acts on
- each active site can only allow specific substrate to fit in
- the specificity of enzyme is attributed to the complementary shape between the substrate and the active site
Chemical reaction of enzymes
Changing one molecule into another molecule requires deforming the reactant molecules before the reaction can proceed
Activation energy
This can be achieved when the reactant molecules absorb energy from their surroundings.
The energy that must be supplied to the reactant molecules for them to react is known as activation energy.
- activation energy is often supplied in the form of heat
- this absorption of thermal energy increases the speed of the reactant molecules, so that they collide more frequently and forcefully in the correct orientation for chemical reaction to occur
- thermal agitation of the atoms within the molecules makes also the bonds more likely to break
Why is heating inappropriate?
Heating speeds up a chemical reaction, but this is inappropriate for a biological system. As high temperatures kill cells and denature proteins. Heating will also speed up all chemical reactions, not just the intended ones, therefore enzymes are used.
Enzymes relation with activation energy
Enzymes lower the activation energy required to start a chemical reaction
Enzymatic reactions
- Effective collision between specific substrate and enzyme at the correct orientation causes the substrate molecules enter the active site of the enzyme
- substrate molecules bind to the active site of the enzyme and forms the enzyme-substrate complex
- the formation of enzyme-substrate complex lowers the activation energy
- chemical reaction occurs and products are formed
- the enzyme-substrate complex dissociates to release the products and the chemically unchanged enzyme is ready for another cycle of chemical reaction
Lock and key hypothesis
- The substrate is a “key” and the enzyme is a “lock”
- the substrate whose shape is complementary to the shape of the active site of the enzyme
- the active site of enzyme has a specific shape in which the substrate fits exactly
- the substrate binds to the active site of the enzyme, forming an enzyme-substrate complex
- once the products are formed, they no longer fit into the active site of the enzyme and are released into the surrounding medium
Induced fit model
- The active site of enzymes is complementary in shape but not a perfect fit to the substrate it catalyses
- however, when the substrate bind to the active site of the enzyme, it induces a change in the shape of the active site.
- this allows the substrate to fit more tightly into the active site
- binding of the substrate to the active site of the enzymes occurs and the enzyme-substrate complex is formed
- chemical reaction occurs, forming products and they are released into the surrounding medium as they no longer fit into the active site of the enzyme
Characteristics of enzymes
- Enzymes speed up chemical reactions by lowering the activation energy. Enzymes are extremely efficient, proceeding 10^3 to 10^8 times faster than uncatalysed reactions.
- enzymes remain chemically unchanged after the reactions, they can be used over and over again. Hence, enzymes are required in small concentrations.
- enzymes are specific in action due to its 3D shape
Effects of varying temperature
Enzymes have an optimum temperature, which is the temperature at which the rate of enzyme activity is at its maximum. Different enzymes have different optimum temperatures with most ranging from 37 - 45 degree Celsius.
Effects of varying temperature (start of curve)
- Rate of enzyme activity is low at low temperatures
- enzymes are less active at low temperatures
- reversible condition- enzymes increase in activity when temperature increases
Effects of varying temperature (optimum temperature)
- As temperature increases, kinetic energy of substrate and enzyme molecules increases.
- enzyme start to be more active.
- substrate and enzyme molecules collide more often. This increases the number of effective collisions and formation of enzyme-substrate complexes and thus the rate of reaction increases.
- the reaction rate doubles for every 10 degree Celsius rise in temperature until optimum temperature is reached
- the rate of reaction is at its maximum at optimum temperature
Effects of varying temperature (end of curve)
- As temperature increases beyond optimum temperature, rate of reaction starts to decrease
- enzyme is denatured. The enzyme loses its 3D shape and active site is unable to bind to the substrate. Once an enzyme is denatured, it is irreversible, and it cannot regain its function even when temperature is lowered
- as temperature continues to increase, more enzyme molecules become denatured, which causes the rate of reaction to decrease further
Effects of varying pH
- Enzymes have an optimum pH, which is the pH at which enzyme activity is at its maximum. Different enzymes have different optimum pH.
- any PH that deviates from the optimum pH will cause the rate of reaction to decease
- At extreme pH, enzyme loses its 3D shape and active site is unable to bind to the substrate.
- once an enzyme is denatured, it is irreversible, and it cannot regain its function even when pH goes back to optimum
Limiting factor
A factor that directly affects the rate of chemical reaction if its quantity is changed. The value of this factor has to be increased in order to increase the rate of the process.
Effects of varying enzyme concentration
- The rate of an enzymatic reaction is dependent on the number of effective collisions between enzyme and substrate molecules
- at lower enzyme concentrations, adding more enzyme increases the rate of chemical reaction (enzyme concentration is limiting )
- when the enzyme concentration increases, more effective collisions occurs, resulting in the formation of more enzyme-substrate complexes
- at the plateau, the rate of reaction becomes constant and enzyme concentration is no longer a limiting factor (substrate concentration is limiting)
- there are not enough substrates to occupy all the active sites of the enzymes
Effects of varying substrate concentrations
- At low substrate concentrations, few substrate molecules are present, hence there are many available enzyme active sites for effective collision to occur
- the rate of reaction increases with an increase in substrate concentration until a point when further increase in substrate concentration will no longer increase the rate of reaction
- the rate of reaction becomes constant and reaches a plateau
- at higher substrate concentrations, increasing the concentration of substrate cannot increase the rate of reaction. Enzyme concentration is the limiting factor
- this is because all active sites of the enzyme molecules are saturated with substrate molecules, and the concentration of product formed per unit time remains the same
