Topic 3: Enzymes Flashcards
A. Introduction to Enzymes
What are Enzymes?
- Enzymes are protein molecules which act as biological catalysts (they greatly increase the rate of a chemical reaction, without themselves being changed at the end of the reaction).
- Without enzymes, biochemical reactions would proceed too slowly to sustain life. Raising the temperature could increase the speed of reaction, but this would be detrimental to the cell.
- Enzymes enable metabolic reactions to proceed rapidly but at low temperatures.
- Enzymes are specific. A single enzyme will generally catalyse only a single reaction. Enzymes serve to control the reactions within each metabolic pathway.
B. Properties of Enzymes
Properties of Enzymes
- Most enzymes: i) are globular proteins; ii) have active sites with a specific shape where the reaction takes place.
- Enzymes function as biological catalysts and hence share the properties of catalysts: effective in small amounts; chemically unaltered at the end of the reaction they catalyse; function by lowering the activation energy required for the reaction to occur.
- Enzymes are extremely efficient. Most enzyme-catalysed reactions are highly efficient, proceeding 10^3 to 10^8 times faster than uncatalysed reactions.
- Enzymes have a high degree of specificity: most enzymes are specific to one type of substrate molecule, some enzymes are specific to a group of substrates which have similar shapes; the specificity of an enzyme is due to the conformation at its active site.
- The activity of enzymes is affected by changes in: a) pH; b) temperature; c) substrate concentration; d) eenzyme concentration
- The activity of enzymes can also be changed by the presence of inhibitors or activators. This implies that the rate of product formation can be controlled according to the needs of the cell.
C. Mode of Action of Enzymes
General Structure of An Enzyme Molecule
- All enzymes have a complex globular shape and are relatively large molecules.
- Only active site of the enzyme molecule comes into direct ocntact with the substrate.
- Substrate binds to the active site of the enzyme, forming the enzyme-substrate complex.
- The shape of the active site is complementary to the shape of the substrate.
C. Mode of Action of Enzymes
Activation Energy (Eₐ)
- Represents the energy barrier that has to be overcome before a reaction can take place to form products.
- The larger the Eₐ, the slower the reaction will be at any particular temperature.
- If Eₐ of a reaction is lowered, the rate of reaction would be increased.
- Enzymes are special biological catalysts, which serve to reduce the active energy required for a chemical reaction to start.
D. How Enzymes Work
Lock-and-Key Hypothesis
- The active site has a specific shape, to which the substrate binds.
- The substrate is imagined to be like a key, whose shape is complementary to the shape of the enzyme active site (or the lock).
- The shape of substrate or substrates is complementary and fit exactly into the shape of active site to form the enzyme-substrate complex.
- Once the reaction is completed with products formed, the products no longer fit into the active site, and are released into the surrounding medium, leaving the active site free to receive new substrate molecules.
D. How Enzymes Work
Induced Fit Hypothesis
- Stemmed from evidence that suggested that some enzymes and their acitive sites were physically flexible structures.
- Shape of substrate is still complementary to shape of active site but does not fit exactly.
- Binding of the substrate to the active site induces a small conformationalchange in the shape of the enzyme (this enables the substrate to fit more snugly into the active site when they form the enzyme-substrate complex, allowing the enzyme to perform its catalytic function more effectively)
- Once the reaction is completed with products formed, the products no longer fit into the active site, and are released into the surrounding medium, leaving the active site free to receive new substrate molecules.
D. How Enzymes Work
Enzymes at the Molecular Level
Collision between substrate and enzyme at the correct orientation causes the substrate to bind to the enzyme molecule at its active site, to form a short-lived enzyme-substrate complex. Within the complex, the chances of reactions occurring are greatly enhanced.
The reaction occurs.
Once reaction has occurred, the products are formed and subsequently released.
The unchanged enzyme molecule is also released. This is then available to catalyse another cycle of reaction.
E. Factors Affecting Rate of Enzyme Action
Factors Affecting Rate of Enzyme Action
Effects of:
A. Temperature
B. pH
C. Substrate concentration
D. Enzyme concentration
E. Factors Affecting Rate of Enzyme Action
Effects of Temperature
- As temperature increases, there is an increase in kinetic energy of enzyme and substrate molecules.
- This results in an increase in the frequency of effective collisions between enzyme and substrate molecules → more enzyme-substrate complexes are formed per unit time.
- The rate of reaction increases with temperature until the optimum temperature is reached.
- If the temperature is increased beyond the optimum temperature, the rate of enzymatic reaction decreases rapidly. This is because the enzyme is denatured.
- Optimum temperature is the temperature at which the enzyme is functioning at its maximum rate.
- The optimum temperature of most mammalian enzymes lie between 30 – 40 °C, but enzymes with higher optimum temperatures exist.
E. Factors Affecting Rate of Enzyme Action
What happens when an enzyme is denatured?
- Since enzymes are made of proteins, excessive heat disrupts the intramolecular bonds (especially the hydrogen bonds and hydrophobic interactions) which stabilize the (tertiary and quaternary) structures of the enzyme.
- The enzyme unfolds and the precise shape of the active site is lost. The enzyme is said to be denatured.
- Enzyme denaturation is usually irreversible.
E. Factors Affecting Rate of Enzyme Action
What happens when an enzyme is placed at freezing point?
If the temperature is reduced to near or below freezing point, the enzymes are inactivated. Enzyme activity is very low (lesser kinetic energy), but they will regain their catalytic influence when higher temperatures are restored.
E. Factors Affecting Rate of Enzyme Action
Effects of pH
- Under conditions of constant temperature, every enzyme functions most particularly over a specific range of pH.
The optimum pH of an enzyme is the pH at which maximum rate of reaction occurs.
- At this pH, the intramollecular bonds which maintain the protein structure of the enzyme are intact.
- The conformation of the active site is most ideal for substrate binding.
- Hence, there will be highest frequency of effective collision between enzyme and substrate molecules, resulting in the greatest number of enzyme-substrate complexes formed per unit time.
At pH higher or lower than the optimum pH, the H+ concentration would have changed.
* This alters the bonds (hydrogen and ionic bonds) that help to maintain the conformation of the enzyme ➔ active site and substrate binding would be affected
Within a certain range, the effects of pH are normally reversible. Restoring pH to the optimum would usually restore the optimum rate of reaction.
At extreme pH changes, the conformation of the enzyme is severely altered. It is hence denatured.
E. Factors Affecting Rate of Enzyme Action
Effects of Substrate Concentration
For a fixed enzyme concentration, the rate of enzyme reaction increases with increasing substrate concentration.
* An increase in number of substrate molecules leads to an increase in the frequency of effective collisions between enzyme and substrate molecules
➔ more enzyme-substrate complexes formed per unit time.
➔ rate of reaction is increased
* The rate of reaction is limited by substrate concentration.
There comes a point when any further increase in substrate concentration produces no significant change in rate of reaction.
* This is because all the active sites of all the enzyme molecules are saturated with substrate molecules.
* Any extra free substrate molecule has to wait until the enzyme-substrate complex has released the products before it may itself enter the active site of the enzyme.
* The rate of reaction is now limited by enzyme concentration.
E. Factors Affecting Rate of Enzyme Action
Effects of Enzyme Concentration
Provided that the substrate concentration is maintained at a high level, and other conditions such as pH and temperature are kept constant, the rate of reaction is proportional to the enzyme concentration.
* As enzyme concentration is increased, the frequency of effective collision between enzyme and substrate molecules increases.
* Greater number of enzyme-substrate complexes is formed per unit time.
At very high enzyme concentrations, if the concentration of substrate molecules is limiting, the increase in enzyme concentration would not result in any further increase in rate of reaction.
