Enzymes Flashcards
What are enzymes?
1)They accelerate the rate of a biochemical reaction
2)Enzymes are not used up (or consumed) in the reaction
3)Enzymes contain one (or more) active site(s), that is/are used for the catalysis reaction
4)Names of enzymes usually end with a suffix ‘ase’
What is an Active site?
(Enzyme catalyzed reaction)
The active site is a pocket in the 3-D structure of the enzyme that binds to the substrate and converts it into product
Side Chains
(Enzyme catalyzed reaction)
1)Side chains of amino acids residues on the active site hold the substrate in the correct position by reversible non-covalent interactions
2)The side chains also mediate the catalysis of the reaction
3)Enzymes bind to substrate to form enzyme-substrate intermediate first. The reaction is catalyzed and the product is released
The Lock and Key Model
1)The substrate (key) fits exactly into the shape of the active site (lock).
2)The substrate binds to the enzyme and reacts.
3)(Likened to inserting a key into a lock).
The Induced Fit Model
1)Initially, the substrate does not fit exactly into the shape of the active site.
2)But the active site adjusts its shape for the substrate to
bind and react.
Characteristics of enzyme catalyzed reactions
1)Enzyme catalyzed reactions are specific
2)The enzyme only binds either to:
-a specific biological substrate
or
-a specific group of similar substrates
(e.g. peptidase break peptide bonds of all proteins)
What is activation energy?
A fixed amount of energy required for a chemical reaction to initiate
Activation energy of reactions
1)Since substrates and products are different substances, they have different energy values
2)Activation energy hinders the transition from one energy values to another
3)It does not affect the energy values of the substrates and products
4)An enzyme/catalyst lowers the activation energy of a reaction by providing an alternative path
-Less energy is thus required for the reaction to proceed
5)Enzymes lower activation energy by forming a transition state with the substrate = the enzyme substrate complex (ES)
-Enzymes help a reaction to proceed more readily, thus increasing the reaction rate
-An enzyme reduces the activation energy of a reaction, but does not eliminate it completely
Factors affecting enzyme activity
a)Environmental factors
-Enzymes are proteins!
-Therefore factors we learned that can affect protein conformation will also affect enzyme activities
b)Presence of inhibitors
-Substances that stop enzyme activity
c)Cofactors and coeznymes
-Molecules that are essential for an enzyme to function
a)Environmental factors that affect enzyme activity
-Factors that influence protein structure will influence enzyme activity
Factors:
1)Temperature
2)pH
3)Detergents
4)Organic solvent
5)salt
6)Mechanical agitation
7)Presence of reducing agents (Beta-mercaptoethanol or DDT)
-Conformation of the enzyme active site is important for it’s function
-When enzymes are exposed to the factors that will cause protein denaturation, the enzymes lose their conformation and therefore their activity
Effect of temperature on enzyme activity
1)Higher temperature helps to overcome the activation energy
-increased the kinetic energy of molecules, or causes the molecules to vibrate/ collide more vigorously
2)Hence, as temperature increases, rate of reaction also increases
3)However, when the temperature becomes too high, the enzyme will start to denature. This causes enzyme activity to decrease
4)Therefore, there exists an optimal temperature for every enzyme where it can function optimally
(optimum temperature for most human enzymes is generally around 25-40 degrees celsius)
Effect of pH on enzyme activites
-Different enzymes work optimally at different pH ranges
1)Each enzyme usually has maximum activity at an optimal pH
2)At the optimal pH, the protein structure exists as the most stable structure for maximum activity
3)When the pH is higher or lower than the optimal
-changes in concentration of hydrogen ions or hydroxide ions
-changes in the charges on the enzyme
-this disrupts the non-covalent interactions on the enzyme, and eventually results in denaturation of the enzyme when the change become too large
b)Inhibitors
1)Inhibit enzyme’s activity by either affecting the binding between enzyme and substrate or by affecting the enzyme itself
2)Since inhibitors inhibits enzymes, they are often also toxic to cells
Use of inhibitors
1)In enzyme research
2)In nature
Example: believed to be a defense mechanism against predators
3)In medicine
Example: inhibitors to kill cancer cells
Types of Reversible inhibitor
-Reversible inhibitor’s action can be controlled by changing the amount of inhibitors available
1)Competitive inhibitors
2)Non-competitive inhibitors
Competitive inhibitors (Reversible inhibitor)
1)Competitive inhibitors structurally resemble the substrate and compete with it for the active site. The inhibitor blocks the substrate from accessing the active site temporarily
2)Competitive inhibitors lowers the substrate binding effectively (affinity) of an enzyme
Non-competitive inhibitors (Reversible inhibitor)
1)Non-competitive inhibitors bind at a location other than the active site for the substrate
2)It does not affect substrate binding but lowers enzyme activity
Irreversible inhibitors
1)Forms strong covalent bonds with the enzyme
2)The site of attack (active site, binding site) becomes inactive permanently. The enzyme thus loses its function permanently
Irreversible inhibitors (Process)
-Since irreversible inhibitors permanently inactivate enzymes, they are often highly toxic to the cell
1)Irreversible inhibitor binds with the enzyme
2)Permanently inactivates the enzyme
3)This killing the cell
C) Cofactors and coenzymes
Molecules that are essential for an enzyme to function
Cofactors
Inorganic substances essential for an eznyme to function
Example: Iron in haemoglobin
Coenzymes
Organic molecules essential for an enzyme to function
Example: coenzyme (A)
Regulation of Enzymes Activities
1)There are thousands of reactions occurring in the cell every moment
2)How does the body coordinate and control all these reactions?
-Since all reactions involve enzymes, we can control the reactions by controlling the activities of the enzyme
3)Our body produces many different molecules to control enzyme activities, either by making the enzymes more active or less active
The main mechanisms of Enzyme Regulation
(A) Allosteric control
(B) Covalent Modification
(C) Availability of enzymes
(D) Oligomerization of proteins
(E) Removal of inhibitors of enzymes
Allosteric Control (A)
-An effector molecule binds reversibly and non-covalently to the enzyme
1)It binds to an allosteric site that is different from the active site
2)The enzyme and it’s active site changes in conformation
3)This causes a change in enzyme activity (activation/ deactivation)
Types of effectors (Allosteric Control - A)
1)Positive effectors (or allosteric activators)
-activates enzyme
2)Negative effectors (or allosteric inhibitors)
-deactivates the enzyme
Covalent Modification (B)
Examples of covalent modification:
1)Phosphorylation
2)Cleavage of proteins
3)Cross-linking of protein subunits
Phosphorylation
1)A phosphate group covalently attached to the enzyme
2)Causes the enzyme to change in shape
3)Enzyme becomes activated
Availability of Enzymes (C)
-In order for an enzymatic reaction to take place, the enzyme must be available
-There are 2 ways enzymes are made unavailable for reacting with the substrates
1)The enzymes could be stored in special compartments within the cell
2)The enzymes are only produced when the cell needs it
Oligomerization of Proteins (D)
-Some enzymes become activated when they form oligomers (a combination of multiple enzymes)
Removal of Inhibitors of enzymes (E)
If an enzyme is inactive because of an inhibitor, the removal of the inhibitor will free the enzyme to react
