Chap 8- Mechanisms and Inhibitors Flashcards
What are the strategies employed to catalyze specific reactions.
- Covalent catalysis
- General acid-base catalysis
- Metal ion catalysis
- Catalysis by approximation and orientation
Covalent catalysis def
In covalent catalysis, the active site contains a reactive group, usually a powerful nucleophile that becomes temporarily covalently modified I the course of catalysis. The proteolytic enzyme chymotrypsin provides an excellent example of this mechanism.
General acid-base catalysis def
In general acid-base catalysis, a molecule other than water plays the role of a proton donor ir acceptor. Chymotrypsin uses a histidine residues as a base catalyst to enhance the nucleophilic power of serine.
Metal ion catalysis def
Metal ions can function catalytically in several ways. For instance, a metal ion may serve as an electro Philip catalyst, stabilizing a negative charge on a reaction intermediate. Alternatively, a metal ion may generate a nucleophile by increasing the acidity of a nearby molecule such as water. Finally, a metal ion may bind to the substrate, increasing the number of interactions with the enzyme and thus the binding energy. Metal ions are required cofactors for many of the enzymes we will encounter.
Catalysis by approximation and orientation def
Many reactions include two distinct substrates. In such cases, the reaction rate may be considerably enhanced by bringing the two substrates into proximity and in the proper orientation on a single binding surface if a; enzyme.
How does temperature affect the rate on enzyme-catalyze reactions?
As the temperature rises, the rate of most reactions increases because it increases the Brownian motion of the molecules, which makes interactions between an enzyme and its substrate more likely. For most enzymes, there is a temperature at which the increase in catalytic activity ceases and there is a precipitous loss of activity.
How does pH affect enzymes
Enzyme activity often varies with pH, the H+ concentration of the environment. The activity of most enzymes displays a bell-shaped curve when examined as a function of pH. The optimal pH where enzymes display maximal activity varies with the enzyme and is correlated with the environment of the enzyme.
Reversible inhibition def and what are the three types?
Reversible inhibition is characterized by rapid dissociation of the enzyme-inhibitor complex.
Three types:
1. Competitive inhibition
2. Uncompetitive inhibition
3. Non competitive inhibition.
Competitive inhibition def
The inhibitors resembles the substrate and binds to the active site of the enzyme. The substrate is then prevented from binding to the same active site. A competitive inhibitor diminishes the rate of catalysis by reducing the proportion of enzyme molecules bound to a substrate. Competitive inhibition can be relieved by increasing the substrate concentration.
Uncompetitive inhibition def
The uncompetitive inhibitors binding site is created only when the enzyme binds the substrate. It is substrate dependent because the inhibitor binds only to the enzyme-substrate complex. Cannot be overcome by the addition of more substrate.
Non competitive inhibition def
The inhibitor and substrate can bind simultaneously to an enzyme molecule at different binding sites. Acts by decreasing the overall number of active enzyme molecules rather than diminishing the proportion;of enzymes that are bound to substrate. Cannot be overcome by increasing the substrate concentration.
How to distinguish the three types of reversible inhibition
You can measure the rates of catalysis at different concentrations of substrate and inhibitor.
Competitive inhibitor- effect is to increase the value of Km, more substrate is needed to obtain same reaction rate. Vmax will remain the same.
Uncompetitive inhibition- Vmax will lower because there are enzyme-substrate-inhibitor complexes that do not form any product. Also lowers the Km value because the inhibitor binds to ES to form ESI, depleting ES. A lower concentration of S is required to form half of the maximal concentration of ES, resulting in reduction of the apparent Km.
Non competitive inhibition when plotted
The value of Vmax is decreased to give a new apparent Vmax value. The value of Km is unchanged. The inhibitor simply lowers the concentration of functional enzyme.
Using double reciprocal plots for distinguishing inhibitors
Competitive- the y intercept, 1/vmax, is unchanged because Vmax is unchanged. Increase in the slope indicates the strength of the binding of a competitive inhibitor.
Uncompetitive- the slope stays the same but the y-intercept is increased. The lines are parallel.
Non competitive- the intercept of the y-axis is decreased, the slope, Km/Vmax, is larger by the same factor
Irreversible inhibitor def
Dissociates very slowly from its target enzyme because it has become tightly bound to the enzyme, either covalently or non covalently.
Examples, penicillin modifies transpeptidase, preventing synthesis or bacterial cell walls. Aspirin acts by modifying cyclooxygenase, reducing the synthesis of inflammatory signals.
What are the four categories of irreversible inhibitors?
1) group-specific reagents
2) affinity labels
3) suicide inhibitors
4) transition-state analogs
Group-specific reagents def
Modify specific R groups of amino acids.
Affinity labels def
Also called substrate analogs are molecules that covalently modify active site residues and are structurally similar to an enzymes substrate. More specific for the active site than group-specific reagents.
Suicide inhibitors def
Chemically modified substrates. They bind to the enzyme as a substrate and is processed by the Normal catalytic mechanism. The mechanism then generates a chemical reactive intermediate that inactivates the enzyme through covalent modification.
Transition-state analogs def
Potential inhibitors of enzymes. The formation of the transition state is crucial to enzyme catalysis. Transition state analogs have inhibitory power.
What type of enzyme is chymotrypsin, and where does it come from?
It is a proteolytic enzyme, which hydrolyzes proteins,and it is Se created from the pancreas in response to a meal.
Where does chymotrypsin cleave peptide bonds?
Selectively on the carboxyl-terminal side of yhe large hydrophobic amino acids such as tryptophan, tyrosine, phenylalanine, methionine, isoleucine.
What is covalent modification?
It is a catalytic strategy used by chymotrypsin where the enzyme employs a powerful nucleophile to attack the unreactive carbonyl of the substratee. The nucleophile becomes covalently attached to the substrate riddle in the course of catalysis.
