chapter 4- Enzymes -_8514192d48fac6a84a7259a7df6e0970
Enzymes
General Overview
Enzymes are proteins that act as catalysts, increasing the rate of chemical reactions without being consumed or altered permanently.
Enzymes – Classification
Enzymes are categorized based on the type of reaction they catalyze.
1. Oxidoreductases
Catalyze the transfer of electrons between molecules.
Reductant: Electron donor
Oxidant: Electron acceptor
Types:
Oxidases: Accept hydrogen/electrons, often involving oxygen.
Dehydrogenases: Transfer hydrogen from substrates.
2. Transferases
Transfer specific chemical groups from one molecule to another.
Examples:
Acyltransferase
Aminotransferase
Glycosyltransferase
Kinases
Methyltransferases
Nucleotidyltransferases
3. Hydrolases
Catalyze the addition of water to split bonds, cleaving molecules.
Examples:
Phosphatase
Peptidase
Lipase
Hydrolase
Amylase
4. Lyases
Break double bonds through addition or removal of groups (e.g., water, ammonia, carbon dioxide).
Examples:
Decarboxylase
Synthase
Aldolase
Cyclase
Endonuclease
5. Isomerases
Catalyze rearrangements or structural changes within a molecule.
Examples:
Epimerase
Mutase
Racemase
6. Ligases (Synthetases)
Catalyze the joining of two molecules using energy from ATP to ADP.
Examples:
Amino acid-RNA ligase: C-O bond formation
Amide synthetases & peptide synthetases: C-N bond formation
Enzymes – Nomenclature
Enzymes typically have names ending in "-ase," following either:
The substrate name (e.g., urease for urea, lactase for lactose).
Their function descriptors (e.g., dehydrogenase, oxidase, decarboxylase).
Common names with no direct relation to substrate (e.g., pepsin, chymotrypsin, trypsin).
Enzymes – Activation Energy
Enzymes speed up reactions by lowering the activation energy.
Changes in the reaction pathway offer a lower energy route for substrate-to-product conversion.
Activation energy is the energy barrier that must be overcome for a reaction to proceed.
The transition state of a reaction represents the highest energy level.
Catalyzed vs. Uncatalyzed Reactions
The rate of uncatalyzed reactions increases with substrate concentration.
Catalyzed reactions show two stages:
Formation of enzyme-substrate complex.
Conversion to product.
As substrate concentration increases, enzyme-active sites may become saturated.
Enzyme-Substrate Interaction
The active site of an enzyme is where substrates bind, featuring pockets for substrate docking made possible by weak, non-covalent interactions.
Two models describing enzyme action:
Lock & Key Model: Enzymes and substrates have fixed shapes for complementary matching.
Induced-Fit Model: The enzyme active site is dynamic, adjusting shape to optimize substrate fit and maximize reaction rate.
Enzyme Specificity
Certain enzymes exhibit specificity, affecting their interaction with substrates:
Absolute specificity: Acts only on one substrate (e.g., urease).
Group specificity: One enzyme acts on similar substrates (e.g., hexokinase).
Relative/linkage specificity: Actions on different substrates with the same bond type (e.g., pepsin).
Stereochemical specificity: Active only with specific isomeric forms (e.g., enantiomers).
Cofactors & Coenzymes
Many enzymes require additional components (cofactors) for activity:
Apoenzymes: Protein portion of the enzyme.
Cofactors: Non-protein components needed for catalysis (ex: metal ions).
Coenzymes: Small organic molecules that temporarily bind and assist (often derived from vitamins).
Environmental Effects on Enzymes
pH: Each enzyme has an optimum pH range for activity. Extreme pH can denature enzymes.
Temperature: Increased temperature enhances reaction rates, but extreme heat can denature enzymes.
Kinetics of Enzyme Activity
Kinetics studies the rates of processes oxidized by enzymes.
Low substrate concentrations lead to increased reaction rates, while high concentrations can saturate enzymes, reaching Vmax and indicating maximal enzymatic activity.
The Km (Michaelis-Menten constant) reflects enzyme affinity for substrates.
Regulation of Enzyme Activity
Enzymes are regulated via:
Production in response to substrate presence.
Allosteric control (binding of effectors that influence activity).
Feedback inhibition (products inhibit earlier enzymes).
Zymogens (inactive precursors activated by proteolysis).
Chemical modifications such as phosphorylation.
Inhibition of Enzyme Activity
Enzyme inhibitors can significantly reduce catalytic activity:
Irreversible inhibitors: Bind tightly to enzymes, preventing E-S complex formation (e.g., arsenic, snake venom).
Reversible inhibitors: Include competitive (compete for active sites) and noncompetitive (bind elsewhere, altering function).
Use of Enzymes in Medicine
Used as diagnostic tools and therapeutic agents.
Example diagnostic enzymes include lipase (related to pancreatitis) and creatine kinase (associated with myocardial infarction).
Isoenzymes have unique properties impacting their diagnostic utility.
Multiple Choice Questions on Enzymes
What is the primary function of enzymes?
A) Decrease the rate of reactions
B) Increase the rate of reactions
C) Change the nature of reactions
D) Consume substrates permanently
Answer: B) Increase the rate of reactions
Enzymes known as oxidoreductases are responsible for:
A) Transferring electrons
B) Adding water to split molecules
C) Rearranging molecular structures
D) Joining two molecules
Answer: A) Transferring electrons
Which of the following enzymes transfers chemical groups between molecules?
A) Lyases
B) Transferases
C) Hydrolases
D) Isomerases
Answer: B) Transferases
The function of hydrolases is to:
A) Break down double bonds
B) Rearrange molecular structures
C) Split molecules using water
D) Catalyze electron transfer
Answer: C) Split molecules using water
Lyases perform which of the following actions?
A) Transfer hydrogen atoms
B) Break bonds by adding or removing groups
C) Join two substrates using ATP
D) Catalyze isomers
Answer: B) Break bonds by adding or removing groups
The enzyme that converts glucose-6-phosphate to fructose-6-phosphate is a(n):
A) Lyase
B) Isomerase
C) Transferase
D) Ligase
Answer: B) Isomerase
Enzymes are named based on:
A) Their molecular weight
B) The substrate they work on or their function
C) Their color
D) Their activation energy
Answer: B) The substrate they work on or their function
The active site of an enzyme is:
A) A fixed structure that does not change
B) The location where substrates bind
C) An external factor affecting enzyme activity
D) The point where enzyme is denatured
Answer: B) The location where substrates bind
Which model suggests that the enzyme active site adjusts its shape to fit the substrate?
A) Lock and Key Model
B) Induced-Fit Model
C) Competitive Inhibition Model
D) Allosteric Regulation Model
Answer: B) Induced-Fit Model
Enzyme specificity refers to:
A) The enzyme's ability to catalyze any reaction
B) The ability of an enzyme to act on a particular substrate
C) The enzyme's shape
D) The energy required for a reaction
Answer: B) The ability of an enzyme to act on a particular substrate
An example of a coenzyme is:
A) Hemoglobin
B) NAD+
C) Amylase
D) Lipase
Answer: B) NAD+
The optimum pH for enzyme activity is:
A) The same for all enzymes
B) Unique for each enzyme
C) Always neutral (pH 7)
D) Varied with temperature
Answer: B) Unique for each enzyme
How is enzyme activity primarily regulated?
A) Environmental temperature only
B) Production in response to substrates
C) Enzymes are always produced at high concentrations
D) They are not regulated
Answer: B) Production in response to substrates
Feedback inhibition occurs when:
A) Substrates enhance enzyme activity
B) Products inhibit earlier enzymes in the pathway
C) Enzymes are denatured
D) Extreme temperatures affect enzymes
Answer: B) Products inhibit earlier enzymes in the pathway
Irreversible inhibitors:
A) Bind loosely to enzymes
B) Prevent E-S complex formation permanently
C) Can be overcome by increasing substrate
D) Are always noncompetitive
Answer: B) Prevent E-S complex formation permanently
Which of the following is true about reversible inhibitors?
A) Always bind permanently to the enzyme
B) Can be competitive or non-competitive
C) Increase reaction rates
D) Are not related to enzyme function
Answer: B) Can be competitive or non-competitive
An example of a diagnostic enzyme is:
A) Amylase
B) Lactase
C) Urease
D) Hexokinase
Answer: A) Amylase
Isoenzymes are important because:
A) They catalyze the same reaction at different rates
B) They are generally inactive
C) They have the same properties
D) They have no clinical significance
Answer: A) They catalyze the same reaction at different rates
Km, or Michaelis-Menten constant, indicates:
A) The energy released during the reaction
B) The affinity of the enzyme for its substrate
C) The maximum velocity of the reaction
D) The rate of product formation
Answer: B) The affinity of the enzyme for its substrate
Which factor can lead to enzyme denaturation?
A) Extreme pH levels
B) High substrate concentration
C) Moderate temperature
D) Proper ion concentration
Answer: A) Extreme pH levels
Ligases are primarily involved in:
A) Adding hydrogen to substrates
B) Joining two molecules using ATP
C) Rearranging bonds within a molecule
D) Breaking molecules apart
Answer: B) Joining two molecules using ATP
How does temperature affect enzyme activity?
A) It has no effect
B) Higher temperature always increases activity
C) Increased temperature enhances rates up to a point
D) Lower temperatures deactivate enzymes
Answer: C) Increased temperature enhances rates up to a point
Coenzymes are often derived from:
A) Proteins only
B) Nucleic acids
C) Vitamins
D) Carbohydrates
Answer: C) Vitamins
Apoproteins are:
A) The non-protein part of enzymes
B) The protein portion of enzymes
C) A type of enzyme substrate
D) Active enzyme forms
Answer: B) The protein portion of enzymes
The formation of an enzyme-substrate complex is:
A) The last step in enzyme activity
B) The first step in catalyzed reactions
C) Irreversible
D) Always results in denaturation
Answer: B) The first step in catalyzed reactions
Note Summary:
These questions cover enzyme classification, function, specificity, kinetics, regulation, and applications, promoting a comprehensive understanding of enzymes.