Enzyme Kinetics: Michaelis-Menten Equation

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Questions and Answers

What does Km represent in the Michaelis-Menten equation?

Maximum reaction velocity

Substrate concentration at half Vmax (correct)

Enzyme concentration

Rate of product formation

The velocity of an enzyme-catalyzed reaction always increases with temperature.

False

What are the two main types of enzyme inhibitors?

Competitive and noncompetitive inhibitors

The maximum rate of reaction, indicated by Vmax, occurs when the enzyme is __________ with substrate.

saturated Signup and view all the answers

Match the enzyme with its optimum pH:

Pepsin = 1-2 Alkaline phosphatase = 9-10 Acid phosphatase = 4-5 Most enzymes = 6-8 Signup and view all the answers

How does increasing the concentration of products affect an enzyme-catalyzed reaction?

Slows down or stops the reaction Signup and view all the answers

Reversible inhibitors permanently bind to the enzyme, preventing its activity.

False Signup and view all the answers

What is the effect of substrate concentration on the velocity of reaction at initial phases?

It increases velocity correspondingly. Signup and view all the answers

Which of the following terms refers to an active enzyme with its nonprotein component?

Holoenzyme Signup and view all the answers

Ribozymes are more commonly encountered than protein catalysts.

False Signup and view all the answers

What term is used to describe coenzymes that are permanently associated with the enzyme?

prosthetic group Signup and view all the answers

The active site of an enzyme contains specific _____ that participate in substrate binding.

amino acids Signup and view all the answers

Match the following coenzymes with their associated vitamins:

NAD+ = Niacin FAD = Riboflavin Coenzyme A = Pantothenic acid Biotin = Biotin Signup and view all the answers

Which of the following best describes the function of cofactors?

Metallic ions that are essential for enzyme activation Signup and view all the answers

The enzyme-substrate (ES) complex immediately dissociates into enzyme and product (P).

False Signup and view all the answers

What is the main application of the Michaelis-Menten equation?

To describe the kinetic properties of enzyme-catalyzed reactions Signup and view all the answers

Calcium is an example of a _____ that can increase lipid enzyme activity.

cofactor Signup and view all the answers

What type of coenzyme only transiently associates with the enzyme?

Co-substrate Signup and view all the answers

What is the primary role of enzymes in biochemical reactions?

Decrease the energy of the reaction Signup and view all the answers

Which class of enzymes is responsible for the transfer of C-, N-, or P- containing groups?

Transferases Signup and view all the answers

Apoenzyme is an active form of an enzyme that requires a coenzyme to function.

False Signup and view all the answers

Define the term 'substrate' in the context of enzyme reactions.

The substance upon which an enzyme acts. Signup and view all the answers

Enzymes can speed up reactions by a factor of __________ compared to uncatalyzed reactions.

10^3 to 10^8 Signup and view all the answers

Match the following classes of enzymes with their functions:

Oxidoreductases = Catalyze oxidation-reduction reactions Hydrolases = Cleavage of bonds by addition of water Lyases = Cleave without adding water Isomerases = Racemization of isomers Signup and view all the answers

Which type of enzyme inhibition involves a molecule competing with the substrate for the active site?

Competitive inhibition Signup and view all the answers

Ligases do not require ATP for their enzymatic reactions.

False Signup and view all the answers

What is the turnover number in enzyme activity?

The number of substrate molecules converted to product per enzyme molecule per second. Signup and view all the answers

The class of enzymes that catalyze the cleavage of bonds without the addition of water is called __________.

Lyases Signup and view all the answers

Which of the following drugs are known to cause vasodilation and reduce blood pressure?

Captopril Signup and view all the answers

Allosteric enzymes are modified by effectors that bind covalently.

False Signup and view all the answers

What type of substrate serves as a positive effector in homotropic regulation?

the substrate itself Signup and view all the answers

The addition or removal of phosphate groups from an enzyme is known as __________.

phosphorylation and dephosphorylation Signup and view all the answers

Match the following terms with their definitions:

Positive effectors = Increase enzyme activity Negative effectors = Inhibit enzyme activity Covalent modification = Addition/removal of chemical groups Proenzyme = Inactive form of an enzyme Signup and view all the answers

Which drug is an example of a competitive inhibitor?

Atorvastatin Signup and view all the answers

Statins increase plasma cholesterol levels.

False Signup and view all the answers

What type of enzyme inhibition occurs when the inhibitor and substrate bind to different sites on the enzyme?

Noncompetitive inhibition Signup and view all the answers

Dihydropteroate synthase is inhibited by ______, which is structurally similar to PABA.

sulphonamide Signup and view all the answers

Match the following drugs with their corresponding inhibition type:

Atorvastatin = Competitive inhibitor Lead = Noncompetitive inhibitor Methotrexate = Competitive inhibitor Iodoacetate = Noncompetitive inhibitor Signup and view all the answers

What is the primary effect of angiotensin-converting enzyme (ACE) inhibitors?

Lower blood pressure Signup and view all the answers

Noncompetitive inhibitors can be relieved by increasing substrate concentration.

False Signup and view all the answers

Which enzyme is inhibited by heavy metals like lead?

Ferrochelatase Signup and view all the answers

Penicillin and amoxicillin are examples of ______ antibiotics.

β-lactam Signup and view all the answers

Which type of inhibitor binds covalently to the enzyme's active site?

Irreversible inhibitor Signup and view all the answers

What is the mechanism of action of statin drugs in relation to cholesterol synthesis?

They act as competitive inhibitors of HMG-CoA reductase. Signup and view all the answers

Noncompetitive inhibitors can be relieved by increasing substrate concentration.

False Signup and view all the answers

Name an example of a competitive inhibitor mentioned in the content.

methotrexate Signup and view all the answers

Dihydropteroate synthase is inhibited by sulphonamide, which is structurally similar to __________.

PABA Signup and view all the answers

Match the following drugs with their type of inhibition:

Penicillin = Inhibits bacterial cell wall synthesis Atorvastatin = Competitive inhibition of cholesterol synthesis Lead = Irreversible noncompetitive inhibition ACE inhibitors = Lower blood pressure by blocking enzyme activity Signup and view all the answers

What are catalytic residues?

Amino acid chains that participate in substrate binding and catalysis Signup and view all the answers

The Michaelis-Menten equation describes the interaction between enzymes and substrates in a linear manner.

False Signup and view all the answers

What is the term for the complex formed when an enzyme binds with its substrate?

enzyme-substrate complex (ES) Signup and view all the answers

An enzyme that has lost its nonprotein component is referred to as an __________.

apoenzyme Signup and view all the answers

Match the following types of enzyme inhibitors with their descriptions:

Competitive Inhibitor = Binds to the active site, competing with the substrate Noncompetitive Inhibitor = Binds to an allosteric site, not the active site Uncompetitive Inhibitor = Binds only to the enzyme-substrate complex Allosteric Inhibitor = Affects enzyme activity by binding at a site other than the active site Signup and view all the answers

Which factor can increase the activity of an enzyme?

Presence of appropriate cofactors Signup and view all the answers

Induced fit refers to the rigid binding of the substrate to the active site of an enzyme.

False Signup and view all the answers

What is the role of coenzymes in enzyme activity?

Coenzymes assist enzymes in catalyzing reactions and are essential for enzyme function. Signup and view all the answers

Which of the following is a common method of regulating enzyme activity through covalent modification?

Phosphorylation and dephosphorylation Signup and view all the answers

Allosteric enzymes are regulated by molecules that bind covalently to the active site.

False Signup and view all the answers

What type of effectors are known to inhibit enzyme activity?

Negative effectors Signup and view all the answers

Enzymes can often be activated through the process of __________, which involves the addition of phosphate groups.

phosphorylation Signup and view all the answers

Match the following types of effectors with their effects:

Homotropic = Inhibits enzyme activity by a different molecule Heterotropic = Increases activity, often the substrate itself Signup and view all the answers

Which enzyme is often inhibited by citrate, making it an example of a heterotropic effector?

Phosphofructokinase-1 Signup and view all the answers

Partial proteolysis results in the conversion of inactive proenzymes into their active forms.

True Signup and view all the answers

What is the primary role of protein kinases in enzyme regulation?

To catalyze phosphorylation Signup and view all the answers

Allosteric enzymes often exhibit __________ regulation, where the substrate itself can enhance enzyme activity.

homotropic Signup and view all the answers

Match the following terms related to enzyme activity with their correct definitions:

Positive effectors = Increase enzyme activity Negative effectors = Inhibit enzyme activity Covalent modification = Addition or removal of functional groups Allosteric regulation = Binding at a site other than the active site Signup and view all the answers

Which of the following factors does NOT influence enzyme activity?

Light intensity Signup and view all the answers

Increase in substrate concentration always leads to an increase in the reaction velocity until saturation is reached.

True Signup and view all the answers

What is the effect of temperature on enzyme activity at high temperatures?

Decreased velocity of reaction Signup and view all the answers

The Michaelis constant (Km) is defined as the substrate concentration at which the reaction velocity is ______ of Vmax.

half Signup and view all the answers

Match the enzyme with its optimum pH:

Pepsin = 1-2 Alkaline phosphatase = 9-10 Acid phosphatase = 4-5 Other enzymes = 6-8 Signup and view all the answers

Which type of inhibition involves the inhibitor competing with the substrate for the active site?

Competitive inhibition Signup and view all the answers

Noncompetitive inhibitors bind to the active site and prevent substrate binding permanently.

False Signup and view all the answers

What is the maximum rate of reaction called when the enzyme is saturated with substrate?

Vmax Signup and view all the answers

An inhibitor that binds irreversibly to an enzyme typically forms ______ bonds.

covalent Signup and view all the answers

What happens to the reaction when product concentration increases significantly?

It stops or reverses Signup and view all the answers

What is the primary role of enzymes in biochemical reactions?

To act as protein catalysts that increase the reaction rate Signup and view all the answers

Holoenzyme refers to the inactive form of an enzyme without its cofactor.

False Signup and view all the answers

What term is used to describe the maximum rate of reaction in an enzyme-catalyzed process?

Vmax Signup and view all the answers

The enzymes called __________ catalyze the transfer of C-, N-, or P-containing groups.

transferases Signup and view all the answers

Match the following classes of enzymes with their functions:

Oxidoreductase = Catalyze oxidation-reduction reactions Hydrolases = Catalyze cleavage of bonds by addition of water Lyases = Cleave bonds without adding water Ligases = Condensation of two molecules requiring ATP Signup and view all the answers

Which type of inhibition occurs when a molecule competes with the substrate for the active site?

Competitive inhibition Signup and view all the answers

Noncompetitive inhibitors bind to the enzyme's active site.

False Signup and view all the answers

What is the meaning of 'turnover number' in enzyme activity?

Number of substrate molecules converted to product per enzyme molecule per second Signup and view all the answers

The substance upon which an enzyme acts is called the __________.

substrate Signup and view all the answers

Which class of enzymes is primarily responsible for breaking down substrates through hydrolysis?

Hydrolases Signup and view all the answers

Study Notes

Michaelis-Menten Equation

Describes the relationship between the initial rate of an enzymatic reaction and the concentration of substrate.
V0 is the initial reaction velocity.
Vmax is the maximal velocity (when the enzyme is saturated with substrate).
Km is the Michaelis constant, which is the substrate concentration at half Vmax.

Factors Influencing Enzyme Activity

Enzyme concentration: Velocity increases proportionally with enzyme concentration when substrate is unlimited.
Substrate concentration: Velocity increases with substrate concentration initially, then plateaus at Vmax as the enzyme becomes saturated.
Product concentration: Increased product concentration slows down, stops or even reverses the reaction.
Temperature: There's an optimal temperature for enzyme activity, with velocity increasing initially, then decreasing at higher temperatures due to enzyme denaturation.
pH: Each enzyme has an optimal pH, with activity decreasing on either side due to changes in the charge of amino acid residues at the active site.

Inhibition of Enzyme Activity

Inhibitors: Substances that decrease the velocity of enzyme-catalyzed reactions.
Irreversible inhibitors: Bind to the enzyme through covalent bonds.
Reversible inhibitors: Bind to the enzyme through noncovalent bonds.
Competitive inhibition: Inhibitor binds to the same active site as the substrate, competing for binding.
Noncompetitive inhibition: Inhibitor binds to a different site on the enzyme, altering its conformation and activity.

Examples of Enzyme Inhibition

Competitive Inhibition:
- Statin drugs (atorvastatin, pravastatin) are competitive inhibitors of HMG-CoA reductase, an enzyme involved in cholesterol synthesis.
Noncompetitive Inhibition:
- Lead can inhibit ferrochelatase, an enzyme involved in heme synthesis, by forming covalent bonds with cysteine residues.
- Insecticides can inhibit acetylcholine esterase, an enzyme involved in neurotransmission, by covalent binding.

Enzyme Inhibitors as Drugs

Many drugs target enzymes to treat various diseases.
Beta-lactam antibiotics (penicillin, amoxicillin) inhibit bacterial cell wall synthesis.
Angiotensin-converting enzyme (ACE) inhibitors (captopril, enalapril, lisinopril) block the conversion of angiotensin I to angiotensin II, a potent vasoconstrictor, lowering blood pressure.

Regulation of Enzyme Activity

Allosteric regulation: Effectors bind to the enzyme at a site other than the active site, altering its activity.
- Homotropic effectors: The substrate itself acts as an effector.
- Heterotropic effectors: A different molecule acts as an effector.
Covalent modification: Adding or removing groups like phosphate or peptide chains modifies enzyme activity.
- Phosphorylation/dephosphorylation: Protein kinases add phosphate groups, phosphoprotein phosphatases remove them.
- Partial proteolysis: Conversion of an inactive proenzyme to an active enzyme by cleaving a peptide bond.

Enzyme Nomenclature and Classification

Each enzyme has a short recommended name and a more complete systematic name.
Class 1: Oxidoreductases: Catalyze oxidation-reduction reactions.
Class 2: Transferases: Catalyze transfer of functional groups like C-, N-, or P-containing groups.
Class 3: Hydrolases: Catalyze hydrolysis reactions by adding water.
Class 4: Lyases: Catalyze cleavage of bonds without adding water.
Class 5: Isomerases: Catalyze the rearrangement of atoms within a molecule.
Class 6: Ligases: Catalyze the joining of two molecules using ATP.

Enzyme Structure and Function

Holoenzyme: The active enzyme with its nonprotein component.
Apoenzyme: The inactive enzyme without its nonprotein component.
Cofactor: A metallic ion that is part of the holoenzyme.
Coenzyme: A small organic molecule that is part of the holoenzyme.
Active site: A special pocket or cleft in the enzyme containing amino acid residues involved in substrate binding and catalysis.
Substrate: The molecule acted upon by the enzyme.
Enzyme-substrate (ES) complex: Formed when substrate binds to the active site.
Induced fit: Conformational changes that occur in the enzyme upon substrate binding.

Key Concepts

Enzymes are biological catalysts that speed up reactions without being consumed.
Enzyme activity is influenced by various factors including substrate concentration, pH, and temperature.
Enzyme inhibition can be competitive or noncompetitive.
Enzyme activity is regulated by allosteric mechanisms, covalent modification, and other mechanisms.

Enzymes

Protein catalysts that increase the rate of reactions and decrease the energy of the reaction without being changed in the overall process
Enzyme-catalyzed reactions are highly efficient, proceeding from 10³-10⁸ times faster than uncatalyzed reactions.
All reactions in the body are mediated by enzymes.
Lack of enzymes will lead to a block in metabolic pathways causing inborn errors of metabolism.
The substance upon which an enzyme acts is called the substrate.
The enzyme will convert the substrate into the product or products.
Turnover number is the number of molecules of substrate converted to product per enzyme molecule per second.
Each enzyme is assigned two names:
- A short, recommended name, convenient for everyday use
- A more complete, systematic name, used when an enzyme must be identified unambiguously.

Classification of Enzymes

Oxidoreductase: Catalyze oxidation-reduction reactions, such as conversion of lactate to pyruvate by lactate dehydrogenase.
Transferase: Catalyze transfer of C-, N-, or P- containing groups, such as conversion of serine to glycine by serine hydroxymethyl transferase.
Hydrolases: Catalyze cleavage of bonds by addition of water, e.g. Urease convert urea to CO₂ and NH₃.
Lyases: Cleave without adding water, catalyze cleavage of C-C, C-S and certain C-N bonds, such as cleavage of pyruvate by pyruvate decarboxylase into acetaldehyde.
Isomerases: Catalyze racemization of optical or geometric isomers, such as conversion of methylmalonyl CoA to Succinyl CoA by methylmalonyl CoA mutase.
Ligases: ATP dependent condensation of two molecules, e.g. acetyl CoA carboxylase.

Holoenzymes

Refer to the active enzyme with its nonprotein component.
Apoenzyme refers to the inactive enzyme without its nonprotein part.
If the nonprotein part is a metal ion such as Zn²⁺ or Fe²⁺, it is called a cofactor.
If the nonprotein part is a small organic molecule, it is called a coenzyme.

Coenzymes

Low molecular weight organic substances, without which the enzyme cannot exhibit any reaction.
Function of coenzymes and cofactors: Essential for the biological activity of the enzyme.
Cofactor is a metallic ion that is present in some enzymes and shows higher activity, for example, calcium activates lipase.
Coenzymes that only transiently associate with the enzyme are called co-substrates.
If the coenzyme is permanently associated with the enzyme and returned to its original form, it is called a prosthetic group, e.g. FAD.
Coenzymes are frequently derived from vitamins. For example, NAD+ contains niacin and FAD contains riboflavin.

Active Sites

Enzyme molecules contain a special pocket or cleft called the active site.
The active site contains amino acid chains that participate in substrate binding and catalysis.
These amino acid chains are called catalytic residues or catalytic groups.
For example, Proteolytic enzymes have a serine residue at the active center, which is called serine proteases.
The specific substrate binds to the active site.

Enzyme-Substrate Interactions

The substrate binds the enzyme, forming an enzyme-substrate (ES) complex.
Binding is thought to cause a conformational change in the enzyme (induced fit) that allows catalysis.
ES is converted to an enzyme-product (EP) complex that subsequently dissociates to enzyme and product.

Michaelis-Menten Equation

A reaction model that explains the formation of an enzyme-substrate complex.
Describes the relationship between the reaction rate, substrate concentration, and the enzyme's catalytic properties.
Equation:
- V₀ = (V_max [S]) / (K_m + [S])
V₀ = initial reaction velocity
V_max = maximal velocity
K_m = Michaelis constant = (k_-1 + k₂) / k₁
[S] = substrate concentration

Factors Affecting Enzyme Activity

Enzyme Concentration:
- Velocity of reaction is increased proportionally with the concentration of the enzyme when substrate concentration is unlimited.
Substrate Concentration:
- As substrate concentration is increased, the velocity is also increased in the initial phases, but the curve flattens afterwards.
- The maximum velocity thus obtained is called Vmax or the point of saturation.
Concentration of Products:
- When product concentration is increased, the reaction is slowed, stopped or even reversed.
Temperature:
- Increase of velocity with temperature.
- Decrease of velocity with higher temperature.
Effect of pH:
- Each enzyme has an optimum pH, on both sides of which the velocity will be drastically reduced.
- The pH decides the charge on the amino acid residues at the active site.
- Usually, enzymes have the optimum pH between 6 and 8.
- Some important exceptions are Pepsin (with optimum pH 1-2), alkaline phosphatase (optimum pH 9-10) and acid phosphatase (4-5).

Inhibition of Enzyme Activity

An inhibitor is any substance that can diminish the velocity of an enzyme-catalyzed reaction.
Irreversible inhibitors bind to the enzyme through covalent bonds.
Reversible inhibitors typically bind to the enzyme through noncovalent bonds.
The most common inhibitions are either competitive or noncompetitive.

Competitive Inhibition

The inhibitor binds to the same active site of the enzyme because the inhibitor is similar in structure to the substrate.
Inhibitor molecules compete with the normal substrate molecules for attaching to the active site of the enzyme.
- E + S → E-S → E + P
- E + I → E-I
If the substrate concentration is enormously high when compared to the inhibitor, then the inhibition is reversed.

Examples of Competitive Inhibition

Succinate Dehydrogenase reaction is inhibited by malonate, which is a structural analog of succinate.
Dihydropteroate Synthase (enzyme in prokaryotes involved in the synthesis of folic acid needed for DNA synthesis) is inhibited by sulphonamide (antibacterial agent), which is structurally similar to PABA.
Dihydrofolate Reductase is inhibited by methotrexate (anticancer agent), structurally similar to folic acid.

Statin Drugs as Competitive Inhibitors

This group of antihyperlipidemic agents inhibits the first committed step in cholesterol synthesis.
This reaction is catalyzed by hydroxymethylglutaryl-CoA reductase (HMG-CoA reductase).
Statin drugs, such as atorvastatin (Lipitor) and pravastatin (Pravachol), are structural analogs of the natural substrate for this enzyme and compete effectively to inhibit HMG-CoA reductase.
By doing so, they inhibit de novo cholesterol synthesis, thereby lowering plasma cholesterol levels.

Noncompetitive Inhibition

The inhibitor and substrate bind to different sites on the enzyme.
An increase in the substrate concentration generally does not relieve this inhibition.
A variety of poisons, such as iodoacetate, heavy metal ions (silver, mercury) and oxidizing agents, act as irreversible noncompetitive inhibitors.

Examples of Noncompetitive Inhibition

Lead:
- Forms covalent bonds with sulfhydryl side chains of cysteine in proteins.
- Ferrochelatase, an enzyme that catalyzes the insertion of Fe²⁺ into protoporphyrin (a precursor of heme) is an example of an enzyme sensitive to inhibition by lead.
Insecticides:
- Their neurotoxic effects are a result of their covalent binding to acetylcholine esterase (enzyme that catalyzes the cleavage of the neurotransmitter, acetylcholine).

Enzyme Inhibitors as Drugs

At least half of the ten most commonly dispensed drugs in the United States act as enzyme inhibitors.
- The widely prescribed β-lactan antibiotics, such as penicillin and amoxicillin, act by inhibiting enzymes involved in bacterial cell wall synthesis.
- Drugs may also act by inhibiting extracellular reactions, as illustrated by angiotensin-converting enzyme (ACE) inhibitors. They lower blood pressure by blocking the enzyme that cleaves angiotensin I to form the potent vasoconstrictor, angiotensin II.

Regulation of Enzyme Activity

Allosteric Regulation:
- Allosteric enzymes are regulated by molecules called effectors (also called modifiers) that bind noncovalently at a site other than the active site.
- Function of allosteric effectors:
  - Alter the affinity of the enzyme for its substrate.
  - Modify the maximal catalytic activity of the enzyme.
- Negative effectors: effectors that inhibit enzyme activity.
- Positive effectors: effector that increase enzyme activity.
Homotropic Effectors:
- When the substrate itself serves as an effector, the effect is said to be homotropic.
- Most often, an allosteric substrate functions as a positive effector.
Heterotropic Effectors:
- The effector may be different from the substrate, in which case the effect is said to be heterotropic.
- Heterotropic effectors are commonly encountered, for example, the glycolytic enzyme phosphofructokinase-1 is allosterically inhibited by citrate, which is not a substrate for the enzyme.
Covalent Modification:
- Many enzymes may be regulated by covalent modification, most frequently by the addition or removal of groups such as peptide chains or phosphate groups.
Phosphorylation and Dephosphorylation:
- It is the addition or removal of phosphate groups from specific serine, threonine, or tyrosine residues of the enzyme.
- Protein phosphorylation is recognized as one of the primary ways in which cellular processes are regulated.
- Phosphorylation reactions are catalyzed by a family of enzymes called protein kinases that use adenosine triphosphate (ATP) as a phosphate donor.
- Dephosphorylation reaction: Phosphate groups are cleaved from phosphorylated enzymes by the action of phosphoprotein phosphatases.
Partial Proteolysis:
- Another type of activation by covalent modification is the conversion of an inactive proenzyme or zymogen to the active enzyme.
- Example: splitting a single peptide bond, and removal of a small polypeptide from trypsinogen, the active trypsin is formed. This results in unmasking of the active center.
- Note: All the gastrointestinal enzymes are synthesized in the form of proenzymes, and only after secretion into the alimentary canal, they are activated. This prevents autolysis of cellular structural proteins.

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Test your understanding of the Michaelis-Menten equation and factors influencing enzyme activity. This quiz covers initial reaction rates, enzyme saturation, and optimal conditions for enzymatic reactions. Perfect for anyone studying biochemistry!