Enzyme Activity and Reaction Velocity

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Questions and Answers

What is the main role of enzymes in biological reactions?

To change the structure of substrates

To provide energy for reactions

To act as catalysts without being consumed (correct)

To increase the temperature of the reaction

Which classification of enzymes involves the transfer of functional groups?

Transferases (correct)

Isomerases

Hydrolases

Lyases

What type of reaction does an oxidoreductase enzyme catalyze?

Isomerization

Hydrolysis

Ligation

Redox reactions (correct)

What feature characterizes the active site of an enzyme?

It is a site where substrate binding and catalysis occur Signup and view all the answers

Which of the following is NOT a type of enzyme classification based on reaction type?

Covalentases Signup and view all the answers

What is an example of a nonprotein moiety that can participate in enzyme catalysis?

Prosthetic group Signup and view all the answers

Which statement about the efficiency of enzymes is correct?

Enzymes can increase reaction rates by factors of up to $10^9$ Signup and view all the answers

Which type of enzyme is responsible for the hydrolytic cleavage of covalent bonds?

Hydrolases Signup and view all the answers

What is the effect of competitive inhibition on Km?

Increases Km Signup and view all the answers

Which statement about Vmax in competitive inhibition is true?

Vmax is unchanged Signup and view all the answers

What distinguishes noncompetitive inhibition from competitive inhibition?

Noncompetitive inhibitors do not affect Km Signup and view all the answers

In a Lineweaver-Burk plot, how do the lines for noncompetitive inhibition appear?

They intersect at the y-axis Signup and view all the answers

Which type of effectors are substrates that function as positive effectors in allosteric enzymes?

Homotropic effectors Signup and view all the answers

What is the effect of noncompetitive inhibition on the apparent Vmax of a reaction?

Apparent Vmax is decreased Signup and view all the answers

What is an example of a competitive inhibitor mentioned?

HMG CoA reductase inhibitors Signup and view all the answers

What characterizes heterotropic effectors in allosteric enzymes?

They differ from substrates Signup and view all the answers

What characterizes holoenzymes in relation to apoenzymes?

They contain nonprotein moieties and are active. Signup and view all the answers

Which coenzyme is derived from riboflavin?

FMN Signup and view all the answers

How does acid-base catalysis differ between specific and general catalysis?

General catalysis involves participation from multiple acids or bases. Signup and view all the answers

What role do coenzymes play in enzymatic reactions?

They are non-protein molecules that participate in catalysis. Signup and view all the answers

What is the primary mechanism by which catalysis by strain operates?

By distorting the enzyme's structure to weaken substrate bonds. Signup and view all the answers

Which of the following metals is classified as a cofactor?

Iron Signup and view all the answers

What is the signature characteristic of covalent catalysis in enzymatic reactions?

A covalent bond is formed between the enzyme and substrate. Signup and view all the answers

In which mechanism is the concentration of substrates directly related to the reaction rate?

Catalysis by proximity Signup and view all the answers

What effect does lowering the activation energy have on the rate of a reaction?

It increases the reaction rate. Signup and view all the answers

Which factor does NOT affect the velocity of enzyme-catalyzed reactions?

Product concentration Signup and view all the answers

At what condition does the reaction velocity become constant and equal to Vmax?

When [S] > Km Signup and view all the answers

What is the significance of the Michaelis constant (Km)?

It measures the affinity between enzyme and substrate. Signup and view all the answers

What occurs when the temperature exceeds the stability limit of human enzymes?

The enzyme undergoes denaturation. Signup and view all the answers

In the context of enzyme kinetics, what does a high Km value indicate?

Low affinity for substrate. Signup and view all the answers

What is the result of plotting the reciprocal of reaction velocity (Vo) against substrate concentration ([S])?

A straight line. Signup and view all the answers

In an enzyme-catalyzed reaction, increasing substrate concentration has which effect until Vmax is reached?

It directly increases reaction velocity. Signup and view all the answers

Which compound allosterically inhibits phosphofructokinase-1?

Citrate Signup and view all the answers

Which process is responsible for activating zymogens?

Proteolytic action of enteropeptidase Signup and view all the answers

What is the typical time required for a change in enzyme synthesis or degradation?

Hours to days Signup and view all the answers

What is the result of product inhibition in enzymatic reactions?

Change in Vmax and/or Km Signup and view all the answers

Which enzyme modification typically activates the enzyme?

Phosphorylation Signup and view all the answers

What role does insulin play in enzyme regulation?

Inducing synthesis of enzymes involved in glucose metabolism Signup and view all the answers

Which of the following is NOT a mechanism of enzyme regulation?

Zymogen degradation Signup and view all the answers

How does allosteric control affect enzyme activity?

Changes in Vmax and/or K0.5 Signup and view all the answers

Study Notes

Enzyme Activity

High activation energy (Eact) in uncatalyzed reactions results in slow reaction rates.
Lower activation energy leads to faster reaction rates.
Enzymes provide an alternative reaction pathway that lowers activation energy and does not affect the free energy of substrates and products.

Factors Affecting Reaction Velocity

Substrate concentration: The rate of an enzyme-catalyzed reaction increases with substrate concentration until a maximal velocity (Vmax) is reached.
Temperature: Increasing temperature increases the rate of both uncatalyzed and enzyme-catalyzed reactions by increasing kinetic energy and the collision frequency of reacting molecules.
- Higher temperatures can lead to denaturation and loss of catalytic activity.
- Human enzymes generally exhibit stability up to 45-55℃.
pH: Most intracellular enzymes exhibit optimal activity at pH values between 5-9.
- Extremes of pH can lead to enzyme denaturation.

Michaelis-Menten Kinetics

Michaelis-Menten Equation:
- Vo = initial reaction velocity
- Vmax = maximal velocity = kcat [E]total
- Km = Michaelis constant = (k-1 + k2)/k1
- [S] = substrate concentration
Assumptions:
- Substrate concentration is greater than enzyme concentration.
- Concentration of the ES complex does not change with time (steady-state assumption).
- Initial reaction velocities are used in the analysis of enzyme reactions.
Km:
- Represents the substrate concentration at which the reaction velocity is equal to ½ Vmax.
- Does not vary with enzyme concentration.
- High Km indicates low affinity.
- Low Km indicates high affinity.

Enzyme Classification by Reaction Type

Oxidoreductases: Catalyze redox reactions.
Transferases: Transfer moieties such as glycosyl, methyl, or phosphoryl groups.
Hydrolases: Catalyze the hydrolytic cleavage of C—C, C—O, C—N, and other covalent bonds.
Lyases (synthases): Catalyze cleavage of covalent bonds by atom elimination, generating double bonds.
Isomerases: Catalyze geometric or structural changes within a molecule.
Ligases (synthetases): Join together (ligation) of two molecules in reactions coupled to the hydrolysis of ATP.

Enzyme Structure and Function

Active Site: Site where catalysis occurs.
- Formed by the folding of proteins.
- Participates in substrate binding and catalysis.
Nonprotein Moieties: Participate in catalysis.
- Prosthetic groups: Tightly and stably incorporated into a protein's structure by covalent bonds or noncovalent forces.
  - Eg. Pyridoxal phosphate, flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), thiamine pyrophosphate, lipoic acid, biotin, transition metals – Fe, Co, Cu, Mg, Mn, Zn.
- Cofactors: Similar to prosthetic groups, but bind weakly and transiently to their cognate enzymes or substrates.
- Coenzymes: Small organic molecules derived from water-soluble B vitamins.
  - Eg. NAD and NADP – nicotinamide, FMN and FAD – riboflavin, Coenzyme A – pantothenic acid.
Holoenzyme: An active enzyme with its nonprotein moiety.
Apoenzyme: An enzyme without its nonprotein moiety.

Mechanism of Enzyme Action

Enzyme binds substrate at the active/catalytic site.
Active site fits the shape of the substrate.
Association between enzyme and substrate is temporary.

Four Mechanistic Strategies During Catalysis

Catalysis by proximity: Increases the frequency of substrate encounters.
Acid-base catalysis: Ionizable functional groups of aminoacyl side chains act as acids or bases.
- Specific acid or base catalysis: Participating acids or bases are protons or hydroxide ions.
- General acid or base catalysis: Reactions responsive to all acids or bases present.
Catalysis by strain: Creates a conformational change in an enzyme that weakens bonds.
- Transition state intermediate: A strained conformation midway in the transformation of substrates to products.
Covalent catalysis: Formation of a covalent bond between enzymes and one or more substrates.
- Ping pong mechanism: First substrate is bound and its product is released prior to the bonding of the second substrate.

Enzyme Inhibitors

Substances that decrease the velocity of an enzyme-catalyzed reaction.
- Reversible inhibitors: Noncovalent bonds.
- Irreversible inhibitors: Covalent bonds.
Competitive inhibition: Inhibitor binds reversibly to the same site that the substrate would normally occupy.
- Effect on Vmax: Unchanged.
- Effect on Km: Increases Km.
- Effect on Lineweaver-Burk plot: Inhibited and uninhibited reactions intersect on the y-axis at 1/Vmax and have different x-intercepts at -1/Km.
Noncompetitive inhibition: Inhibitor and substrate bind at different sites on the enzyme.
- Inhibitor can bind either free enzyme or the enzyme-substrate complex.
- Effect on Vmax: Decreases apparent Vmax.
- Effect on Km: Unchanged Km.
- Effect on Lineweaver-Burk plot: Unchanged Km, decreased apparent Vmax.

Allosteric Enzymes

Effectors: Bind noncovalently at a site other than the active site (allosteric site).
- Negative effectors: Inhibitors.
- Positive effectors: Increase enzyme activity.
Homotropic effectors:
- Substrates as effectors.
- Function as positive effectors.
- Sigmoidal curve.
- Positive cooperativity.
- Eg. Hemoglobin.
Heterotropic effectors:
- Effectors different from substrate.
- Exemplified by feedback inhibition.
- Eg. Phosphofructokinase-1 is allosterically inhibited by citrate (not a substrate).

Covalent Modification

Phosphorylation and dephosphorylation:
- Phosphorylation: Protein kinases use ATP as a phosphate donor, activating the enzyme.
- Dephosphorylation: Phosphoprotein phosphatase deactivates the enzyme.
Activation of zymogens:
- Activation via proteolytic action of enteropeptidase (enterokinase) of trypsinogen → trypsin.
- Trypsin catalyzes the subsequent conversion of numerous other pancreatic zymogens.

Enzyme Synthesis

Cells can regulate the amount of enzyme present by altering the rate of enzyme synthesis.
Increase or decrease of enzyme synthesis leads to alteration in the total population of active sites.
Eg. Insulin production in response to hyperglycemia – induction of enzymes involved in glucose metabolism.

Mechanisms of Enzyme Regulation

Regulation Event: Typical Effector, Results, Time Required for Change
- Substrate availability: Substrate - Change in velocity - Immediate
- Product inhibition: Reaction product - Change in Vmax and/or Km - Immediate
- Allosteric control: Pathway end product - Change in Vmax and/or K0.5 - Immediate
- Covalent modification: Another enzyme - Change in Vmax ad/or Km - Immediate to minutes
- Synthesis or degradation of enzyme: Hormone or metabolite - Change in the amount of enzyme - Hours to days

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Description

Explore the fundamental concepts of enzyme activity and the factors affecting reaction rates in this quiz. Learn how activation energy, substrate concentration, temperature, and pH influence enzyme-catalyzed reactions. Test your knowledge of how enzymes operate and their importance in biological processes.