Biochemistry Enzyme Models - Fischer's Theory

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Questions and Answers

What principle underlies Emil Fischer’s lock-and-key model?

Enzymes and substrates have complementary molecular geometries (correct)

All enzymes can bind multiple types of substrates

Enzymes change shape to accept substrates

Enzymes operate independently of substrate shape

Which of the following best describes Koshland’s induced-fit hypothesis?

Enzymes always retain a fixed shape regardless of substrates

Enzymes have rigid structures that do not allow flexibility

Enzymes only function with specific substrates

Enzymes change conformation upon substrate binding (correct)

What type of enzyme is trypsin categorized as, based on its specificity?

A ribozyme

An allosteric enzyme

An absolute enzyme (correct)

An isoenzyme

Which of the following is NOT a characteristic of cofactors?

They are always organic molecules (C) Signup and view all the answers

In the context of enzyme functionality, what are allosteric enzymes characterized by?

They undergo conformational changes in response to effector molecules (D) Signup and view all the answers

Which statement accurately describes ribozymes?

They are RNA molecules that can catalyze chemical reactions (B) Signup and view all the answers

What determines the ability of an enzyme to discriminate between D and L isomers?

The enzyme's absolute specificity (B) Signup and view all the answers

Which of the following statements about metalloenzymes is true?

They include metal ions such as Zn, Fe, and Mg (B) Signup and view all the answers

What is the primary effect of increasing substrate concentration on enzyme activity?

It leads to more frequent collisions between substrate and enzyme. (B) Signup and view all the answers

How does high salt concentration affect enzyme function?

It can lead to denaturation of the enzyme. (A) Signup and view all the answers

What happens when an enzyme becomes saturated?

The enzyme can no longer bind additional substrate molecules. (D) Signup and view all the answers

What is the main effect of competitive inhibition on enzyme activity?

It prevents substrate binding by occupying the active site. (B) Signup and view all the answers

What is a consequence of enzyme denaturation?

It leads to a permanent loss of enzyme activity. (D) Signup and view all the answers

What would be the outcome on Km and Vmax in the presence of a competitive inhibitor?

Km is increased; Vmax is unchanged. (B) Signup and view all the answers

What can alter the ionic interactions between an enzyme and its substrate?

Alterations in salinity or salt concentration. (B) Signup and view all the answers

Which enzyme operates optimally at a pH of 8?

Trypsin (A) Signup and view all the answers

What effect does salinity have on enzyme functions?

Each enzyme has an optimal salt concentration. (A) Signup and view all the answers

What does an increase in enzyme concentration lead to?

An increase in reaction rate until substrate becomes limiting. (C) Signup and view all the answers

What type of inhibition does Disulfiram exhibit when blocking enzyme action in chronic alcoholism?

Competitive inhibition (B) Signup and view all the answers

What is the result of alterations in substrate availability due to high salt concentration?

Decreased overall reaction rate due to less soluble substrates. (B) Signup and view all the answers

Which scenario describes non-competitive inhibition?

Inhibitor binds to a site other than the active site, changing enzyme shape. (C) Signup and view all the answers

What relationship exists between substrate concentration and enzyme activity prior to saturation?

Increasing substrate concentration leads to a proportional increase in reaction rate. (A) Signup and view all the answers

Why is regulation of enzyme activity essential for organisms?

To prevent all enzyme reactions from occurring simultaneously. (C) Signup and view all the answers

What is a characteristic of optimal pH for most human enzymes?

It is generally between 6 and 8. (B) Signup and view all the answers

What is the effect on Vmax when allosteric enzymes are regulated negatively?

Vmax is decreased (A) Signup and view all the answers

Which type of effector acts as its own substrate in allosteric enzymes?

Homotropic effector (C) Signup and view all the answers

What is a characteristic of uncompetitive inhibition?

It binds only to enzyme-substrate complexes (B) Signup and view all the answers

How does the Km value change in the presence of uncompetitive inhibitors?

Decreases (C) Signup and view all the answers

What type of modification is most commonly seen in the regulation of enzymes?

Covalent modifications (C) Signup and view all the answers

Which amino acid residues are most often involved in covalent modification?

Serine, threonine, and tyrosine (A) Signup and view all the answers

In the context of allosteric enzyme regulation, what do positive effectors do?

Enhance the enzyme activity (B) Signup and view all the answers

What is the impact of noncompetitive inhibition on Km and Vmax?

Km is unchanged; Vmax is decreased (D) Signup and view all the answers

What role do protein kinases play in phosphorylation reactions?

They catalyze phosphorylation reactions. (D) Signup and view all the answers

How does phosphorylation affect enzyme activity?

The effect depends on the specific enzyme. (C) Signup and view all the answers

What is the significance of enzyme induction and repression?

They alter the total population of active sites in tissues. (C) Signup and view all the answers

Why are plasma enzymes important in clinical diagnosis?

Increased levels often indicate tissue damage. (C) Signup and view all the answers

What does the Michaelis-Menten equation primarily describe?

The relationship between substrate concentration and enzyme activity. (B) Signup and view all the answers

What can be inferred from increased alanine aminotransferase (ALT) levels in the plasma?

It indicates potential liver damage. (A) Signup and view all the answers

What is the role of ATP in phosphorylation reactions?

It acts as a phosphate donor in phosphorylation. (C) Signup and view all the answers

Which aspect is NOT typically affected by enzyme phosphorylation?

Enzyme irreversibility. (A) Signup and view all the answers

Study Notes

Emil Fischer’s Lock-and-Key Model

Proposed in 1895, it describes enzyme-substrate interaction based on complementary molecular geometries.
Enzymes depicted as rigid structures that explain enzyme specificity.
Limitations revealed as enzyme structures were better understood; not all enzymes fit the rigid model.

Cofactors

Vital for enzyme activity; includes organic or metal-based compounds.
Types of cofactors:
- Metalloenzymes: Contain metal ions like Zn, Fe, Mg, and Cu.
- Coenzymes: Organic molecules such as NAD, NADP, and ATP.
- Prosthetic Groups: Covalently bonded cofactors.

Absolute Specificity

Certain enzymes accept only one type of molecule.
Example: Trypsin can discriminate between D and L isomers.

Koshland’s Induced-fit Hypothesis

Introduced in 1958, proposing enzymes are flexible and change shape upon substrate binding.
Suggests substrate binding alters enzyme conformation, facilitating the catalytic process.
Emphasizes weak intermolecular interactions sustaining protein structure.

Factors Affecting Enzyme Function

Substrate Concentration

Higher substrate concentrations increase reaction rates but levels off when enzymes are saturated.

Enzyme Concentration

Increased enzyme levels enhance reaction rates until substrate availability limits activity.

Competitive Inhibition

Inhibitors compete for the active site, affecting enzyme's 3D structure without permanently denaturing it.
Km increases while Vmax remains unchanged.

Optimal pH

Most human enzymes have optimal activity between pH 6-8; exceptions include:
- Pepsin (stomach): pH 2-3
- Trypsin (small intestine): pH 8

Salinity

Each enzyme has an optimal salt concentration for maximal activity.
High salinity can alter internal ionic bonds, affecting enzyme function and stability.

Enzyme Inhibition Examples

Penicillin: Inhibits bacterial enzymes required for cell wall synthesis.
Disulfiram: Treats alcoholism by blocking alcohol-metabolizing enzymes.

Regulation of Enzyme Activity

Essential for maintaining metabolic coordination within organisms.
Km remains unchanged for non-competitive inhibitors, while Vmax decreases.

Allosteric Regulation

Allosteric enzymes have two binding sites: active and regulatory.
Effectors can be positive (enhancing activity) or negative (reducing activity).

Covalent Modification

Enzymes can be regulated via phosphorylation (adding a phosphate group) or dephosphorylation (removing it).
Regulation via the addition of phosphate groups predominantly affects serine, threonine, and tyrosine residues.

Clinical Applications of Enzymes

Enzyme activity serves as a diagnostic tool; deviations in enzyme levels in plasma can indicate tissue damage.
Example: Elevated alanine aminotransferase (ALT) levels can signal liver damage.

Michaelis-Menten Equation

Describes the kinetics of enzyme-catalyzed reactions: V = Vmax[S] / (KM + [S])
As substrate concentration increases, the reaction velocity approaches Vmax.

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Description

Explore Emil Fischer's Lock-and-Key model from 1895, which illustrates how enzymes and substrates interact due to their complementary molecular geometries. This model explains the specificity of enzymes, although later structural evidence has revealed its limitations.