ENZYMES, BIOCHEM

Exam

Q: What is the main function of enzymes?

A: Catalyze biochemical reactions in cells, increasing reaction rates without being consumed.

Q: What is the substrate of an enzyme?

A: The molecule upon which an enzyme acts

Q: Where does catalysis occur in an enzyme?

A: At the active site, a specific region of the enzyme.

Q: Are all enzymes proteins?

A: Nearly all, but some RNA molecules (ribozymes) also have catalytic activity.

Q: What is meant by enzyme specificity?

A: Ability to select a particular substrate and catalyze a specific reaction.

Q: Give examples of types of enzyme specificity.

A: Absolute (one substrate), Group-specific (similar substrates), Broad (hydrolases like papain).

Q: Name the six main enzyme classes.

A: Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases.

Q: What is a holoenzyme?

A: Apoenzyme (protein) + cofactor (inorganic ion or organic molecule)

Q: Give examples of enzyme cofactors.

A: Mg²⁺, Zn²⁺, Fe²⁺, Mn²⁺, Cu²⁺, Ni²⁺.

Q: Give examples of coenzymes and their dietary precursors.

A: NAD⁺ (niacin), FAD (riboflavin), CoA (pantothenic acid), Biocytin (biotin).

Q: How do metal ions assist enzymes?

A: Orient substrates, mediate redox reactions, stabilize charges.

Q: Describe the enzyme-substrate complex (ES).

A: Temporary complex formed when enzyme binds substrate at the active site

Q: What is the transition state in enzyme catalysis?

A: The high-energy intermediate between substrate and product.

Q: How do enzymes affect activation energy and reaction equilibrium?

A: Lower activation energy; equilibrium position remains unchanged.

Q: Explain the difference between the Fischer “lock-and-key” and Koshland “induced-fit” models.

• A: Lock-and-key: rigid active site; Induced-fit: active site adapts to substrate.

Q: What factors influence enzyme reaction rates?

A: Substrate concentration, enzyme concentration, temperature, pH, inhibitors, activators.

Q: Define Michaelis constant (Km).

A: Substrate concentration at which reaction rate is half of Vmax.

Q: Difference in kinetics when [S] << Km vs. [S] >> Km.

A: << Km: rate depends on [S]; >> Km: rate depends on [E].

Q: Competitive vs non-competitive inhibition.

A: Competitive: inhibitor binds active site, increases Km, Vmax unchanged. Non-competitive: binds elsewhere, Vmax decreases, Km unchanged.

Q: What is allosteric regulation?

A: Modulation of enzyme activity via binding at a site other than the active site.

Q: Homotropic vs heterotropic allosteric modulators.

A: Homotropic: substrate itself is modulator; Heterotropic: different molecule.

Q: Example of feedback inhibition in enzymes.

A: Isoleucine inhibits threonine dehydratase in L-threonine → L-isoleucine pathway.

Q: How do covalent modifications regulate enzymes?

A: Phosphorylation, acetylation, or other modifications reversibly alter activity.

Q: Give examples of enzymes regulated by phosphorylation.

A: Glycogen phosphorylase, pyruvate dehydrogenase, acetyl-CoA carboxylase.

Q: What are isozymes?

A: Enzymes with different amino acid sequences that catalyze the same reaction.

Q: Example of tissue-specific isozyme and clinical relevance.

A: LDH-H4 in heart (increased in myocardial infarction), CK-MB in heart damage.

Q: What is the difference between functional and non-functional plasma enzymes?

A: Functional: secreted actively for physiological roles; Non-functional: intracellular enzymes released due to cell damage.

Q: Give examples of functional plasma enzymes.

A: Coagulation enzymes, LCAT.

Q: How does enzyme localization relate to metabolic pathways?

A: Enzymes are localized where their pathways occur (cytoplasm, mitochondria, specialized cells).

Q: Why do hexokinase and glucokinase have different kinetics?

A: Hexokinase: low Km, active in muscle; Glucokinase: high Km, regulates liver glucose uptake.