Question 1

Which of the following best describes the mechanism of action for suicide inhibitors?

Accepted Answer

They mimic the initial steps of catalysis, forming a reactive complex that covalently modifies and destroys the enzyme.

Question 2

Botulinum toxin (BOTOX) acts as an irreversible inhibitor by:

Accepted Answer

Cleaving SNAP-25, a protein involved in neurotransmitter release.

Question 3

Chymotrypsin is classified as a serine protease because:

Accepted Answer

It uses a serine residue in its active site to form a nucleophilic alkoxide for peptide bond cleavage.

Question 4

Which amino acids form the catalytic triad in chymotrypsin's active site?

Accepted Answer

$$\mathrm{Ser195}$$, $$\mathrm{His57}$$, $$\mathrm{Asp102}$$

Question 5

In the mechanism of RNase A, which residue acts as a general base to extract a proton from the 2'OH of RNA during the first step?

Accepted Answer

$$\mathrm{His12}$$

Question 6

Enolase catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyruvate. What is the role of $$\mathrm{Mg$$^{2+}$$}$$ ions in this reaction?

Accepted Answer

They acidify the C2 proton, facilitating its removal and completing the dehydration.

Question 7

Lysozyme catalyzes the cleavage of the glycosidic bond between which two sugars in bacterial cell walls?

Accepted Answer

GlcNac and MurNac

Question 8

During the lysozyme Sn2 mechanism, which residue attacks the C1 of MurNac to form a covalent intermediate?

Accepted Answer

$$\mathrm{Asp52}$$

Question 9

Which statement best describes feedback inhibition in metabolic pathways?

Accepted Answer

The final product of a pathway allosterically inhibits an enzyme early in the pathway.

Question 10

Allosteric regulation of enzymes can involve which of the following mechanisms?

Accepted Answer

Reversible binding/release events or covalent modifications such as phosphorylation.

Question 11

In the PhoQP system in bacteria, what is the function of PhoQ?

Accepted Answer

It is an allosteric kinase that senses $$\mathrm{Mg$$^{2+}$$}$$ levels and signals low $$\mathrm{Mg$$^{2+}$$}.$$

Question 12

Which amino acids are most commonly phosphorylated in eukaryotic proteins?

Accepted Answer

Tyr, Ser, and Thr

Question 13

What is the effect of adding a phosphate group ($$\mathrm{PO_4$^{3-}}) to a $$protein?

Accepted Answer

It adds a bulky negative charge, which can affect structure and ionic interactions, potentially activating or inactivating the protein.

Question 14

A zymogen is best described as:

Accepted Answer

An inactive precursor form of an enzyme that requires proteolytic cleavage for activation.

Question 15

Which of the following statements about caspases is TRUE?

Accepted Answer

Caspases are proteases synthesized as inactive proenzymes and activated during apoptosis.

Question 16

Which of the following is a multi-step, two-stage catalytic process that does NOT form a covalent intermediate?

Accepted Answer

RNase A catalysis

Question 17

A protein can be phosphorylated at multiple sites. What is the consequence of this for protein function?

Accepted Answer

The number and location of phosphates can affect protein function or cellular location.

Question 18

Which of the following best explains why serine, histidine, and aspartate form a catalytic triad in serine proteases?

Accepted Answer

Their side chains form a hydrogen-bonding network that alters the pKa of histidine, enabling serine to act as a nucleophile.

Question 19

Which of the following is TRUE regarding allosteric enzymes?

Accepted Answer

Allosteric enzymes can be regulated positively or negatively, and most regulatory enzymes are allosteric.

Enzyme Mechanisms and Regulation in Biochemistry

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