Passage
The innate immune system relies heavily on the phagocytic action of neutrophils, mobile white blood cells that engulf and degrade bacteria and viruses as the first line of host defense against invading pathogens. One mechanism for eliminating viruses engulfed in the phagosome involves the degradation of viral proteins by neutrophil serine proteases (NSPs) .NSPs, which form part of the chymotrypsin family of serine proteases, cleave viral proteins using a catalytic triad relay system. Originally discovered in granules released by neutrophils, NSP4 is a novel monomeric serine protease that has been studied to elucidate the catalytic mechanism of NSPs. Prior experimental data suggest that NSP4 alters its tertiary structure upon substrate binding and cleaves viral proteins using the acylation reaction shown in Figure 1.
Figure 1 Degradation of viral proteins by acylation of NSP4Computational methods were used to identify the energetic parameters of reactions between NSP4 and synthetic viral proteins. To ensure that simulated reactions resembled physiological conditions, algorithms were adjusted to account for the enzyme being in an aqueous environment. The rate-determining step in the acylation reaction was determined by calculating the changes in free energy in both the enzyme and substrate (Figure 2) .
Figure 2 Free energy profiles of synthetic viral protein cleavageKnowledge of amino acid frequency distribution at an enzymatic active site may facilitate the design of novel active sites and enzyme-specific inhibitors. Researchers analyzed the composition of amino acids that form the NSP4 active site during protein folding to study the characteristics of residues involved in active site formation (Figure 3) .
Figure 3 Frequency distribution of amino acid residues at the NSP4 active site
Adapted from Stapels DA, Geisbrecht BV, Rooijakkers SH. Neutrophil serine proteases in antibacterial defense. Curr Opin Microbiol. 2015;23:42-8.
-Which of the following is the most accurate statement regarding NSP4 binding of viral proteins?

Question

Passage
The innate immune system relies heavily on the phagocytic action of neutrophils, mobile white blood cells that engulf and degrade bacteria and viruses as the first line of host defense against invading pathogens. One mechanism for eliminating viruses engulfed in the phagosome involves the degradation of viral proteins by neutrophil serine proteases (NSPs) .NSPs, which form part of the chymotrypsin family of serine proteases, cleave viral proteins using a catalytic triad relay system. Originally discovered in granules released by neutrophils, NSP4 is a novel monomeric serine protease that has been studied to elucidate the catalytic mechanism of NSPs. Prior experimental data suggest that NSP4 alters its tertiary structure upon substrate binding and cleaves viral proteins using the acylation reaction shown in Figure 1.

Figure 1 Degradation of viral proteins by acylation of NSP4Computational methods were used to identify the energetic parameters of reactions between NSP4 and synthetic viral proteins. To ensure that simulated reactions resembled physiological conditions, algorithms were adjusted to account for the enzyme being in an aqueous environment. The rate-determining step in the acylation reaction was determined by calculating the changes in free energy in both the enzyme and substrate (Figure 2) .

Figure 2 Free energy profiles of synthetic viral protein cleavageKnowledge of amino acid frequency distribution at an enzymatic active site may facilitate the design of novel active sites and enzyme-specific inhibitors. Researchers analyzed the composition of amino acids that form the NSP4 active site during protein folding to study the characteristics of residues involved in active site formation (Figure 3) .

Figure 3 Frequency distribution of amino acid residues at the NSP4 active site
Adapted from Stapels DA, Geisbrecht BV, Rooijakkers SH. Neutrophil serine proteases in antibacterial defense. Curr Opin Microbiol. 2015;23:42-8.
-Which of the following is the most accurate statement regarding NSP4 binding of viral proteins?

Quizplus · Accepted Answer

11ecba2d_0dd7_7015_a3bf_cdd8f7d3202b_MD0008_00 Two competing theories exist regarding enzyme-substrate interaction, the lock and key theory and the induced fit model. The lock and key theory proposes that an enzyme's active site is already in the proper structural conformation to allow a substrate to bind readily and form an active enzyme-substrate (ES) complex. This theory proposes that no conformational changes are necessary for catalysis to occur. The induced fit model states that a conformational change is induced when the substrate binds the active site, resulting in the formation of a functional ES complex that is said to be in the induced form. Enzymes are highly specific for their substrates, and only the correct substrate can induce the required complementary change in active site conformation. Shifting an enzyme's tertiary or quaternary structure to alter the shape of the active site requires energy. The passage states that NSP4 alters its tertiary structure upon substrate binding, so NSP4 most likely undergoes a conformational change that requires energy upon substrate binding. (Choice B) Quaternary structure is only present in multimeric proteins that have multiple subunits. The passage states that NSP4 is a monomer (one subunit), so it does not have quaternary structure. (Choice C) When the correct substrate enters and is bound by an enzyme's active site, heat is released (not consumed) from the formation of weak bonds (hydrogen, hydrophobic, ionic bonds) between the two molecules. (Choice D) Cofactors are molecules required for enzyme catalysis. Enzymes that require cofactors for activity are called apoenzymes when the cofactor is absent whereas those containing cofactors are known as holoenzymes. The passage does not indicate that NSP4 needs a cofactor for activity. Educational objective: The lock and key model proposes that the active site of an enzyme is already in the correct conformation to bind to a substrate. In contrast, the induced fit theory states that the initial binding of a substrate causes the active site to undergo a conformational change that complements the structure of the substrate.

Passage The Innate Immune System Relies Heavily on the Phagocytic Action