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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.
-Considering that NSPs are proteases, which of the following statements best describes the reaction catalyzed by NSPs?

Question

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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.
-Considering that NSPs are proteases, which of the following statements best describes the reaction catalyzed by NSPs?

Quizplus · Accepted Answer

11ecba2d_0dd6_3794_a3bf_45091decc2bc_MD0008_00 Peptides are linear sequences of amino acids linked by amide bonds (-CO-NH-) that join the α-carboxyl group of one amino acid to the α-amino group of another amino acid. Proteins that have completed their biological purpose must be degraded by proteases so that their constituent amino acids can be reused to generate new peptides. To cleave an amide bond during protein degradation, a water molecule (H₂O) must be added using acids (H⁺) or bases (OH⁻) in a reaction known as amide hydrolysis. Figure 1 shows the acylation process (R-C=O addition) of amide hydrolysis by NSP4, which cleaves the amide bond of the viral peptide by adding a hydrogen atom to the nitrogen. The hydrogen atom displaces the C-N single bond when electrons from the oxygen on the carbonyl carbon reform the double bond. The acyl group (R₁-C=O) attached to NSP4 is subsequently converted into carboxylic acid when electrons from an OH⁻ group attack the electrophilic (electron-attracting) carbonyl carbon. (Choices A, C and D) Hydrolysis reactions only involve consumption of water to break bonds. They do not involve water production, which occurs in condensation reactions such as peptide bond formation. Educational objective: Proteins can be degraded by protease enzymes. During peptide hydrolysis, a water molecule is used to cleave the C-N bond in the amide linkage using an acid (H⁺) or base (OH⁻). In contrast, the reverse of amide hydrolysis is amide bond condensation, which involves the formation of a water molecule from two amino acids to form a larger peptide.

Passage The Innate Immune System Relies Heavily on the Phagocytic Action