Passage
Oxidative stress occurs in cells when reactive oxygen species (ROS) such as H₂O₂, superoxide (O₂⁻) , and hydroxyl radicals (^•OH) interact destructively with biological molecules. Several studies have indicated that histidine and certain peptides that contain histidine can decrease oxidative stress.A common marker of oxidative stress in cells is leakage of the enzyme lactate dehydrogenase (LDH) into the extracellular space. Researchers incubated cultured rat cells (PC12 cells) with H₂O₂ in the presence of different tripeptides, each containing one alanine, one glycine, and one histidine residue. Every possible sequence was tested, but the researchers found that the sequence GAH was the most effective in limiting extracellular LDH activity. After obtaining these preliminary results, the following experiments were conducted.Experiment 1PC12 cells were incubated with or without 5 mM H₂O₂ and with varying concentrations of GAH for 4 hours. Cells were centrifuged, and the supernatant (ie, extracellular material) was extracted and assayed for LDH activity (Figure 1) .
Figure 1 Extracellular LDH activity from PC12 cells exposed to H₂O₂ and varying concentrations of GAH. p-values are relative to 5 mM H₂O₂ and 0 mg/mL GAH.Experiment 2LDH was purified and assayed for activity in the presence of 5 mM H₂O₂ and varying concentrations of GAH (Figure 2) .
Figure 2 Activity of purified LDH with varying concentrations of GAHExperiment 3PC12 cells were incubated with or without 5 mM H₂O₂ in the presence and absence of 1 mg/mL GAH. Cells were then stained with the fluorescent marker TUNEL, which binds strongly to apoptotic cells. The percentage of apoptotic cells in each condition is shown in Figure 3.
Figure 3 Percentage of apoptotic cells in the presence or absence of H₂O₂ and GAH
-The side chain of the C-terminal amino acid residue in GAH is derived from which structure?

Question

Passage
Oxidative stress occurs in cells when reactive oxygen species (ROS) such as H₂O₂, superoxide (O₂⁻) , and hydroxyl radicals (^•OH) interact destructively with biological molecules. Several studies have indicated that histidine and certain peptides that contain histidine can decrease oxidative stress.A common marker of oxidative stress in cells is leakage of the enzyme lactate dehydrogenase (LDH) into the extracellular space. Researchers incubated cultured rat cells (PC12 cells) with H₂O₂ in the presence of different tripeptides, each containing one alanine, one glycine, and one histidine residue. Every possible sequence was tested, but the researchers found that the sequence GAH was the most effective in limiting extracellular LDH activity. After obtaining these preliminary results, the following experiments were conducted.Experiment 1PC12 cells were incubated with or without 5 mM H₂O₂ and with varying concentrations of GAH for 4 hours. Cells were centrifuged, and the supernatant (ie, extracellular material) was extracted and assayed for LDH activity (Figure 1) .

Passage Oxidative stress occurs in cells when reactive oxygen species (ROS) such as H<sub>2</sub>O<sub>2</sub>, superoxide (O<sub>2</sub><sup>−</sup>) , and hydroxyl radicals (<sup>•</sup>OH) interact destructively with biological molecules. Several studies have indicated that histidine and certain peptides that contain histidine can decrease oxidative stress.A common marker of oxidative stress in cells is leakage of the enzyme lactate dehydrogenase (LDH) into the extracellular space. Researchers incubated cultured rat cells (PC12 cells) with H<sub>2</sub>O<sub>2</sub> in the presence of different tripeptides, each containing one alanine, one glycine, and one histidine residue. Every possible sequence was tested, but the researchers found that the sequence GAH was the most effective in limiting extracellular LDH activity. After obtaining these preliminary results, the following experiments were conducted.Experiment 1PC12 cells were incubated with or without 5 mM H<sub>2</sub>O<sub>2</sub> and with varying concentrations of GAH for 4 hours. Cells were centrifuged, and the supernatant (ie, extracellular material) was extracted and assayed for LDH activity (Figure 1) . <strong>Figure 1</strong> Extracellular LDH activity from PC12 cells exposed to H<sub>2</sub>O<sub>2</sub> and varying concentrations of GAH. p-values are relative to 5 mM H<sub>2</sub>O<sub>2</sub> and 0 mg/mL GAH.Experiment 2LDH was purified and assayed for activity in the presence of 5 mM H<sub>2</sub>O<sub>2</sub> and varying concentrations of GAH (Figure 2) . <strong>Figure 2</strong> Activity of purified LDH with varying concentrations of GAHExperiment 3PC12 cells were incubated with or without 5 mM H<sub>2</sub>O<sub>2</sub> in the presence and absence of 1 mg/mL GAH. Cells were then stained with the fluorescent marker TUNEL, which binds strongly to apoptotic cells. The percentage of apoptotic cells in each condition is shown in Figure 3. <strong>Figure 3</strong> Percentage of apoptotic cells in the presence or absence of H<sub>2</sub>O<sub>2</sub> and GAH -The side chain of the C-terminal amino acid residue in GAH is derived from which structure? A) B) C) D)

Figure 1 Extracellular LDH activity from PC12 cells exposed to H₂O₂ and varying concentrations of GAH. p-values are relative to 5 mM H₂O₂ and 0 mg/mL GAH.Experiment 2LDH was purified and assayed for activity in the presence of 5 mM H₂O₂ and varying concentrations of GAH (Figure 2) .

Figure 2 Activity of purified LDH with varying concentrations of GAHExperiment 3PC12 cells were incubated with or without 5 mM H₂O₂ in the presence and absence of 1 mg/mL GAH. Cells were then stained with the fluorescent marker TUNEL, which binds strongly to apoptotic cells. The percentage of apoptotic cells in each condition is shown in Figure 3.

Figure 3 Percentage of apoptotic cells in the presence or absence of H₂O₂ and GAH
-The side chain of the C-terminal amino acid residue in GAH is derived from which structure?

Quizplus · Accepted Answer

11ecba2d_0f16_a6b8_a3bf_4d9891c805c5_MD0008_00 Amino acids consist of both a backbone structure and a side chain. The backbones of all biologically relevant amino acids contain an amino group (N-terminus), an α-carbon, and a carboxyl group (C-terminus). The side chain is bonded to the α-carbon, and each of the 20 standard amino acids has a unique side chain structure, which determines the identity of the amino acid. Peptides form when the C-terminus of one amino acid or peptide reacts with the N-terminus of another to form an amide bond. The newly formed peptide will have one N-terminus and one C-terminus. The amino acid residue closest to the N-terminus is the N-terminal residue, and the residue closest to the C-terminus is the C-terminal residue. Additional residues may be found between the N- and C-terminal residues. The tripeptide GAH consists of an N-terminal glycine, an alanine, and a C-terminal histidine. The side chain of histidine is derived from the molecule imidazole, a five-member aromatic ring that contains two nitrogen atoms separated by one carbon atom. Therefore, the side chain of the C-terminal residue in GAH is derived from imidazole, shown in Choice B. (Choice A) This is the structure of indole and is part of the side chain of tryptophan rather than histidine. (Choice C) This structure is phenol. It is part of the side chain of tyrosine, not histidine. (Choice D) This molecule is a cyclic secondary amine and is part of the structure of proline, not histidine. Educational objective: Amino acids consist of a side chain connected to a backbone structure that contains an amino group (N-terminus), an α-carbon, and a carboxyl group (C-terminus). Peptides are chains of amino acids that form when the amino group of an amino acid or peptide reacts with the carboxyl group of another amino acid or peptide. Like amino acids, peptides have an N-terminus and a C-terminus.

Passage Oxidative Stress Occurs in Cells When Reactive Oxygen Species (ROS)