Passage
Antimicrobial peptides are an important component of innate immunity. Since their discovery in 1939, over 5,000 peptides with antimicrobial properties have been isolated from organisms in every domain of life. They have demonstrated activity against enveloped viruses, bacteria, fungi, and even tumor cells and have recently gained interest for their potential in fighting antibiotic-resistant bacteria. Most known antimicrobial peptides are positively charged, but some with negative charges have also been discovered.Frogs are known to secrete a variety of antimicrobial peptides as a major source of immunity. Secretions from a new species of frog were collected and the peptides were separated by cation exchange chromatography. Four of the isolated peptides demonstrated strong antibacterial activity and were characterized by isoelectric focusing and sequencing:
Figure 1 Results of isoelectric focusing of four antimicrobial peptidesTable 1 Amino Acid Compositions of Four Antimicrobial Peptides
Adapted from Wang Z, Wang G. APD: the Antimicrobial Peptide Database. Nucleic Acids Res. 2004;32(Database issue) :D590-2.
-Which amino acid residue is most likely responsible for the decreased isoelectric point of Peptide 2 relative to Peptide 3?

Question

Passage
Antimicrobial peptides are an important component of innate immunity. Since their discovery in 1939, over 5,000 peptides with antimicrobial properties have been isolated from organisms in every domain of life. They have demonstrated activity against enveloped viruses, bacteria, fungi, and even tumor cells and have recently gained interest for their potential in fighting antibiotic-resistant bacteria. Most known antimicrobial peptides are positively charged, but some with negative charges have also been discovered.Frogs are known to secrete a variety of antimicrobial peptides as a major source of immunity. Secretions from a new species of frog were collected and the peptides were separated by cation exchange chromatography. Four of the isolated peptides demonstrated strong antibacterial activity and were characterized by isoelectric focusing and sequencing:

Figure 1 Results of isoelectric focusing of four antimicrobial peptidesTable 1 Amino Acid Compositions of Four Antimicrobial Peptides

Adapted from Wang Z, Wang G. APD: the Antimicrobial Peptide Database. Nucleic Acids Res. 2004;32(Database issue) :D590-2.
-Which amino acid residue is most likely responsible for the decreased isoelectric point of Peptide 2 relative to Peptide 3?

Quizplus · Accepted Answer

11ecba2d_0df1_fd17_a3bf_5dc4804d19ef_MD0008_00 A peptide's isoelectric point (pI) is the pH at which it has no net charge and therefore does not migrate in an electric field. This point is determined by the ionizable amino acid side chains present in the peptide. Seven side chains (R, K, H, Y, C, E, and D) are "ionizable," meaning their charges can be altered as protons are added or removed with varying pH: Ionizable side chains that contain oxygen or sulfur (Y, C, E, and D) are negatively charged when deprotonated and tend to decrease pI. For side chains that can be negatively charged, a lower pKa correlates with a stronger effect on pI. Therefore, aspartate and glutamate cause the greatest decreases in pI. Ionizable side chains that contain nitrogen (R, K, and H) are positively charged when protonated and tend to increase pI. Side chains that can be positively charged have a greater effect if the pKa is high, and therefore arginine and lysine cause the greatest increases in pI. All other side chains are nonionizable and do not affect pI. Peptides 2 and 3 each contain two lysine (K) residues, but Peptide 2 also contains a glutamate (E) residue, which decreases its pI. (Choice A) Threonine (T) does not have an ionizable side chain, so it does not affect a peptide's pI. (Choice B) Histidine (H) can contribute only a positive charge to a peptide, so it can only increase pI. (Choice C) Aspartate (D) can decrease pI, but it is not present in either Peptide 2 or Peptide 3. Educational objective: The isoelectric point is the pH at which a peptide is neutrally charged. Ionizable residues that can carry a negative charge, such as aspartate and glutamate, tend to decrease the pI. Those that can be positively charged, such as lysine and arginine, tend to increase it.

Passage Antimicrobial Peptides Are an Important Component of Innate Immunity. Since